Effects of Interseeding Cover Crops into Corn and Soybean on Biomass Production, Grain Yields and Ecosystem Services: A Review

Abstract

Planting cover crops can improve soil health and help to sustain agricultural crop yields. In northern climates where corn (Zea mays L.) and soybean (Glycine max L.) are grown, cover crop biomass production can be low. This has led to people investigating the potential of interseeding cover crops into the growing main crop. This paper sought to determine biomass production and the benefit to grain yields, weed control and soil properties from interseeding cover crops into corn and soybean. This review included 70 studies published prior to 15 March 2024. Interseeded cover crops that were winter-hardy such as cereal rye (Secale cereale L.) produced more biomass in the spring (1.04 Mg ha⁻¹ average biomass production) than any of the interseeded cover crops did in fall (0.35 Mg ha⁻¹ average biomass production), primarily at crop row spacings of 76 cm. Factors that affected cover crop biomass production were crop stage, planting method, tillage practice, irrigation and row spacing. There was not a consistent widely planted cover crop species that produced the most biomass. Interseeded cover crops reduced weed biomass by 46% compared to weed control and generally did not affect crop grain yields when planted after V4 crop stage. Interseeded cover crops reduced soil nitrate concentration but generally did not affect other soil properties including soil water content. However, most of these studies planted cover crops at the same site for less than three years. Early interseeded cover crops generally did not perform better than interseeded winter-hardy cover crops planted around crop physiological maturity.

1. Introduction

Cover crops are planted for soil and water conservation, to fix nitrogen for the following crop, reduce nutrient leaching, provide additional biomass for forage, increase diversity in the cropping system and for weed control [1,2]. Cropping systems in some regions have unfavorable conditions for cover crops to grow between grain harvest and planting the following crop. In areas such as the Northern Midwest and Northeastern United States, corn and soybeans are two of the more commonly grown crops [2,3]. These crops are generally planted in late April to early June and harvested for grain in mid-September–November in these regions [4,5]. The climate in these regions of the United States is cold during the fallow period of these crops, which leads to limited time for cover crops to grow in this cropping system [4]. Also, there are limited cover crop species that can produce significant biomass during the fallow period between these two annual crops. Species selection is limited to winter-hardy cover crops such as cereal rye (Secale cereale L.), hairy vetch (Vicia villosa L.) and red clover (Trifolium pratense L.) [6,7]. The poor growing conditions for many cover crop species during the fallow period limits the diversity of cover crop species that can be added to the cropping system and biomass that can be produced by the cover crop.

Cover crops are traditionally planted following harvest of the main crop and terminated by frost, tillage or herbicides applied at or prior to planting [8]. Another way that cover crops can be integrated into a cropping system is to plant them during the growing cycle of the main crop [9]. Cover crops can be planted with the main crop to provide nitrogen to the main crop, control weeds, provide cover for soil and water conservation and to produce forage that can be grazed by livestock once the main crop is harvested [1,10,11,12,13]. Cover crops are frequently planted by a grain drill after harvest [8]. It is difficult to plant with a drill into growing or standing crops, so alternative planting equipment has been developed to interseed cover crops into growing or standing crops. High-clearance drills have been developed that can plant between the rows of growing crops during the vegetative (V) stages [14]. Cover crops have been drill-interseeded up to the V10 stage in corn and during early V stages of soybean growth before the crop canopy begins to close [15,16]. This allows some light to get through the crop canopy to help establish the cover crop. Shade-tolerant cover crops such as annual ryegrass (Lolium multiflorum L.), clovers (Trifolium sp. L.), cereal rye and radish (Raphanus sativus L.) have been the most frequently used cover crops when interseeding into corn and soybeans [6,7,8,9,10,11,12,13,14,15,16,17]. Also, cover crops can be broadcast into crops at later reproductive stages (R stages) by high-clearance tractors and from planes, helicopters and drones [8,17]. When the crop reaches physiological maturity, some of the leaves begin to dry out, opening the crop canopy again [17]. Winter-hardy cover crops such as cereal rye, winter camelina (Camelina sativa L.) and hairy vetch are usually used when broadcasting cover crops into standing crops later in the growing season [17]. This provides additional time for the cover crop to become established prior to winter temperatures occurring and may increase biomass production compared to postharvest cover crop planting.

Interest in cover crops has increased in recent years as agricultural producers have become more interested in soil health and regenerative agriculture [18,19]. Also, cover crops can reduce some of the negative effects of agriculture to the environment by reducing loss of phosphorous and nitrogen by leaching and from runoff [19,20,21,22,23,24]. Benefits to the ecosystem from cover crops generally increase as biomass production from the cover crop increases [2,23,25]. However, in northern climates there is limited time for cover crop growth when full-season crops such as corn and soybeans are grown [2,3,4]. That has led to investigation and development of techniques to plant cover crops into a standing crop to increase the duration of cover crop growth [9,14,15,16]. Little is known about how much biomass is produced, what effect it has on grain yields and what ecosystem services are provided when cover crops are interseeded into growing corn and soybean crops.

Previously published studies, looking at cover crop and weed biomass production, grain yields and soil properties when interseeding cover crops into corn and soybeans between planting and harvest, were analyzed and are listed in

Table 1. Studies included in this review, including paper, location, cover crop species, crop stage when interseeded, planting technique, tillage practice, average annual precipitation, average annual temperature, crop row spacing, crop seeding rate and crop. NA means not available, CP is chisel plow, CT is conventional tillage, MP is moldboard plow, NT is no till, RT is reduced tillage, ST is strip tillage, V is vegetative crop growth stage and R is reproductive crop growth stage.

Paper	Location	Cover Crop Species	Crop Stage	Planting Technique	Tillage Practice	Average Annual Precipitation	Average Annual Temperature	Crop Row Spacing	Crop Seeding Rate	Crop
Abdin et al., 1998 [26] and Abdin et al., 2000 [27]	L’Assomption and MacDonald, QC, Canada	Cereal rye, hairy vetch, red clover, white clover, subterranean clover, yellow sweet clover, black medic, Persian clover, strawberry clover, crimson clover, alfalfa, berseem clover and mixtures	NA	Broadcast	MP	808	5.3	76	80,000	Corn
Adler et al., 2020 [28]	Leonard, MO, USA	Wheat + radish + turnip mixture	R6	Broadcast	NT	990.6	10.3	38	444,600	Soybean
Alonso-Ayuso et al., 2020 [29]	Aranjuaz, Spain	Annual ryegrass, barrel medic, yellow sweet clover and common vetch	V4 and V5	Drill	NA	334	15.1	76	80,000	Corn
Antosh et al., 2022 [30]	Mandan, ND, USA	Cereal rye + triticale + turnip + cowpea mixture	V4, V6 and V8	Drill	NT	455.9	5.8	76	59,280	Corn
Baributsa et al., 2008 [11]	Hickory Corners, MI, USA	Chickling vetch and red clover	V5–V7	Broadcast	CP	1003	10.1	76	37,500–75,000	Corn
Barker et al., 2018 [31]	Concord, Brule, Clay Center and Meade, NE, USA	Cereal rye and cereal rye + black oats + winter pea + hairy vetch + balansa clover + radish + forage collard mix	NA	Broadcast	NT	693, 508, 731 and 748	8.7, 10.1, 10.3 and 9.9	76	NA	Corn and soybean
Batildas 2019 [32]	Clay Center and Lincoln, NE, USA	Cereal rye, radish, hairy vetch and cereal rye + radish + hairy vetch mixture	Planting, V8, R5, R8	Broadcast	NT	731 and 734	10.3 and 10.8	76	84,000 and 67,000	Corn
Belfry and Van Eard, 2016 [5]	Cedar Springs, Chatham-Southwest, Doyles, Erie Beach, Erieau, Leonard, Lighthouse Cove, Raglan and Van Horne, ON, Canada	Alfalfa, radish, cereal rye, crimson clover, red clover, hairy vetch, oat, pea, soybean and mixtures	V4–V12	Broadcast	CT and NA	963	9.7	76	84,000	Corn
Berti et al., 2017 [33]	Fargo and Prosper, ND and Morris, MN, USA	Winter camelina	Planting–silking in corn and planting-R2 in soybean	Broadcast and drill	NA	571, 572, 673	5.7, 5.7, 5.7	61 and 76 in corn and 31 and 61 in soybean	86,450 in corn and 432,250 in soybean	Corn and soybean
Bich et al., 2014 [34]	Andover, Aurora and Trail City, SD, USA	Clover + lentil + winter wheat mixture	V3–V6	Broadcast and drill	NT and CT	601, 616 and 414	6.4, 6.2 and 7.4	76	71,630, 79,040 and 61,570	Corn
Bishop and Nelson, 2018 [35]	Novelty, MO, USA	Pennycress	V4–V6 and R5-R6	Broadcast	NT	990	10.3	76	74,100	Corn
Black et al., 2023 [13]	Cherry Grove, Goodhue and Faribault, MN, USA	Annual ryegrass + red clover + crimson clover + bayou kale + radish + turnip mixture	V3–V4	Broadcast	RT	863, 863, 828	6.4, 6.4, 6.7	76, 152	90,000	Corn
Blanco-Canqui et al., 2017 [36]	Bellwood, NE, USA	Cereal rye	NA	Broadcast	ST	756	9.3	NA	NA	Corn
Brockmueller et al., 2022 [37]	Arlington and East Troy, WI, USA	Cereal rye	Planting	Drill	CT	869	8.8	76	531,265	Soybean
Brooker et al., 2020 [9]	East Lansing and Richville, MI, USA	Annual ryegrass, crimson clover and radish	V2, V3, V4, V5, V6 and V7	Broadcast	CP	784	8.4	76	79,000	Corn
Brooker et al., 2020 [38]	Clayton, Hart, Hickory Corners, Hillman and Springport, MI, USA	Annual ryegrass, crimson clover, oilseed radish	V3 and V6	Broadcast	NT and CT	925, 929, 1003, 762 and 833	8.9, 7.6, 10.1, 6.1 and 9	76	79,100, 70,395, 69,160, 71,630, 69,160, 79,100 and 83,980	Corn
Carabajal–Capitan et al., 2021 [39]	Lingle, WY, USA	Turnip, pea, winter wheat and mixtures	V6	Broadcast	CT	355	8.4	76	84,000	Corn
Caswell et al., 2019 [40]	Beltsville, MD, Rock Springs, PA and Aurora, NY, USA	Crimson clover, ladino clover, red clover, yellow sweet clover, hairy vetch and Kentucky bluegrass	V5–V8 in corn and V3 to R1 in soybean	Drill	NT	1191, 1103 and 992	9.94, 13.61 and 9.0	76	69,000 and 79,000 in corn and 432,000 in soybean	Corn and soybean
Chen et al., 2006 [41]	Lamberton, Waseca and Rosemount, MN, USA	Alfalfa, red clover, annual ryegrass	Planting and 2 weeks after planting	Broadcast	ST	709, 908 and 886	6.8, 7.1 and 6.8	76	432,000	Soybean
Chikoye et al., 2006 [42]	Ibadin, Nigeria	Velvet bean	NA	Sown	NT	1467	25.9	50 and 76	53,000 and 80,000	Corn
Curran et al., 2018 [43]	Many sites in NY, PA and MD, USA	Annual ryegrass, red clover + crimson clover+ hairy vetch mixture and annual ryegrass + red clover + crimson clover+ hairy vetch mixture	V3–V7	Drill	NT and CT	992–1191	9–13.61	76	69,000–87,000	Corn
Curtright et al., 2023 [44]	East Lansing and Richville, MI, USA	Annual ryegrass, crimson clover, radish and annual ryegrass + crimson clover + radish mixture	V3 and V6	Broadcast	NA	805 and 845	8.9 and 8.7	76	NA	Corn
Donaldson 2019 [45]	Linn County, OR, USA	Oats, winter and spring wheat and cereal rye	V5–V6	Drill	CT	1219	12.3	76	NA	Corn
Fakhari et al., 2015 [46]	Ardabil, Iran	Cereal rye, berseem clover and hairy vetch	Planting	Broadcast	CT	80.7	10.1	76	NA	Corn
Francis, 2020 [47]	Arlington, WI, USA	Red clover	V4–V5	Drill	NT	869	8.8	76	NA	Corn
Franzen et al., 2020 [48]	Rutland and Gardener, ND, USA	Cereal rye + radish + winter camelina mixture	V6–V8	Broadcast	NT and CT	561 and 519	5.8 and 5.5	NA	NA	Corn
Goettl et al., 2024 [49]	Leonard and Rutland, ND, USA	Flax + faba bean + oats + winter pea + crimson clover + buckwheat + phacelia + forage radish, cereal rye + red clover + alfalfa, and sorghum + millet + cereal rye + red clover + alfalfa	V4, V5	Drill	NT	579 and 561	6.1 and 5.5	152	34,000 and 32,000	Corn
Hines et al., 2023 [50]	Kimberly, ID, USA	Red clover + yellow sweet clover + crimson clover + annual ryegrass + radish and red clover + annual ryegrass + Italian ryegrass + radish + rapeseed + turnip	V5	Broadcast and broadcast + incorporated	CT	281	8.6	76, 118, 152	38,500	Corn
Hively and Cox, 2001 [51]	Fayette, NY, USA	White clover, red clover, barrel medic, alfalfa, ryegrass and red fescue	NA	Broadcast	CT	1093	9.3	76	500,000	Soybean
Isbell et al., 2022 [52]	Pennsylvania Furnace, PA, USA	Annual ryegrass	V5–V6	Drill	CP	1006	10.1	76	81,150	Corn
Johnson et al., 1998 [53]	Ames, IA, USA	Oats, cereal rye and oat + cereal rye mixture	NA	Broadcast	NT	909	9.80	76	NA	Soybean
Johnson et al., 2021 [54]	Fargo, ND, USA	Winter camelina and cereal rye	R7	Broadcast	NT	571	5.7	30.5	469,300	Soybean
Jones et al., 1998 [55]	Hickory Corners, MI, USA	Annual ryegrass + red clover mixture	V6	Broadcast	CT	1003	10.1	76	NA	Corn
Kandel et al., 2021 [56]	Fargo and Casselton, ND, USA	Cereal rye and winter camelina	R6	Broadcast	NT	571	5.7	30.5 and 61	469,300	Soybean
Kaspar et al., 2001 [57]	Ames, IA, USA	Cereal rye and oat	NA	Broadcast	CT	909	9.8	76	NA	Soybean
Kaspar et al., 2006 [58]	Ames, IA, USA	Cereal rye, oat and cereal rye + oat mixture	NA	Broadcast	NA	909	9.8	76	387,000	Soybean
Koehler-Cole and Elmore 2020 [59]	Clay Center and Meade, NE, USA	Cereal rye and hairy vetch	NA	Broadcast	NT	731 and 738	10.3 and 9.9	76	84,000 and 65,000 in corn and 372,000 and 435,000 in Soybean	Corn and soybean
Koehler-Cole et al., 2023 [60]	Concord, Clay Center and Meade, NE, USA	cereal rye, hairy vetch + winter pea mix, cereal rye + hairy vetch + winter pea + radish mixture and cereal rye + black oat + forage collard + radish + hairy vetch + winter pea + clover mixture	R5.5 in corn and R6 and 7 in soybean	Broadcast	NT	731, 693 and 748	8.7, 10.3 and 9.9	76	NA	Corn and soybean
Koehler-Cole et al., 2020 [61]	Concord, Clay Center and Meade, NE, USA	Cereal rye and cereal rye + radish + hairy vetch + winter pea mixture	R5.5	Broadcast	NT	731, 693 and 748	8.7, 10.3 and 9.9	76	NA	Corn
Liu et al., 2019 [62]	Lamberton, MN, USA	Cereal rye, pennycress and winter camelina	NA	Broadcast and incorporated	RT	708	6.9	76	86,484 and 373,116	Corn and soybean
Marcos et al., 2023 [63]	Ames, Crawfordville and Sutherland, IA, USA	Cereal rye	R6	Broadcast	CT	909, 950 and 881	9.8, 10.3 and 8.11	76	NA	Soybean
Mohammed et al., 2020a [64] and Mohammed et al., 2020b [65]	Ames, IA, Prosper, ND and Morris and Rosemount, MN, USA	Pennycress, cereal rye and winter camelina	R4, R5 and R6	Broadcast	CT	909, 572, 673 and 886	9.8, 5.7, 5.7 and 6.8	76	79,000, 82,000 and 89,000 in corn and 445,000, 460,000, 430,000 and 376,000 in soybean	Corn and soybean
Moore and Mirsky, 2020 [66]	Beltsville, MD, USA	Cereal rye, annual ryegrass, red clover + crimson clover + hairy vetch mixture and annual ryegrass + red clover + crimson clover + hairy vetch mixture	V4–V6 and R6	Drill and broadcast	NT	1191	13.6	76	69,000	Corn
Nelson et al., 2022 [67]	Novelty, MO, USA	Cereal rye and annual ryegrass	R6, R6.5, R7 and R8	Broadcast	NT	990.6	10.3	76	419,000	Soybean
Nelson et al., 2011 [68]	Novelty, MO, USA	Cereal rye	Planting	Broadcast	CT	990.6	10.3	19 and 76	346,000, 494,000 and 741,000	Soybean
Nguyen et al., 2022 [69]	Hickory Corners, MI, USA	Oats + winter pea + radish mixture and annual ryegrass + rapeseed + crimson clover mixture	V5–V6	Broadcast	CT	1003	10.1	76	69,000	Corn
Noland et al., 2018 [70]	Lamberton and Waseca, MN, USA	Hairy vetch, pennycress, red clover, cereal rye and oat + pea + radish mixture	V7	Broadcast and drill	NT	709 and 908	6.8 and 7.1	76	86,500	Corn
Nunes et al., 2018 [71]	Aurora, NY, USA	Annual ryegrass + red clover + crimson clover + hairy vetch mixture	NA	Drill	NT and MP	916	8.9	NA	NA	Corn
Peterson et al., 2019 [72]	Fargo and Prosper, ND, USA	Winter camelina, winter pea, radish, cereal rye and winter camelina + winter pea + radish + cereal rye mixture	R4 and R6	Broadcast	CT	571	5.7	76	505,000	Soybean
Peterson et al., 2021 [15]	Beltsville and Glendale, MD, USA	Cereal rye, cereal rye + hairy vetch mixture, cereal rye + pea mixture, cereal rye + red clover mixture, and cereal rye + crimson clover mixture	R5	Drill	NT	1191 and 1132	13.61 and 12	76	434,450	Soybean
Rabin et al., 2023 [73]	Lamberton and Waseca, MN, USA	Annual ryegrass, annual ryegrass + crimson clover mixture and annual ryegrass + crimson clover + radish mixture	R5–R6 in corn and R7–R8 in soybean	Broadcast	NT, ST and CT	709 and 908	6.8 and 7.1	76	89,000 in corn and 371,000 in soybean	Corn and soybean
Reinbott et al., 2004 [74]	Columbia, MO, USA	Oat, hairy vetch, pea, oat + hairy vetch mixture, and oat + pea mixture	R7	Broadcast	NT	1082	12.5	76	NA	Soybean
Ruis et al., 2020 [75]	Concord, Clay Center and Mead, NE, USA	Cereal rye and cereal rye + radish + hairy vetch + winter pea mixture	R5.5	Broadcast	NT	731, 693 and 748	8.7, 10.3 and 9.9	76	NA	Corn and soybean
Rusch et al., 2020 [76]	Grand Rapids, Lamberton and Waseca, MN, USA	Annual ryegrass, annual ryegrass + crimson clover mixture, annual ryegrass + crimson clover + radish mixture, cereal rye, cereal rye + crimson clover mixture and cereal rye + crimson clover+ radish mixture	V6 and R5	Broadcast	ST	734, 709 and 908	4.6, 6.9 and 7.1	76	86,000	Corn
Schmidt et al., 2021 [77]	Casselton, Hickson and Prosper, ND, USA	Cereal rye, winter camelina and oilseed radish	V7 and R4	Drill and broadcast	CT	593, 571 and571	5.3, 5.6 and 5.7	76	NA	Corn and soybean
Sedghi et al., 2023 [78]	Talbot and Kent County, MD, USA	Cereal rye + clover + brassica mixture	NA	Broadcast	NT, RT and CT	1105–1120	13.6–14.1	NA	NA	Corn and soybean
St. Aime et al., 2023 [16]	Pendleton, SC, USA	Buckwheat, pigeon pea, white clover and buckwheat + pigeon pea + white clover mixture	V4, V7 and V10	Broadcast	CT	1220	16.2	76	80,000	Corn
Stanton and Haramoto, 2021 [79]	Princeton, KY, USA	Orchard grass and annual ryegrass	V5	Drill	NT	1296	14.7	76	74,000	Corn
Thelen et al., 2004 [80]	Hickory Corners, MI, USA	Cereal rye	Planting and V4	Drill	CT	1003	10.1	19 and 76	446,000, 889,200 and 1,333,800	Soybean
Thompson et al., 2020 [81]	Lexington, IL, USA	Cereal rye + radish mixture	NA	Broadcast	NT	998	10.6	NA	NA	Corn and soybean
Uchino et al., 2009 [82], 2012 [83] and 2015 [1]	Sapporo, Hokkaido, Japan	Cereal rye and hairy vetch	2 week before planting, planting and 2 weeks after planting	Seed Tape	CT	1229	6.2	6-	74,000 in corn and 83,333 and 100,000 in soybean	Corn and soybean
Wallace et al., 2017 [84]	Aurora, NY, Beltsville and Landisville, MD and Pennsylvania Furnace, PA, USA	Annual ryegrass and red clover	V5	Drill	NT	917, 1191, 1091 and 1006	9.1, 13.6, 11.5 and 10.1	76	69,000–87,000	Corn
Wallace et al., 2021 [14]	Lancaster, Mifflin and Union County, PA, USA	Annual ryegrass + orchard grass + forage radish mixture	V5–V7	Broadcast and drill	RT and MP	1087, 1051 and 1005	11.5, 10.8 and 10.1	76	65,000–81,250	Corn
Wilson et al., 2019 [85]	Rosemount, MN, USA	Cereal rye	NA	Broadcast	CT	886	6.8	76	NA	Soybean
Youngerman et al., 2018 [86]	Aurora, NY, Beltsville, MD and Pennsylvania Furnace, PA, USA	Cereal rye + annual ryegrass + hairy vetch + red clover mixture	V5	Drill	MO	917, 1191 and 1006	9.1, 13.6 and 10.5	76	37,100, 74,100 and 111,200	Corn

. The objectives of this review are the following. (1) Determine how much cover crop biomass is produced and what factors affect biomass production when they are interseeded into corn and soybeans. (2) Analyze the effects of interseeded cover crop on grain yields, weed control and soil properties. The hypothesis is that planting cover crops during the early V stages of the crop would produce the most biomass, provide the best weed control and supply the greatest benefits to soil properties, but may have a negative effect on crop grain yields. Cover crop mixtures would produce more biomass than single cover crop species that were interseeded and would offer the greatest benefit to weed control and soil properties. Legume cover crops would have larger benefits for corn yield than other species, whereas cover crop mixtures would be most beneficial to soybean yields.

2. Materials and Methods

A search of Google Scholar was conducted looking for papers that investigated cover crops planted into corn and soybean prior to harvest. Search terms entered into Google Scholar included corn or soybeans combined with “cover crop” and “interseeding”, “cover crop and broadcasting”, and “cover crop and preharvest planted”. Papers were included from both organic and conventional cropping systems. Papers were only included in this analysis that planted cover crops into growing or standing corn or soybeans under the same tillage and fertilization practices. Papers included in this analysis included open access papers and ones from journals to which South Dakota State University Library subscribes. Web Plot Digitizer Version 5 was used to extract data from graphs [87]. Average annual temperature and precipitation were collected from worldclimate.com or usclimatedata.com [88,89].

Many papers reported results of biomass production or grain yield averaged across planting dates or reported data on cover crop species averaged across planting dates in the analysis. In the comparison of biomass production from cover crop species and the effect of cover crop species on grain yields, data were averaged across planting dates. In the results analyzing the effect of cover crop planting dates on biomass production and grain yields, data were averaged across cover crop species for the given planting date. When comparing different planting methods, tillage practices and mean annual temperature and precipitation, data were averaged across cover crop species and planting dates. Data presented in the text and figures represent the averaged value across studies with standard error as error bars or ±. Data were included in the analysis as presented in the manuscript, although some papers present data averaged across site years and locations, whereas others present data with individual site years. The high number of observations in the analysis is due to some studies including many cover crop species and planting date combinations. An ANOVA was conducted with Bonferroni t-tests of differences between means to compare different factors that may affect cover crop biomass production, grain yields and weed biomass production in SAS 9.4 [90,91]. Scientific names of cover crops/crops included in this review are cereal rye (Secale cereale L.), hairy vetch (Vicia villosa L.), red clover (Trifolium pratense L.), white clover (Trifolium repens L.), subterranean clover (Trifolium subterraneum L.), yellow sweet clover (Melilotus officinalis L.), black medic (Medicago lupulina L.), Persian clover (Trifolium resupinatum L.), strawberry clover (Trifolium fragiferum L.), crimson clover (Trifolium incarnatum L.), alfalfa (Medicago sativa L.), berseem clover (Trifolium alexandrinum L.), wheat (Triticum aesivium L.), radish (Raphanus sativus L.), turnip (Brassica rapa L.), annual ryegrass (Lolium multiflorum L.), common vetch (Vicia sativa L.), barrel medic (Medicago truncatula L.), triticale (Triticosecale species L.), cowpea (Vigna unguiculata L.), chickling vetch (Lathyrus sativus L.), black oats (Avena strigosa L.), oats (Avena sativa L.), winter pea (Pisum sativum L.), balansa clover (Trifolium michelianum L.), forage collard (Brassica oleracea L.), soybean (Glycine max L.), corn (Zea mays L.), winter camelina (Camelina sativa L.), pennycress (Thlaspi arvense L.), lentil (Vicia lens L.), bayou kale (Brassica oleracea sabellica L.), ladino clover (Trifolium repens L.), Kentucky bluegrass (Poa pratensis L.), velvetbean (Mucuna pruriens L.), flax (Linum usitatissimum L.), faba bean (Vicia faba L.), buckwheat (Fagopyrum esculentum L.), sorghum (Sorghum bicolor L.), millet (Pennisetum glaucum L.), phacelia (Phacelia tanacetifolia L.), Italian ryegrass (Lolium multiflorum L.), red fescue (Festuca rubra L.), pigeon pea (Cajanus cajan L.) and orchard grass (Dactylis glomerata L.).

3. Interseeded Cover Crop Biomass Production

3.1. Average Cover Crop Biomass Production

In this review, 38 papers reported fall biomass production and 34 papers reported spring biomass production from interseeded cover crops (Table 1). Thirty-two of the studies reported fall biomass production and 24 reported spring biomass production from cover crops that were interseeded into corn (Table 1). Also, 14 papers were found that determined fall and 18 that determined spring cover crop biomass production when cover crops were interseeded into soybeans (Table 1). Average fall biomass production in corn was 0.30 ± 0 02 Mg ha⁻¹ and spring cover crop biomass production was 0.96 ± 0.078 Mg ha⁻¹. In soybeans, average fall biomass production was 0.45 ± 0.045 Mg ha⁻¹ and spring biomass production was 1.12 ± 0.092 Mg ha⁻¹. The biomass production across studies on interseeded cover crops is less than that reported by a review that found an average cover crop biomass production of 1.5 to 3 Mg ha⁻¹ across the climatic conditions of studies that were included in this review using a variety of cover crop planting methods [92]. This suggests that cover crop performance in these interseeded cover crops may not be as strong as cover crop biomass production averaged across cropping systems in the geographic locations included in this review.

3.2. Cover Crop Species and Mixtures

Certain cover crop species may perform better when they are planted into growing crops or a combination of different species may perform better. In previous research, there were 15 individual cover crop species that were interseeded into corn and 16 planted into soybeans along with many different mixtures of species (Figure 1 and Figure 2). In corn, the most common cover crop species used were annual ryegrass, cereal rye, hairy vetch, radish and red clover (Figure 1A,B). In soybean, the most commonly planted cover crop species were annual ryegrass, cereal rye, oats and winter camelina (Figure 2A,B). Many studies also planted cover crop mixtures containing multiple species (Table 1). The mixtures vary across studies, so mixtures were combined by the number of species that they contained, with mixtures being categorized as two-species, three-species or a diverse mixture with four or more species.

In corn, fall biomass production was the highest in pea, winter wheat and diverse mixture, and lowest in the pennycress and winter camelina cover crops (Figure 1A). Species/mixture diversity affected both fall and spring biomass production at a probability (p) < 0.0001. However, the two cover crop species that produced the most biomass had only two observations, making it difficult to conclude that they performed better than the other species. The widely planted species, including cereal rye, hairy vetch, annual ryegrass, radish and red clover, produced a similar amount of fall biomass (0.20–0.35 Mg ha⁻¹) (Figure 1A). Red clover produced the most spring biomass, although this was averaged across different corn seeding rates during four years at a location in Michigan ([11] and Figure 1B). Cereal rye produced similar spring biomass to annual ryegrass and alfalfa, but produced more biomass than hairy vetch, winter camelina and mixtures (Figure 1B). It is worth noting that most of the mixtures that overwintered contained cereal rye in them, and much of the biomass may have consisted of cereal rye (Table 1). Some of the other species in these mixtures were not winter-hardy and may have only contributed to biomass in fall but not the following spring. Diverse mixtures produced the most fall biomass, which agrees with the hypothesis, but in spring cover crop mixtures did not produce more biomass than individual species.

In soybeans, the cover crop species of pea, radish, diverse mixtures and cereal rye produced the most, whereas clovers and annual ryegrass produced the least fall biomass (Figure 2A). Fall biomass production by different cover crop species was significantly varied at p < 0.0001 but not with a p value = 0.1046 for spring cover crop biomass production. Cereal rye, annual ryegrass and mixtures were the most common cover crops interseeded into soybean in previous studies (Figure 2A,B). Cereal rye, hairy vetch and the two-species mixture produced more spring biomass than any of the clover species, pennycress and winter camelina. All of the two-species mixtures that overwintered contained either cereal rye or hairy vetch in the seed mix, which likely made up much of the biomass that was produced. Cereal rye and hairy vetch are two of the more winter-hardy cover crops available, which may be why they performed well in these cover crop studies in temperate climates [17]. In soybeans, cover crop mixtures produced similar biomass to many of the commonly planted individual cover crop species, which does not completely agree with the hypothesis that cover crop mixtures would produce the most biomass. However, compared to a review which analyzed biomass production in many agroecosystems [79], the biomass production in studies included in this review was much lower. This shows that interseeded cover crops generally performed worse than average cover crop performance in the regions where these interseeding studies were conducted.

3.3. Crop Stage When Interseeded

Planting cover crops into a certain crop growth stage can affect shading of the interrow area of the main crop and competition between the planted cover crop and the main crop. Seeding cover crops into the main crop at the right time minimizes competition between the cover crop and main crop but allows for adequate cover crop biomass production. Some of the studies did not include crop growth stage when cover crop was planted, making it difficult to perform this comparison (Table 1). Winter-hardy cover crops such as cereal rye, hairy vetch and winter camelina were most commonly planted during the R stages (R4–R8), whereas mixtures and less cold-tolerant species such as annual ryegrass, radish and clover species were planted earlier in the growing season during the V stages (Table 1). In corn, fall cover crop biomass production was significantly affected at p = 0.0081 and spring biomass at p < 0.0001. Planting cover crops at planting and the V4 stage produced the most fall biomass in corn, although high biomass produced when interseeded at planting in one irrigated location led to the high average value ([46] and Figure 3A). These results were expected in the hypothesis due to a greater time for cover crops to grow when they are planted during the early V stages. The highest biomass production the following spring occurred when cover crops were planted at the R6 stage (Figure 3B). However, many of the studies that planted cover crops at earlier crop stages planted cover crop species that would winterkill. Also, some of the crop species may have not been able to outcompete corn once the corn plants became large and the cover crop plants died. Most of the studies that planted cover crops during the R stages used cover crop species or contained species in the mixture that would survive throughout the winter at the location where they were planted. That is likely why cover crops planted at the later stages of corn growth produced more spring biomass.

Cover crops were interseeded at fewer growth stages in soybeans compared to corn (Figure 4A). Most of the studies broadcast-seeded cover crops into soybeans during the R stages, apart from some studies which looked at the potential of interseeded cover crops to control weeds [37,68,80,83]. The highest fall biomass was produced when cover crops were planted into soybeans at the R4 and R6 stage, whereas lowest biomass was produced when cover crops were planted at the V4 and R5 stages (Figure 4A). However, fall biomass was not significantly affected by growth stage (p = 0.5582), while spring biomass was affected by the crop stage at which cover crops were interseeded (p = 0.0017). The highest spring biomass was produced when cover crops were planted into soybeans at the R5 and R8 stages (Figure 4B). These findings do not agree with the hypothesis that planting cover crops during early V stages would produce more fall biomass in soybeans. Broadcasting winter-hardy cover crops into both corn and soybeans between the R4 and R8 stages generally produced more biomass the following spring than planting any cover crop species at earlier growth stages did in the fall. This finding suggests that planting summer annual cover crops during V stages of the corn and soybean crops does not improve cover crop biomass production compared to planting a winter-hardy cover crop when the crop is near physiological maturity and the canopy begins to open back up. A potential drawback to using fall biomass measurement as an indication of performance of cover crops planted in May or June is that the cover crops may germinate but then die when the crop canopy closes. The biomass from cover crops may already be broken down by the time cover crop biomass is measured in the fall. However, this also may indicate poor performance of the cover crop species planted in shaded conditions or summer temperatures.

3.4. Cover Crop Seeding Method

Traditionally, cover crops are planted postharvest using a drill to ensure good seed-to-soil contact. When cover crops are broadcast, the seeds fall on the soil surface and may be eaten by animals or have poor contact with the soil [39,79]. Many of the papers broadcast cover crops into growing crops due to difficulty drill-seeding at later stages of crop growth and lack of equipment available to drill the seed (Table 1). In corn, average fall cover crop biomass was 0.28 ± 0.03 Mg ha⁻¹ when cover crops were broadcast, 0.45 ± 0.11 Mg ha⁻¹ when broadcast and incorporated and 0.28 ± 0.03 Mg ha⁻¹ when drilled. In soybean, average fall biomass was 0.46 ± 0.046 Mg ha⁻¹ when broadcast, 0.26 ± 0.056 Mg ha⁻¹ when broadcast and incorporated and 0.61 ± 0.13 Mg ha⁻¹ when drilled. However, in studies where cover crop species overwintered, spring biomass produced under corn was 1.12 ± 0.11 Mg ha⁻¹ when broadcast and 0.74 ± 0.13 Mg ha⁻¹ when drilled, and in soybean was 0.97 ± 0.08 Mg ha⁻¹ when broadcast and 2.23 ± 0.38 Mg ha⁻¹ when drilled. Fall (p = 0.1417) biomass production was not affected by planting method but spring was at p = 0.0227 in corn. In soybeans, drilled cover crops produced more biomass the following spring, although many of the observations were from one study in Maryland [15]. In soybeans, fall biomass production was not significantly affected by planting method p = 0.0998 but spring biomass was at p < 0.0001. Caution should be taken when analyzing spring biomass production due to the drilled technique being used more often during V stages, and broadcast technique being more often used during the R stages (Table 1). Some studies also compared broadcast cover crops to drill-planted cover crops that were planted at the same date and location [14,70,77]. One study at two locations in Minnesota found that drilling the cover crop produced more fall cover crop biomass than broadcasting [70]. However, in this study planting method did not affect spring pennycress or cereal rye biomass, while planting the cover crop by drill increased red clover and winter camelina spring biomass production [70]. Another study conducted at three locations in North Dakota found that drilled cover crops produced more fall biomass than broadcast cover crops when planted at V7 but not the R4 stage [77]. Also, a study conducted in three counties in Pennsylvania found that drilled cover crops at the V5–V7 stage produced more fall biomass than broadcast cover crops [14]. Although averaged across studies biomass production was similar with the different planting methods, drill-seeded cover crops produced more biomass when they were planted at the same time and location in some studies.

3.5. Tillage Practice

Tillage practice may affect how cover crops perform when they are interseeded into growing crops. Tillage affects the amount of crop residue on the surface and may affect the seed-to-soil contact. Average fall biomass production was higher under conventional tillage practices, including chisel plow and moldboard plow, compared to no tillage (Figure 5). Many of the studies did not specify the types of tillage implements they used, so these were grouped into conventional tillage in the comparison. Broadcasting cover crops on top of soil under no till produced the least amount of fall biomass, whereas drilling or broadcasting into conventional till produced the highest biomass (Figure 5). Under no tillage, drill seeding produced more fall biomass compared to broadcast seeding, but biomass production was similar across planting techniques under tilled soils (Figure 5A). Across planting methods, tillage type statistically affected fall (p < 0.0001) and spring biomass (<0.0001) production. Spring biomass production was higher under chisel plow (2.47 ± 0.43 Mg ha⁻¹ across 23 observations) than no tillage, but biomass averaged across tilled plots (1.03 ± 0.087 Mg ha⁻¹ across 108 observations) was similar to that under no tillage (1.09 ± 0.074 Mg ha⁻¹ across 189 observations). When comparing the different planting methods by tillage combinations, fall and spring biomass production were significantly affected at p < 0.0001. This suggests that interseeded cover crops produce more fall biomass when they are broadcast onto soils that are managed with tillage techniques that disturb the soil surface. However, spring biomass production was similar across the tillage practices.

3.6. Cover Crop Seeding Rate

The seeding rate of cover crops also may affect how much biomass is produced. Most studies only planted a cover crop species at one seeding rate. Across studies, seeding rate was not correlated to biomass production. One study planted winter camelina and cereal rye at full and 75% seeding rate [54]. Spring biomass production was similar in winter camelia when seeded at 5.04 kg ha⁻¹ (0.097 Mg ha⁻¹) and 6.72 kg ha⁻¹ (0.103 Mg ha⁻¹) and cereal rye when seeded at 67.2 kg ha⁻¹ (0.321 Mg ha⁻¹) and 50.4 kg ha⁻¹ (0.304 Mg ha⁻¹) [54]. Another study that broadcast-seeded cereal rye into soybean at R7 and postharvest-drilled the cover crop found spring biomass production was 0.57 Mg ha⁻¹ when seeded at 46 kg ha⁻¹, 0.66 Mg ha⁻¹ when seeded at 67 kg ha⁻¹ and 0.77 Mg ha⁻¹ when seeded at 88 kg ha⁻¹, averaged across three sites [63]. Increasing seeding rate increased the spring biomass production in two of the six site years in this Iowa, USA study [63]. An additional study broadcast-planted cereal rye and hairy vetch into corn at the R5 stage and soybean at the R7 or 8 stage [59]. This study found hairy vetch biomass production was similar at three seeding rates (45, 60 and 90 kg ha⁻¹) in both corn and soybean across two years and locations in Nebraska [59]. It found that rye biomass was higher at seeding rates of 101 kg ha⁻¹ (1.66 Mg ha⁻¹) and 135 kg ha⁻¹ (1.8 Mg ha⁻¹) than at 67 kg ha⁻¹ (0.95 Mg ha⁻¹) when broadcast into corn, but no differences were found when broadcast into soybeans [59]. These findings show that in some cases using higher seeding rates can lead to an increase in biomass production, but more studies are needed to see how other species perform when they are interseeded into growing crops at different seeding rates. However, seeding cover crops at a higher seeding rate costs more for the producers, and did not cause a large increase in cover crop biomass production in the three previous studies.

3.7. Corn or Soybean Row Spacing and Seeding Rate

A majority of studies found in this review planted corn and soybeans at 76 cm row spacing (Table 1). However, some producers are interested in planting corn in wider rows to increase cover crop biomass and improve ecosystem services provided from the cover crop. Three studies measured fall cover crop biomass production when corn was planted at 152 cm row spacing [13,49,50], and one measured cover crop performance when corn was planted at 118 cm row spacing [50]. Across studies, average fall cover crop biomass production under 76 cm row spacing was 0.28 ± 0.02 Mg ha⁻¹, under 112 cm was 1.58 ± 0.36 Mg ha⁻¹ and under 152 cm was 1.17 ± 0.36 Mg ha⁻¹. Another study measured winter camelina cover crop biomass when planted into corn at 61 and 76 cm row spacing and found no significant interaction between corn row spacing and percent soil cover or biomass production [33]. Three studies planted cover crops into soybeans at different row spacings [33,56,68]. A study that broadcast cereal rye into soybeans at planting with 19 and 76 cm row spacing found that there was no difference in cover crop biomass between the two row spacings [68]. Another study found no differences in percent soil cover and spring and fall biomass production of cereal rye and winter camelina planted into R6-stage soybean at 30.5 and 61 cm row spacing [56]. An additional study found no effect of soybean row spacing on winter camelina biomass production at 31 and 61 cm row spacing [33]. These data suggest that cover crop biomass production was similar when comparing row spacings of 76 cm or less, but that more biomass may be produced when interseeding cover crop into corn planted at 118 and 152 cm row spacing.

The main crop seeding rate can affect the density of corn or soybean plants per hectare and how much light can penetrate through the canopy for the cover crop to use [86]. It would be expected that increasing the grain crop seeding rate might reduce cover crop biomass production and light penetration through the plant canopy. Most studies only used one corn or soybean seeding rate at a location, and there was no correlation between seeding rate and cover crop biomass production in the spring or the fall across studies. Two studies analyzed cover crop biomass production when seeded into corn planted at different seeding rates [11,86]. One study planted two different cover crop species into corn seeded at 37,500, 55,000, 65,000 and 75,000 seeds ha⁻¹ [11]. This study found that chickling vetch biomass production was higher in the 37,500 seeds ha⁻¹ seeding rate (0.51 Mg ha⁻¹) than the 55,000 seeds ha⁻¹ (0.28 Mg ha⁻¹), 65,000 seeds ha⁻¹ (0.31 Mg ha⁻¹) and 75,000 seeds ha⁻¹ (0.20 Mg ha⁻¹) seeding rates across four site years [11]. Also, this study found that fall red clover biomass production was 0.37 Mg ha⁻¹ at 37,500 seeds ha⁻¹, 0.26 Mg ha⁻¹ at 55,000 seeds ha⁻¹, 0.24 Mg ha⁻¹ at 65,000 seeds ha⁻¹ and 0.18 Mg ha⁻¹ at 75,000 seeds ha⁻¹ across three site years [11]. The chickling vetch did not overwinter, but the red clover produced 4.25 Mg ha at 37,500 seeds ha⁻¹, 4.16 Mg ha⁻¹ at 55,000 seeds ha⁻¹, 4.29 Mg ha⁻¹ at 65,000 seeds ha⁻¹ and 3.6 Mg ha⁻¹ at 75,000 seeds ha⁻¹, which was not significantly different [11]. The other study that interseeded a cereal rye, annual ryegrass, hairy vetch and red clover cover crop mixture into corn planted at 37,100, 74,100 and 111,200 seeds ha⁻¹ found cover crop biomass decreased as corn planting population increased [86]. Mean cover crop biomass production was 0.28 Mg ha⁻¹ when corn was seeded at 37,500 seeds ha⁻¹, 0.19 Mg ha⁻¹ when seeded at 74,100 seeds ha⁻¹ and 0.084 Mg ha⁻¹ when seeded at 111,200 seeds ha⁻¹ [86]. Two studies also interseeded cover crops into soybean planted at different seeding rates [68,80]. A study in Michigan planted cereal rye cover crop into soybeans seeded at 444,600, 889,200 and 1,333,800 seeds ha⁻¹ and found no difference in cover crop biomass production across three site years [80]. Another study planted cereal rye cover crop into soybean planted at 494,000 and 741,000 seeds ha⁻¹ and found no difference in cover crop biomass production at the two seeding rates [68]. Some of the previous studies found that reducing seeding rate can increase biomass production in corn but that there also may be reduced grain yields from reducing the crop seeding rate. The two studies that measured cover crop biomass production in soybeans found that cover crop biomass production was not affected by soybean seeding rate.

3.8. Irrigation

Cover crops planted as companion crops into corn or soybean may compete for available water in the soil. Irrigation water additions during the summer may improve performance of the interseeded cover crop. Irrigation has been shown to increase biomass production of cover crops in previous studies [93,94]. Unfortunately, no studies analyzed cover crop performance under both irrigated and dryland conditions at the same location. Across studies, average fall biomass production was 0.35 ± 0.012 Mg ha⁻¹ in dryland (576 observations) and was 0.60 ± 0.16 Mg ha⁻¹ under irrigation (60 observations). Average spring cover crop biomass production was 1.1 ± 0.07 Mg ha⁻¹ under dryland (288 observations) and was 0.68 ± 0.16 Mg ha⁻¹ under irrigated management (31 observations). These data suggest that irrigation increases fall cover crop biomass production but decreases spring biomass production when interseeding cover crops into corn and soybeans. The lower spring biomass production under irrigated management may be due to the lack of irrigation between the main crop harvest and planting of the following crop, and these studies occurring in semi-arid or arid environments. However, studies comparing cover crop performance under both irrigated and dryland conditions at the same location are necessary to see if this is true.

3.9. Average Annual Precipitation and Temperature

The temperature and precipitation at a location also may affect cover crop biomass production. Average annual precipitation in studies included in this review ranged from 80 mm to 1243 mm (Table 1). When comparing the effect of average annual precipitation on cover crop biomass production, irrigated studies were not included. Average annual precipitation was grouped into classes of <500 mm, 500–750 mm, 750–1000 mm and >1000 mm. The average annual precipitation did not affect fall cover crop biomass production (Figure 6A). However, annual average precipitation had large effects on spring biomass production, with an increase in biomass production as the average annual precipitation increased (Figure 6A). Spring cover crop biomass production was statistically affected in the precipitation group at p < 0.0001 but fall biomass was not (p = 0.9716). It would be expected that spring and fall cover crop biomass production would increase as precipitation increases due to a greater amount of soil moisture being available. The finding that spring cover crop biomass increased in wetter climates agrees with this. Warmer average annual temperature can lead to an increased duration of cover crop growth during the fallow period between main crop harvesting and planting the following crop. Average annual temperature was grouped into <6 °C, 6–10 °C and >10 °C. Fall and spring cover crop production were both affected by temperature group at p < 0.0001. Fall biomass production was greatest when the average annual temperature was <6 °C, whereas spring biomass production was highest when the average annual temperature was >10 °C (Figure 6B). It would be expected that a warmer climate with more favorable temperatures during cover crop season would produce more cover crop biomass, which was partially shown by the results of the review. Based on the findings of this review, higher mean annual precipitation and temperature do not increase fall cover crop biomass production but do increase biomass production the following spring. This suggests that light penetration through the main crop canopy may have a larger effect on fall cover crop biomass production than the climatic conditions. However, more favorable conditions for cover crop growth, such as greater precipitation and temperature, increased spring biomass production when cover crops were planted that could survive through the winter period.

3.10. Pre- vs. Postharvest Planting

Several studies have compared planting cover crops by postharvest drilling to preharvest broadcast planting of the cover crop into the standing crop [29,32,33,52,60,63,66,67]. Preharvest broadcasting gives the cover crop a longer period of time to establish in the fall, which may increase spring cover crop biomass production. Most of the studies in this comparison broadcast cover crop species that will overwinter into corn and soybeans at the R4–R6 growth stages, which is when the crop canopy begins to open back up again (Table 1 and [17]). This planting technique allows an extra 1–2 months for the cover crop to establish and grow prior to harvesting the main crop for grain. However, broadcast seed may render poorer seed-to-soil contact than cover crop seed that is drilled. It is unclear whether better seed-to-soil contact or extra time for cover crop growth in fall increases spring cover crop biomass production. In corn, average biomass production from preharvest-planted cover crops was 0.81 Mg ha⁻¹ and postharvest-drilled biomass production was 0.83 Mg ha⁻¹ (Figure 7A). In soybean, average biomass production from preharvest-planted cover crop was 1.34 Mg ha⁻¹ and postharvest-drilled cover crop was 0.79 Mg ha⁻¹ (Figure 7A). In corn, spring biomass production was not different between the two planting methods (p = 0.8580), but it was in soybeans (p = 0.0315). This finding suggests that in corn, preharvest-broadcast and postharvest-drilled cover crops produce similar biomass, but that in soybeans preharvest-broadcast cover crops produce more biomass.

Some cover crop species may perform better when they are preharvest-broadcast vs. postharvest-drilled. Some seeds may be better at penetrating through the crop canopy and growing underneath the canopy of the main crop. Cereal rye, hairy vetch and mixtures were the most used cover crops when comparing preharvest-planted to postharvest-drilled cover crops. Regardless of species, biomass production was similar whether cover crop was preharvest-planted or postharvest-drilled (Figure 7B). In these studies, species significantly affected biomass production at p = 0.0115 but species by planting time interaction did not affect biomass production (p = 0.9935). In some cases a study will find greater biomass production from preharvest-planted cover crops [32,63], but in other instances postharvest-drilled cover crops have produced more biomass [32,52]. Having adequate moisture available to germinate the winter-hardy cover crops is likely more important to establish a cover crop stand than the method by which the cover crop is planted. Based upon the findings in this review, preharvest planting of winter-hardy cover crops into standing crops produces similar biomass to postharvest-drilled winter-hardy cover crops.

4. Effect of Interseeded Cover Crop on Main Crop Yield

4.1. General Effect on Grain Yield

Cover crops may compete with the main crop that is planted for resources. Different cover crop species and planting dates may have greater positive or negative effects on the main crop that is being grown. Average corn grain yields were 9.95 Mg ha⁻¹ ± 0.33 Mg ha⁻¹ under no cover crop where weeds were controlled and 9.71 Mg ha⁻¹ ± 0.16 Mg ha⁻¹ where cover crops were planted. Average soybean yields were 2.81 Mg ha⁻¹ ± 0.12 Mg ha⁻¹ without cover crops but with weed control, and 2.81 Mg ha⁻¹ ± 0.08 Mg ha⁻¹ with cover crops. This shows that interseeding cover crops may not affect corn or soybean grain yields. However, in certain cases cover crop species and/or planting date did affect grain yields, which will be further discussed.

4.2. Crop Growth Stage When Interseeded

At certain stages of crop growth, interactions between the main crop and weeds or the added cover crop may negatively affect main crop grain yields. Competition between the cover crop and main crop at certain growth stages may negatively affect crop grain yields. Cover crops were planted into corn at many more growth stages than soybean (Figure 8A,B). Planting cover crops at the same time as corn and at the V4 stage reduced grain yield relative to having no cover crops (Figure 8A). The growth stage at which cover crops were planted affected relative yield in corn and soybean at p < 0.0001. In soybean, planting cover crops at the same time as the planting and V4 stage reduced grain yield, but planting cover crops at the R5 stage and after did not have a negative effect on grain yield (Figure 8B). However, studies are missing that determined grain yields between the V4 and R5 stage of soybeans (Figure 8B). These findings suggest that planting cover crops at early stages of plant growth (V4 or earlier) can have negative effects on crop grain yields, which agrees with the hypothesis. In the earlier V stages of growth, weed control is more important than at later stages of growth [95,96]. The critical weed-free period is up to the V3 stage in soybeans and the V14 stage in corn [95,97]. Planting cover crops before the critical stage for weed control in soybeans reduced grain yields, but in corn, cover crops did not always reduce grain yields when planted before the critical growth stage for weed control. In some instances, planting cover crops during early growth stages of the crop may have negative effects on grain yield and have an effect on the main crop similar to weeds.

4.3. Interseeded Cover Crop Species

Some cover crop species that can fix nitrogen may benefit corn yield by providing additional nitrogen, whereas other cover crops may compete with the main corn or soybean crop for resources. None of the cover crop species benefited crop grain yields compared to control plots where weeds were managed in both corn and soybeans (Figure 9A,B). However, berseem clover, black medic, buckwheat, cereal rye, hairy vetch, Persian clover, radish and winter camelina cover crops tended to reduce corn grain yields across the observations (Figure 9A). Soybean yields were reduced when interseeded with alfalfa, cereal rye and red clover cover crop species (Figure 9B). Cover crop species significantly reduced relative soybean yield at p = 0.0011 but did not affect relative corn yield at p = 0.0578. This shows that planting some species of cover crops as companion crops during the main crop growing season may lead to reduced grain yields. A negative effect of certain cover crop species on grain yield does not agree with the hypothesis. This may be due to limited cover crop biomass production from many of the species that were planted, leading to minimal symbiotic relationship formed between the cover crop and corn or soybeans.

4.4. Average Annual Precipitation and Temperature

Climate also may affect how much interseeding cover crops affect the main crop grain yield. In drier climates, cover crops may take up some of the water that would otherwise be used by the main crop. Relative grain yield was 1.00 ± 0.019 under an average annual precipitation of <500 mm, 0.88 ± 0.026 when precipitation was between 500 and 750 mm, 0.97 ± 0.011 when precipitation was between 750 and 1000 mm and 0.96 ± 0.0095 when precipitation was >1000 mm. This shows that in drier climates (500–750 mm), crop grain yields were reduced more than in climates that received more annual precipitation. Some of the studies that interseeded cover crops when average annual precipitation was <500 mm were conducted with irrigation, which may be why there was not a reduction in grain yields in the lowest average annual precipitation group [29,46,50]. Temperature also may affect how interseeded cover crops affect grain yields. Relative grain yields were 0.95 ± 0.011 when average annual temperature was <6 °C, 0.98 ± 0.0087 when average annual temperature was between 6 and 10 °C and 0.92 ± 0.013 when average annual temperature was >10 °C. Average annual temperature did not have a consistent effect on grain yields, but precipitation did affect relative grain yield.

4.5. Corn and Soybean Row Spacing and Seeding Rate

Some agricultural producers are interested in planting wide-row corn and interseeding cover crops to improve the performance of the cover crop by allowing more light to penetrate the main crop canopy [13,50]. This can potentially produce more cover crop biomass and provide greater benefits to soil properties from the cover crop. Two studies were found that determined crop yield when comparing wide-row corn planted with cover crops to conventional 76 cm corn row spacing [13]. One of the studies found that corn grain yields were 11.5 Mg ha⁻¹ with no cover crop at 76 cm row spacing, 11 Mg ha⁻¹ with cover crop and 76 cm row spacing and 9.4 Mg ha⁻¹ with cover crop and 152 cm row spacing, averaged across six site years [13]. The other study found that corn silage yield at 70% moisture was 78.5 Mg ha⁻¹ with 76 cm row spacing with no cover crop, 80.8 Mg ha⁻¹ with 118 cm row spacing and a cover crop and 68.2 Mg ha⁻¹ with 152 cm row spacing and a cover crop [50]. In a study that compared corn grain yields with interseeded cover crops under 61 and 76 cm row spacing, corn grain yield was higher under 61 cm than 76 cm across cover crop treatments [33]. However, planting cover crops at the planting stage reduced grain yields relative to no cover crop at both row spacings, but not at other planting dates [33]. Two studies analyzed soybean grain yield with cover crops under different row spacings and found no interaction between soybean row spacing and grain yields [33,56]. These two studies show that using wider row spacing to allow for more cover crop growth may reduce corn yield. However, additional cover crop biomass production can provide forage for livestock to graze, which may offset the loss in production from planting corn at wider row spacing if producers are integrating livestock into the cropping system or enhanced ecosystem services from the cover crop occurs.

Two studies analyzed the effects of seeding rate and cover crops on corn [11,86] and soybean [68,80] grain yields. One of the studies found that corn yield tended to increase as corn seeding rate increased from 35,000 to 75,000 seeds ha⁻¹, but that cover crops did not affect grain yield at any of the seeding rates [11]. The other study looking at corn seeding rate and cover crop interaction also found no effect of the cover crop on grain yield at any seeding rate [86]. However, at one of the three study locations, the low seeding rate (37,100 seeds ha⁻¹) resulted in a lower grain yield than the medium (74,100 seeds ha⁻¹) and high (111,200 seeds ha⁻¹) seeding rates [86]. This study attributed the lack of differences in corn yield between seeding rates to using a flex-ear corn variety [86]. In the two studies that analyzed the effects of soybean seeding rate and cover crops on grain yield, the yield generally increased when seeding rate increased [68,80]. However, grain yields were usually less in these studies under any seeding rate when comparing weeded plots to the cover crop plots [68,80]. Based on these few previous studies, grain yields do not appear to be affected by the interaction between corn seeding rate and cover crops; however, in soybeans, the two studies found that interseeding cereal rye cover crop into soybeans may affect grain yield at any seeding rate.

4.6. Following-Year Grain Yields

Some cover crop species interseeded into corn and soybeans may affect the yield of crops planted the following year. Cover crops may reduce nutrient availability and soil water content, or not completely be terminated when the following crop is planted and act like weeds. Relative grain yields generally were not increased following cover crops in the studies included in this review (Figure 10A,B). Corn yield was slightly lower relative to having no cover crop in the season following interseeding of alfalfa, annual ryegrass, cereal rye and red clover cover crops (Figure 10A). When soybeans were planted the year following annual ryegrass interseeded into corn, grain yields were reduced (Figure 10B). However, cover crop mixtures did not reduce crop grain yields (Figure 10A, B). Relative grain yield of corn planted the year following interseeded cover crops was significantly affected by cover crop species at p = 0.0002, as was that of soybean at p < 0.0001. This shows that, in some cases, certain interseeded cover crop species will not benefit the following crop yields. Climate may affect the performance of crops planted following cover crops. Cover crops may take up some of the nutrients and reduce soil water available for the main crop to use. Higher annual precipitation may reduce the effect of the cover crop on the following crop yield. Mean annual temperature and average annual precipitation did not have a consistent effect on relative crop grain yield following interseeded cover crops included in this review.

5. Weed Control from Interseeded Cover Crops

5.1. Weed Biomass Reduction from Interseeded Cover Crop

Weeds can have negative effects on crop production by competing with desired crops being grown for resources. Weeds are traditionally controlled mechanically by tillage and cultivation and chemically with herbicides. Planting cover crops may be a biological control method to control weeds, as cover crops provide symbiotic benefits to the growing crop by controlling undesirable weeds, while not affecting grain yield [98]. Across studies, planting cover crops reduced weed biomass compared to weedy plots. In corn, average weed biomass was 813 kg ha⁻¹ ± 428 kg ha⁻¹ in plots with no weed control, 279 kg ha⁻¹ ± 46 kg ha⁻¹ in cover crop plots, and 58 kg ha⁻¹ ± 14 kg ha⁻¹ in plots that were either mechanically or chemically weeded. In soybeans, average weed biomass production was 1621 kg ha⁻¹ ± 655 kg ha⁻¹ in the plots with no weed control, 846 kg ha⁻¹ ± 225 kg ha⁻¹ under cover crop and 422 kg ha⁻¹ ± 376 kg ha⁻¹ in weeded plots. This shows that, across cover crop species, weeds were reduced by 66% in corn and 48% in soybeans.

5.2. Cover Crop Planting Date Effect on Weed Biomass

When cover crops are interseeded into the main crop, the growth stage of the crops may affect how well they suppress undesirable weeds. Some of the studies that measured weed biomass did not include the V stage of the crop when interseeding, so planting time was categorized into planting, 1–3 weeks after planting and >3 weeks after planting. Cover crops most effectively suppressed weeds when planted >3 weeks after planting (91 kg ha⁻¹ ± 28.2 kg ha⁻¹ weed biomass across 17 observations) compared to 1–3 weeks after planting (244 kg ha⁻¹ ± 29.6 kg ha⁻¹ weed biomass across 47 observations) and at planting (1581 kg ha⁻¹ ± 410 kg ha⁻¹ weed biomass across 10 observations). Planting time significantly affected weed biomass at p < 0.0001. According to the previous studies included in this review, planting cover crops after planting was more effective at controlling weeds than planting cover crops at the same time as main crop planting. This agrees with the hypothesis that weeds would be most effectively controlled when planting cover crops at early V stages of the main crop. This may be from cover crops being planted after mechanical or chemical weed control has been done following the planting of the main crops [14].

5.3. Cover Crop Species Effect on Weed Biomass

Some cover crops may be more effective at controlling weeds than others due to how shade-tolerant they are, how rapidly they grow and the ability of the cover crop to outcompete weeds [99]. There were many previous cases where the effectiveness of a cereal rye cover crop in suppressing weeds was analyzed (Figure 11). However, fewer studies analyzed how effective other cover crops were at reducing weed biomass (Figure 11). Cereal rye and cover crop mixtures had higher weed biomass production than many of the other cover crops. However, these data might change if there were more studies analyzing the performance of other cover crop species in controlling weeds. Weed biomass production under different cover crop species was not significantly varied at a p value = 0.0590. This does not agree with the hypothesis that cover crop mixtures would be best at suppressing weeds. Studies are too few to fully support this due to many of the cover crops only being tested in one study with four site years [27]. In one study where much of the data came from cereal rye, this cover crop showed similar performance in controlling weeds to the other cover crop species [27].

5.4. Grain Yield Compared to Weedy Control

In the previous section on grain yields, the grain yields under cover crops were compared to weed control using either tillage or herbicides across all studies. Here only relative grain yields are included from studies with a weedy and weeded control. Relative grain yield is still determined compared to the weeded plots with no cover crop. Relative to weeded plots with no cover crop, grain yields were lowest in the weedy plots, cereal rye and radish cover crop treatments (Figure 12). There was a significant effect of weed management/cover crop species on relative grain yield at p = 0.0231. Other cover crop species had relative grain yields between 0.89 and 1.0. These findings show that interseeding cover crops can reduce weed biomass and slightly but not significantly increase crop grain yields relative to the weedy control. However, conventional weeding methods had higher grain yield relative to weedy control and some of the cover crop species.

6. Soil Properties

6.1. Soil Chemical Properties

Among the main purposes of planting cover crops are soil and water conservation and the improvement of soil health [2]. There are many previous studies that have analyzed the effect of cover crops on soil properties, but most of these studies have used traditional cover crop planting methods such as drill planting during the fallow period of the main crop [23,24,25,100,101,102]. Few of these studies analyzed how interseeding cover crops into corn and soybeans affects soil properties. Previous studies that measured soil properties after interseeding cover crops into growing crops were conducted for periods of four years or less, which is a relatively short duration of time (

Table 2. Effects of interseeding cover crops on soil chemical properties compared, where NT is no till, MP is moldboard plot, ST is strip till, VT is vertical tillage, CT is conventional tillage, CP is chisel plow, D is disk, RT is reduced tillage, CEC is cation exchange capacity, C is carbon, CO₂ is carbon dioxide, Ca is calcium, Mg is magnesium, K is potassium, Na is sodium, N is nitrogen, NO₃ is nitrate, NH₄ is ammonium, P is phosphorus, PMN is potentially mineralizable nitrogen, SOC is soil organic carbon, R is reproductive stage and V is vegetative crop stage.

Study	Location	Cover Crop Species	Crop	Cover Crop Duration	Tillage	Crop Planting Stage	Soil Depth	Major Findings
Alonso-Ayuso et al., 2020 [29]	Aranjuaz, Spain	Annual ryegrass, barrel medic, yellow sweet clover and common vetch	Corn	1	CT	V4 and V5	0–40	No effect in 2018 site year. Inorganic N concentration higher in control (21.3 kg N ha−1) and common vetch (14.3 kg N ha−1) than in barrel medic (7.1 kg N ha−1), yellow sweet clover (7.5 kg N ha−1) and annual ryegrass (3.1 kg N ha−1) in the other year.
Adler et al., 2020 [28]	Leonard, MO, USA	Wheat + radish + turnip mixture	Soybean	2	NT	R6	0–15	No effect on pH, CEC, Ca, Mg, K, Na, P, PMN and SOC.
Belfry and Van Eard, 2016 [5]	Van Horne, ON, Canada	Alfalfa, radish and alfalfa + hairy vetch + oat + radish mixture	Corn	1	CT	V4–V6 and V10–V12	0–60	No effect on NH4. In the fall, radish (15.4 mg kg−1) cover crop had lower NO3 than mixture (10.6 mg kg−1). In the following spring, NO3 was less in alfalfa (6.05 mg kg−1) than radish (8.36 mg kg−1) and no cover crop (9.45 mg kg−1).
Blanco-Canqui et al., 2017 [36]	Bellwood, NE, USA	Cereal rye	Corn	3	ST	NA	0–10	No effect on soil pH, NO3, K, P and SOC.
Curtright et al., 2023 [44]	East Lansing, MI, USA	Annual ryegrass, crimson clover, radish and annual ryegrass + crimson clover + radish mixture	Corn	1 and 2	NA	V3 and V6	0–10	No effect on microbial biomass C, dissolved organic C, NO3, NH4 and dissolved organic N, but greater microbial biomass N under annual ryegrass + crimson clover + radish mixture (34.3 mg microbial biomass N kg−1 soil), radish (33.6 mg microbial biomass N kg−1 soil) and control (33.5 mg microbial biomass N kg−1 soil) than crimson clover (29.4 mg microbial biomass N kg−1 soil) cover crop.
Curtright et al., 2023 [44]	Richville, MI, USA	Annual ryegrass, crimson clover, radish and annual ryegrass + crimson clover + radish mixture	Corn	1 and 2	NA	V3 and V6	0–10	No effect on microbial biomass C, dissolved organic C, NO3, NH4 and dissolved organic N, but greater microbial biomass N under annual ryegrass + crimson clover + radish mixture (34.3 mg microbial biomass N kg−1 soil), radish (33.6 mg microbial biomass N kg−1 soil) and control (33.5 mg microbial biomass N kg−1 soil) than crimson clover (29.4 mg microbial biomass N kg−1 soil) cover crop.
Donaldson, 2019 [45]	Linn County, OR, USA	Oats, cereal rye, winter wheat	Sweet corn	1	CT	V5–V8	0–60	In 2017 no effect of cover crop on NO3 in 0–30 cm depth. In 2018 NO3 content was less under winter wheat (2.9 ppm) cover crop than control (4.8 ppm). No differences in residual NO3 at 30–60 cm depth.
Francis, 2020 [47]	Arlington, WI, USA	Red clover	Corn	1	NT	V4–V5	0–60	No differences in soil NO3, NH4 and available N in fall or mid-summer sampling. In one of three years NO3 and available N were higher with no cover crop (4.5 mg kg−1 NO3 and 9.0 mg kg−1 available N) than in cover crop (2.2 mg kg−1 NO3 and 7.7 mg kg−1 available N) plots in spring.
Hively and Cox, 2001 [51]	New York, USA	White clover, red clover, barrel medic, alfalfa, ryegrass, red fescue	Soybean	1	MP	Last cultivation	0–30	In 1996 at planting and 1997 at V5 stage there was no difference in soil NO3. In 1996 NO3 was less in rye (11 mg kg−1) planted after harvest than white (18 mg kg−1) and red clover (18 mg kg−1), but no differences between preharvest planted cover crops and control. In 1997 NO3 was higher in red clover (7 mg kg−1) and rye after harvest (7 mg kg−1) than white clover (4 mg kg−1) at planting but no difference in any cover crop species from the control.
Isbell et al., 2022 [52]	Rock Springs, PA, USA	Annual ryegrass	Corn	1	CP	V5–V6	Measured at 5 times in 0–10 cm depth and in following spring at 0–80 cm depth	No difference in 0–10 cm soil inorganic N under corn grain, but there was less inorganic N under annual ryegrass than control at one of five sampling dates under corn silage. Annual ryegrass (2.4 mg N kg−1 soil) reduced inorganic N compared to control (5.3 mg N kg−1 soil) in 40–80 cm depth under grain and in 0–80 cm depth under silage (4.6 mg N kg−1 soil in control and 1.9 mg N kg−1 soil under annual ryegrass). Soil CO2 burst was higher under annual ryegrass (247 mg C kg−1 soil) than no cover crop (214 mg C kg−1) soil the following June, but no difference in December sampling or at either date under grain.
Jones et al., 1998 [55]	Hickory Corners, MI, USA	Annual ryegrass + red clover mixture	Corn	1	CT	V6	0–30	No difference in NO3 in June 1994, but higher NO3 under cover crop (10.6 mg kg−1) than no cover crop (7.6 mg kg−1) in June 1995.
Kaspar et al., 2006 [58]	Ames, IA, USA	Cereal rye, oat and cereal rye + oat mixture	Corn-soybean	4–6	NT and CT	NA	0–5	No effect of cover crop on soil organic carbon when planting cover crop every other year only following soybean.
Koehler-Cole et al., 2020 [61]	Concord, NE, USA	Cereal rye and cereal rye + hairy vetch + winter pea + radish mixture	Corn	1–3	NT	NA	0–20	Cover crops did not affect soil NO3 in 2016, but NO3 was less under both cover crop treatments (2 mg kg−1 with cover crops and 4 mg kg−1 with no cover crop) in 2017 and rye cover crop treatment in 2018 (2 mg kg−1 with cereal rye cover crop and 4 mg kg−1 with no cover crop and mixture).
Koehler-Cole et al., 2020 [61]	Clay Center, NE, USA	Cereal rye and cereal rye + hairy vetch + winter pea + radish mixture	Corn	1–3	NT	NA	0–20	Less NO3 under both cover crop treatments in 2016 (2 mg kg−1 with cover crops and 4 mg kg−1 with no cover crop) and 2017 (9 mg kg−1 with mixture, 11 mg kg−1 with cereal rye 19 mg kg−1 with no cover crop) and rye cover crop treatment in 2018 (9 mg kg−1 under cereal rye and 13 mg kg−1 under mixture and no cover crop).
Koehler-Cole et al., 2020 [61]	Mead, NE, USA	Cereal rye and cereal rye + hairy vetch + winter pea + radish mixture	Corn	1–3	NT	NA	0–20	No effect on soil NO3 in 2017 and 2018 but lower NO3 under rye cover crop (1 mg kg−1) than control (5 mg kg−1) treatment in 2016.
Koehler-Cole et al., 2023 [60]	Concord, NE, USA	Cereal rye, hairy vetch + winter pea mixture, cereal rye + hairy vetch + winter pea + radish mixture and cereal rye + black oat + forage collard + radish + hairy vetch + winter pea + clover mixture	Corn-soybean and continuous corn	4	NT	NA	0–20	No effect of cover crop on soil NO3 and organic C under continuous corn or corn-soybean rotation. In corn-soybean rotation P was higher in hairy vetch + winter pea mixture (40 mg kg−1) and cereal rye + black oat, forage collard + radish + hairy vetch + winter pea + clover mixture (40 mg kg−1) than the other treatments (29 mg kg−1 in cereal rye + hairy vetch + winter pea + radish mixture and cereal rye and 31 mg kg−1 in control) and K was higher in the hairy vetch + winter pea mixture (313 mg kg−1) than the cereal rye + hairy vetch + winter pea + radish mixture (260 mg kg−1), cereal rye (263 mg kg−1) and control (271 mg kg−1) treatment. In continuous corn K and P were less under rye (23 mg kg−1 P and 258 mg kg−1 K) and winter pea + hairy vetch mixture (27 mg kg−1 P and 261 mg kg−1 K) than control (33 mg kg−1 P and 297 mg kg−1 K) and other two cover crop treatments (35 mg kg−1 P and 297 mg kg−1 K under cereal rye + hairy vetch + winter pea + radish mixture and 35 mg kg−1 P and 300 mg kg−1 K under cereal rye + black oat + forage collard + radish + hairy vetch + winter pea + clover mixture).
Koehler-Cole et al., 2023 [60]	Clay Center, NE, USA	Cereal rye, hairy vetch + winter pea mixture, cereal rye + hairy vetch + winter pea + radish mixture and cereal rye + black oat + forage collard + radish + hairy vetch + winter pea + clover mixture	Corn-soybean and continuous corn	4	NT	NA	0–20	No effect of cover crop on soil P and organic C under continuous corn or corn-soybean rotations. In corn-soybean rotation and continuous corn NO3 was higher in the hairy vetch + winter pea mixture (12 mg kg−1) and control (11 mg kg−1) than the other three treatments (8 mg kg−1 under cereal rye and 9 mg kg−1 under cereal rye + hairy vetch + winter pea + radish mixture and cereal rye + black oat + forage collard + radish + hairy vetch + winter pea + clover mixtures). Under corn-soybean rotation K concentration was the same across treatments, but in continuous corn cereal rye + black oat + forage collard + radish + hairy vetch + winter pea + clover mixture (302 mg kg−1) and hairy vetch + winter pea mixture (302 mg kg−1) had lower K than the control (334 mg kg−1).
Koehler-Cole et al., 2023 [60]	Mead, NE, USA	Cereal rye, hairy vetch + winter pea mixture, cereal rye + hairy vetch + winter pea + radish mixture and cereal rye + black oat + forage collard + radish + hairy vetch + winter pea + clover mixture	Corn-soybean and continuous corn	4	NT	NA	0–20	No effect of cover crop on soil P and organic C under continuous corn or corn-soybean rotation. In corn-soybean rotation NO3 was higher in control (5 mg kg−1), hairy vetch + winter pea mixture (6 mg kg−1) and cereal rye + black oat + forage collard + radish + hairy vetch + winter pea + clover mixture (5 mg kg−1) than the cereal rye cover crop (2 mg kg−1). In continuous corn NO3 concentration was lower under cereal rye cover crop (3 mg kg−1) than the winter pea + hairy vetch mixture (5 mg kg−1). Under corn-soybean rotation K concentration was the same across treatments, but in continuous corn cereal rye (402 mg kg−1) increased K compared to control (366 mg kg−1).
Liu et al., 2019 [62]	Lamberton, MN, USA	Winter camelina, pennycress and cereal rye	Corn and soybean	1 and 2	RT	NA	0–30	In one of two springs all three cover crop species reduced NO3 in corn (20.4 kg ha−1 under control, 3.7 kg ha−1 under cereal rye, 9.8 kg ha−1 under winter camelina and 9.6 kg ha−1 under pennycress) and winter camelina (6.9 kg ha−1) and cereal rye (3.9 kg ha−1) cover crops reduced soil NO3 compared to control (22.4 kg ha−1) under soybean. No effect on soil NO3 in the fall. Soil N mineralization was higher under cereal rye cover crop than control under corn and under cereal rye and winter camelina cover crop than control under soybean.
Mohammed et al., 2020b [64]	Ames, IA, USA	Pennycress, winter camelina and cereal rye	Corn and soybean	1 and 2	CT	R4–R6 in corn and R6–R8 in soybean	0–60	No effect of any cover crop on soil NO3 and NH4.
Mohammed et al., 2020 [64]	Prosper, ND, USA	Pennycress, winter camelina and cereal rye	Corn and soybean	1 and 2	CT	R4–R6 in corn and R6–R8 in soybean	0–60	No effect of cover crops on soil NH4. Cereal rye (6.8 mg kg−1) and pennycress (7.0 mg kg−1) cover crops reduced spring soil NO3 compared to control (14.4 mg kg−1) and winter camelina (12.0 mg kg−1) when interseeded into corn and fall and spring NO3 when interseeded into soybean (10.7 mg kg−1 under no cover crop, 5.4 mg kg−1 under pennycress, 4.5 mg kg−1 under cereal rye and 7.4 mg kg−1 under winter camelina).
Mohammed et al., 2020 [64]	Rosemount, MN, USA	Pennycress, winter camelina and cereal rye	Soybean	1 and 2	CT	R4–R6 in corn and R6–R8 in soybean	0–60	No effect of cover crops on soil NH4. In fall cereal rye (2.4 mg kg−1) had lower NO3 in 15–60 cm depth than winter camelina (3.7 mg kg−1) but no differences occurred in 0–15 cm depth. In spring sampling cereal rye (0.9 mg kg−1) had lower NO3 than control (2.7 mg kg−1), winter camelina (4.1 mg kg−1) and pennycress (3.1 mg kg−1) cover crop in 0–15 cm depth but no differences in 15–60 cm depth.
Mohammed et al., 2020 [64]	Morris, MN, USA	Pennycress, winter camelina and cereal rye	Corn and soybean	1 and 2	CT	R4–R6 in corn and R6–R8 in soybean	0–60	No effect of cover crop on NH4 and NO3 in corn. Lower NO3 in fall under cereal rye camelina (1.9 mg kg−1), pennycress camelina (2.6 mg kg−1) and winter camelina (2.7 mg kg−1) cover crop than no cover crop (4 mg kg−1).
Nguyen et al., 2022 [69]	Hickory Corners, MI, USA	Oats + winter pea + radish mixture and annual ryegrass + rapeseed + crimson clover mixture	Corn	1	CT	V5–V6	0–10	No effect of cover crop on soil CO2 burst, microbial biomass C and NH4 concentration. Higher potentially mineralizable N under both cover crop mixtures (0.19 mg N kg−1 soil day−1 under oats + winter pea + radish mixture and 0.18 mg N kg−1 soil day−1 under annual ryegrass + rapeseed + crimson clover mixture) than control (0.14 mg N kg−1 soil day−1). Lower NO3 under annual ryegrass + rapeseed + crimson clover mixture than control and oat + winter pea + radish mixture.
Noland et al., 2018 [70]	Lamberton, MN, USA	Hairy vetch, cereal rye, red clover, pennycress and oat + pea + radish mixture	Corn	1	NA	V7	0–30	Lower NO3 under cereal rye (9.7 kg N ha−1) and pennycress (20.1 kg N ha−1) than red clover (28.8 kg N ha−1) cover crop and control (28.7 kg N ha−1).
Noland et al., 2018 [70]	Waseca, MN, USA	Hairy vetch, cereal rye, red clover, pennycress and oat + pea + radish mixture	Corn	1	NA	V7	0–30	Lower NO3 under pennycress (14.1 kg N ha−1), cereal rye (8.8 kg N ha−1), hairy vetch (11.2 kg N ha−1) and red clover (12.4 kg N ha−1) than oat + pea + radish mixture (31.7 kg N ha−1) and control (27.6 kg N ha−1).
Nunes et al., 2018 [71]	Aurora, NY, USA	Annual ryegrass + red clover + crimson clover + hairy vetch mixture	Corn	4	NT and MP	NA	0–15	No effect on soil active C, total N, P, K, Mg and Mn. Cover crop increased soil organic matter (28.8 g kg−1 with cover crop and 27.4 g kg−1 with no cover crop), CO2 burst (0.625 mg CO2 g−1 soil with cover crop and 0.48 mg CO2 g−1 soil with no cover crop), iron (0.602 mg kg−1 with cover crop and 0.528 mg kg−1) with no cover crop and zinc (1.53 mg kg−1 with cover crop and 0.88 mg kg−1 with no cover crop) concentration.
Peterson et al., 2019 [72]	Prosper, ND, USA	Winter camelina, winter pea, radish, cereal rye and winter camelina + radish + winter pea + cereal rye mixture	Soybean	1	CT	R4 and R6	0–60	In 2016 winter pea (20.2 kg ha−1) cover crop when planted at R4 stage, and winter pea (23.72 kg ha−1), cereal rye (21.52 kg ha−1) and winter pea + cereal rye + radish + winter pea mixture (21.82 kg ha−1) planted at R6 stage reduced fall soil NO3 compared to control (61.72 kg ha−1). No effect of cover crops on NO3 in fall and spring of 2017, spring 2018 and post wheat harvest NO3 in both years.
Peterson et al., 2019 [72]	Fargo, ND, USA	Winter camelina, winter pea, radish, cereal rye and winter camelina + radish + winter pea + cereal rye mixture	Soybean	1	CT	R4 and R6	0–60	No effect on fall, spring or postharvest soil NO3
Ruis et al., 2020 [75]	Concord, NE, USA	Cereal rye and cereal rye + winter pea + hairy vetch + radish mixture	Corn and corn-soybean	4	NT	NA	0–20	No effect of cover crops on soil organic C and total N. No effect on particulate organic matter under corn-soybean, but preharvest cover crop (19.7 g kg−1) increased particulate organic matter under continuous corn compared to control (16.5 g kg−1).
Ruis et al., 2020 [75]	Mead, NE, USA	Cereal rye and cereal rye + winter pea + hairy vetch + radish mixture	Corn and corn-soybean	4	NT	NA	0–20	No effect of cover crop on soil total organic C, total N and particulate organic matter.
Ruis et al., 2020 [75]	Clay Center, NE, USA	Cereal rye and cereal rye + winter pea + hairy vetch + radish mixture	Corn and corn-soybean	4	NT	NA	0–20	No effect of cover crop on soil organic C, total N and particulate organic matter.
Sedghi et al., 2023 [78]	Ralbot and Kent County, MD, USA	Cereal rye + clover + brassica mixture	Corn-soybean	1	NT, VT and CP	NA	0–30	Cover crops reduced NH4 (11.1 mg N kg−1 soil with no cover crop and 9.6 mg N kg−1 soil with cover crop) but did not affect soil NO3 and organic matter the following spring.
Wallace et al., 2021 [84]	Lancaster County, PA, USA	Annual ryegrass + orchard grass + radish mixture	Corn	1	MP	V5–V7	0–20	No effect on soil inorganic N in fall, but postharvest planted cereal rye (3.3 mg N kg−1) had lower inorganic N soil than drill-interseeded treatment (4.9 mg N kg−1 soil).
Wallace et al., 2021 [84]	Mifflin County, PA, USA	Annual ryegrass + orchard grass + radish mixture	Corn	1	MP and D	V5–V7	0–20	No effect of cover crop on soil inorganic N in fall and following spring.
Wallace et al., 2021 [84]	Union County, PA, USA	Annual ryegrass + orchard grass + radish mixture	Corn	1	MP	V5–V7	0–20	No effect of cover crop on fall inorganic N but drill-interseeded (12.8 mg N kg−1 soil) cover crops had higher spring inorganic N than no cover crop (8.8 mg N kg−1 soil).

,

Table 3. Effects of interseeding cover crops on soil physical properties. NT is no till, MP is moldboard plot, ST is strip till, RT is reduced tillage, WSA is percent water stable aggregates, DAS is dry aggregate stability, V is vegetative stage, R is reproductive stage and NA is not available.

Study	Location	Cover Crop Species	Crop	Cover Crop Duration	Tillage	Crop Planting Stage	Soil Depth	Effect on Soil Property
Adler et al., 2020 [28]	Leonard, MO, USA	Wheat + radish + turnip mixture	Soybean	2	NT	R6	0–15	No effect on WSA
Batildas, 2019 [32]	Clay Center, NE, USA	Radish + cereal rye + hairy vetch mixture	Corn	1	NT	Planting, V8 and R5	0–120	Averaged across dates at 30 cm depth water content was less under no cover crop (0.31 cm3 cm−3) than when cover crop was seeded at V8 stage (0.35 cm3 cm−3). No difference in water content at other depths. Across dates soil temperature was less at 12 cm depth when cover crop was planted at planting (21.1 °C) compared to no cover crop (21.2 °C) and V8 (21.3 °C) and R5 (21.2 °C) growth stages.
Batildas, 2019 [32]	Lincoln, NE, USA	Radish + cereal rye + hairy vetch mixture	Corn	1	NT	Planting, V8 and R5	0–120	Across dates at 15 cm depth planting cover crop at planting (0.36 cm3 cm−3), V8 (0.36 cm3 cm−3) and R5 (0.35 cm3 cm−3) stages reduced water content compared to control (0.38 cm3 cm3). At 30 cm depth water content was less when cover crop was planted at V8 (0.35 cm3 cm−3) and R5 (0.36 cm3 cm−3) stage than at main crop planting (0.37 cm3 cm−3) or no cover crop (0.37 cm3 cm−3) treatments. At 45 cm depth water content was less when cover crop was planted at corn planting (0.33 cm3 cm−3) than V8 (0.35 cm3 cm−3) and R5 (0.36 cm3 cm−3) stages. Across dates planting cover crop at corn planting (21.4 °C) reduced soil temperature at 12 cm depth compared to no cover crop (22.1 °C) and cover crop planted at V8 (22.1 °C) and R5 (22.1 °C) growth stages.
Blanco-Canqui et al., 2017 [36]	Bellwood, NE, USA	Cereal rye	Corn	3	ST	NA	0–10	No effect of cover crop on WSA, DAS, water content and bulk density when sampled each spring
Goettl et al., 2024 [49]	Leonard and Rutland, ND, USA	Flax + faba bean + oats + winter pea + crimson clover + buckwheat + phacelia + forage radish, cereal rye + red clover + alfalfa and sorghum + millet + cereal rye + red clover + alfalfa.	Corn	1	NT	V4, V5	5–15	No effect of cover crop on soil water content between July and crop harvest
Kaspar et al., 2001 [57]	Ames, IA, USA	Cereal rye and oat	Corn-soybean	1, 2 and 3	NT and CT	NA	0–7.6	In year 3 cereal rye cover crop (13 g m−2 s−1) had higher infiltration rate than control (10.9 g m−2 s−1), but oat (11.8 g m−2 s−1) cover crop was not different from either. Cereal rye cover crop (0.317 cm3 cm−3) had higher soil water content than control (0.293 cm3 cm−3) and oat (0.287 cm3 cm−3) at planting. No differences in infiltration and water content after year 1 and 2 and bulk density between treatments.
Kaspar et al., 2006 [58]	Ames, IA, USA	Cereal rye, oat and cereal rye + oat mix	Corn-soybean	4–6	NT and CT	NA	0–5	No effect of cover crop on bulk density when planting cover crop every year only following soybean.
Mohammed et al., 2020 [64]	Ames, IA, USA	Pennycress, winter camelina and cereal rye	Corn and soybean	1 and 2	CT	R4–R6 in corn and R6–R8 in soybean	0–60	No effect of any cover crop on soil water content.
Mohammed et al., 2020 [64]	Prosper, ND, USA	Pennycress, winter camelina and cereal rye	Corn and soybean	1 and 2	CT	R4–R6 in corn and R6–R8 in soybean	0–60	No effect of any cover crop on soil water content.
Mohammed et al., 2020 [64]	Rosemount, MN, USA	Pennycress, winter camelina and cereal rye	Soybean	1 and 2	CT	R4–R6 in corn and R6–R8 in soybean	0–60	No effect of any cover crop on soil water content.
Mohammed et al., 2020 [64]	Morris, MN, USA	Pennycress, winter camelina and cereal rye	Corn and soybean	1 and 2	CT	R4–R6 in corn and R6–R8 in soybean	0–60	No effect of any cover crop on soil water content.
Nguyen et al., 2022 [69]	Hickory Corners, MI, USA	Oats + winter pea + radish mixture and annual ryegrass + rapeseed + crimson clover mixture	Corn	1	CT	V5–V6	0–10	No effect of cover crop on soil water content or <0.25 mm aggregates. Less >2 mm, but more 0.25–2 mm aggregates under annual ryegrass + rapeseed + crimson clover mixture (31.6% > 2 mmm and 62.6% between 0.25 and 2 mm) than control (41.5% > 2 mm and 53.2% between 0.25 and 2 mm).
Nunes et al., 2018 [71]	Aurora, NY, USA	Annual ryegrass + red clover + crimson clover + hairy vetch mixture	Corn	4	NT and MP	NA	0–15	Cover crop (0.260 cm3 cm−3) increased available water compared to control (0.252 cm3 cm−3) but did not affect WSA, penetration resistance and infiltration.
Ruis et al., 2020 [75]	Concord, NE, USA	Cereal rye and cereal rye + winter pea + hairy vetch + radish mixture	Corn and corn-soybean	4	NT		0–20	No effect of cover crop on bulk density, wet aggregate stability and sorptivity, but cover crop mixture (1.11 MPa) reduced cone index compared to cereal rye (1.24 MPa) and control (1.29 MPa) under continuous corn. In corn-soybean, cover crops did not affect wet aggregate stability and bulk density, but the cover crop mixture (1.07 MPa) reduced cone index compared to control (1.18 MPa) and cereal rye cover crop (1.21 MPa).
Ruis et al., 2020 [75]	Mead, NE, USA	Cereal rye and cereal rye + winter pea + hairy vetch + radish mixture	Corn and corn-soybean	4	NT		0–20	No effect of cover crop on bulk density, sorptivity, cone index and wet aggregate stability in both cropping systems.
Ruis et al., 2020 [75]	Clay Center, NE, USA	Cereal rye and cereal rye + winter pea + hairy vetch + radish mixture	Corn and corn-soybean	4	NT		0–20	No effect of cover crop on bulk density, sorptivity and wet aggregate stability. In continuous corn cone index was less under cereal rye (2.01 MPa) and no cover crop (2.64 MPa) than the cover crop mixture (3.80 MPa), and bulk density was less under cover crop mixture (1.12 g cm−3) than no cover crop (1.20 g cm−3). Under corn-soybean rotation cover crop mixture (4.14 MPa) had higher penetration resistance than no cover crop (2.8 MPa).
Rusch et al., 2020 [76]	Lamberton, MN, USA	Cereal rye	Corn	1	CT	R5	0–30	No effect of cover crop on soil water content the following spring.
Rusch et al., 2020 [76]	Grand Rapids, MN, USA	Cereal rye	Corn	1	CT	R5	0–30	No effect of cover crop on soil water content the following spring.
Rusch et al., 2020 [76]	Waseca, MN, USA	Cereal rye	Corn	1	CT	R5	0–30	No effect of cover crop on soil water content the following spring.
Sedghi et al., 2023 [78]	Ralbot and Kent County, MD, USA	Cereal Rye + Clover +Brassica	Corn-soybean	1	NT, RT and CT	NA	0–30	Cover crops did not affect soil water content.
St. Aime et al., 2023 [16]	Pendleton, SC, USA	Buckwheat, pigeon pea, white clover and buckwheat + pigeon pea + white clover mixture	Corn	1	CT	V4–V10	0–20	Water content was generally not affected by cover crops when measured in the fall.

and

Table 4. Summary of previous studies that analyzed the effects of interseeding cover crops on soil biological properties. NT is no tillage, CT is conventional tillage, CP is chisel plow, V is vegetative crop stage, N is nitrogen, PLFA is phospholipid fatty acid and NA is not available.

Study	Location	Cover Crop Species	Crop	Cover Crop Duration	Tillage	Crop Planting Stage	Soil Depth	Effects
Bich et al., 2014 [34]	Aurora, SD, USA	Winter wheat + lentil + crimson clover mixture	Corn	1	CT	V3–V5	0–15	No effect of cover crop on soil microbial biomass, fungi and bacteria concentration.
Bich et al., 2014 [34]	Andover, SD, USA	Winter wheat + lentil + crimson clover mixture	Corn	1	NT	V3–V5	0–15	No effect of cover crop on soil microbial biomass, fungi and bacteria concentration.
Bich et al., 2014 [34]	Trail City, SD, USA	Winter wheat + lentil + crimson clover mixture	Corn	1	NT	V3–V5	0–15	No effect of cover crop on soil microbial biomass, fungi and bacteria concentration.
Curtright et al., 2023 [44]	East Lansing, MI, USA	Annual ryegrass, crimson clover, radish and annual ryegrass + crimson clover + radish mixture	Corn	1 and 2	NA	V3 and V6	0–10	No effect of cover crop on soil β-glucosidase, N-acetyl-glycosaminidase, peptidase and phosphatase concentration.
Curtright et al., 2023 [44]	Richville, MI, USA	Annual ryegrass, crimson clover, radish and annual ryegrass + crimson clover + radish mixture	Corn	1 and 2	NA	V3 and V6	0–10	No effect of cover crop on soil β-glucosidase, N-acetyl-glucosaminidase, peptidase and phosphatase.
Isbell et al., 2022 [52]	Rock Springs, PA, USA	Annual ryegrass	Corn	1	CP	V5–V6	0–10	In June there was more PLFA biomass under cover crop than no cover crop treatment. Under corn grain with high N PLFA biomass was 60.7 nmol PLFA g−1 soil with no cover crop and 75.1 nmol PLFA g−1 soil with interseeded cover crop. Under corn silage PLFA biomass was 58.7 nmol PLFA g−1 soil with no cover crop and 70.8 nmol PLFA g−1 soil with interseeded cover crop. Also, there were some differences in the abundance of different soil microbial groups in both December and June sampling.

).

There are many soil chemical properties that were measured following interseeding cover crops into corn and soybean. Planting cover crops may take up essential plant nutrients, reducing their availability to the main crop and affecting biochemical cycling in the soil. Unfortunately, many of the soil properties were only measured under a couple of different interseeded cover crop species (Table 2). That is why the comparison in this section primarily focuses on comparing cover crop treatments to the no-cover-crop control. The most commonly measured soil properties following interseeding cover crops were soil inorganic nitrogen, nitrate and ammonium. In many instances, planting at least one of the species of cover crop affected soil nitrate concentration compared to no cover crop (16 of 24 locations) (Table 2). However, some of these studies found that nitrate was reduced at one time but not at all times that soil nitrate concentration was measured (Table 2). Although soil ammonium concentration was measured in fewer cases (9), cover crops did not affect soil ammonium concentration in 89% of them (Table 2). About half of the time (3 of 6), planting cover crops did not affect soil total inorganic nitrogen concentration. Cover crops take up inorganic nitrogen when they are growing and convert the nitrate into organic N in the cover crop [24]. This is only negative if the cover crop reduces grain yields of the crop it is planted with or reduces the yield of the following crop. In Section 4, cover crops generally did not reduce crop grain yields. However, in some of the studies where interseeded cover crops reduced the yield of the crop they were planted with or the following crop, it may have been due to the cover crop taking up some of the available nitrogen in the soil.

Some previous studies also measured total nitrogen and potentially mineralizable nitrogen following planting cover crops (Table 2). Two times planting cover crops increased potentially mineralizable nitrogen with at least one of the species that was planted, but another time planting cover crops did not have any effect. However, in the four cases that measured soil total nitrogen, interseeding cover crop did not affect total nitrogen (Table 2). Also, one study measured dissolved organic nitrogen at two locations and found no effect of the cover crop [44]. This study also found that microbial biomass nitrogen was higher under cover crop mixture, radish and control than the crimson clover cover crop [44]. It is difficult to determine whether certain factors affected soil nitrogen dynamics due to the limited studies available measuring some of the soil nitrogen-related properties. However, interseeding grass cover crops tended to reduce nitrate concentration in some of the studies listed in Table 2. One of the purposes of planting cover crops is to reduce nutrient leaching and uptake free nutrients [20,23,24]. However, cover crops did not increase the total amount of nitrogen stored in the soil in the studies included in our review.

One of the reasons cover crops are planted is to return additional carbon to the soil and increase soil organic carbon concentration. Soil organic carbon is important in soil aggregate formation and retaining nutrients [103]. Previous studies showed minimal benefits of interseeded cover crops to organic carbon, organic matter or particulate organic matter (Table 2). The studies included in this review were conducted for short periods of time, which may be why there were minimal benefits to soil carbon in these studies (Table 2). Also, cover crop biomass production in many of these studies was generally low (<1.1 Mg ha⁻¹ average biomass production). The relatively low level of carbon additions from the cover crops in these studies may be why there was a lack of benefits to soil organic carbon. However, a previous review found that in many cases planting cover crops increased soil organic carbon concentration, but that biomass production was a factor that affected how much carbon was sequestered [100]. Also, three studies measured soil properties that indicate how effectively the soil processes soil carbon, and two studies measured how much of certain pools of carbon was contained in the soil. Two of the studies found no benefit of cover crops to microbial biomass carbon, and one found no improvement in soil CO₂ burst and dissolved organic carbon [44,69]. However, two studies found that planting cover crops can increase soil CO₂ burst [52,71]. This shows that interseeding cover crops may improve how quickly soil biology processes carbon, but that it does not increase the total soil organic carbon concentration when planted for a short duration of time.

A few studies also measured other soil chemical properties, including soil cation exchange capacity, pH and concentrations of many essential nutrients for plant growth. Cover crops uptake the same nutrients that plants need but may also prevent them from leaching downward in the soil [23]. Cover crops also may improve biochemical cycling of organic matter that will make the nutrients plant-available [101]. Interseeding cover crops generally did not affect soil phosphorous concentration (5 of 6 locations) and had inconsistent effects on soil potassium (Table 2). Soil potassium was increased by some species in one, was lower under some of the species in two, and was not affected by cover crops at the other three locations (Table 2). Although they were only measured in a few cases, interseeding cover crops did not affect soil pH, cation exchange capacity, magnesium, calcium, sodium and manganese concentration, but increased iron and zinc concentration in one study (Table 2). These studies show that in most cases interseeding cover crops did not affect the concentration of most essential plant nutrients besides the nitrate form of nitrogen. The finding that interseeded cover crops generally did not affect soil chemical properties does not agree with the hypothesis that soil properties would be better improved by cover crop mixtures compared to individual species and planting at early V growth stages compared to later R stages. However, there were limited studies available that compared the effect of individual species to cover crop mixtures at the same location. More studies are needed to conclude how long it takes and what cover crop species or mixtures and planting times are best to improve soil properties when interseeding into corn and soybeans.

6.2. Soil Physical Properties

Soil physical properties are important for maintaining soil structure, water infiltration, storage and plant availability and reducing soil erosion [104]. Many fewer studies measured soil physical properties when cover crops were interseeded into corn and soybean compared to soil chemical properties (Table 3). Also, these studies primarily measured soil water content. This is not surprising, because many producers are concerned about cover crops taking up soil water that could be used by the main crop [102]. In a few of the studies, cover crops planted during at least one of the planting dates reduced soil water content compared to the control (Table 3). However, most of the time interseeding cover crops into corn or soybean did not affect soil water content (Table 3). It is worth noting that water content was only measured a few times during the cover crop/main crop cycle in many of these studies [36,57,64]. This finding is important because many would assume that interseeding cover crops into corn or soybean during the main crop growing season would reduce soil water content, which was generally not found in the papers included in this review. Also, soil temperature was measured at two locations and planting cover crops at planting reduced soil temperature, but not at the V8 or R5 stages. This may have occurred from the cover crop biomass increasing cover on the soil surface and increasing the soil surface albedo, which reduces heat adsorption by the soil [25,105]. However, there is only one study that measured soil temperature, which is not enough data to make a conclusion [32].

Some other studies measured how interseeding cover crops affects other soil physical health measurements [36,57,71,75]. These studies found that interseeding cover crops generally did not improve wet aggregate stability, infiltration, bulk density, penetration resistance and cone index (Table 3). However, in one case each interseeded cover crop increased infiltration [57] and plant-available water [71], but reduced bulk density and cone index at one of the locations in another study [75]. In a minority of the cases, interseeding cover crops into corn or soybeans improves soil physical health, but more studies are needed that measure many of these soil physical properties to see if this is true. However, it may take a longer duration of planting interseeded cover crops to see benefits to many of these soil physical properties. A previous review found that increasing the duration that cover crops are planted increased the improvement to soil physical properties provided by cover crops [25]. Also, the studies included in this review found minimal improvement in soil organic carbon concentration from interseeded cover crops, which may be why there also was limited improvement in soil physical properties.

6.3. Soil Biological Properties

Compared to soil physical and chemical properties, many fewer studies measured soil biological properties when interseeding cover crops. However, soil biology is important for creating pores in the soil, decomposing organic materials and making nutrients available to plants [106]. One study measured soil microbial biomass, fungi and bacteria when interseeding cover crops into corn at three locations in South Dakota and found no benefit from the cover crop ([34] and Table 4). However, a study conducted in Pennsylvania found that interseeding annual ryegrass cover crop into corn increased the microbial biomass and changed the soil microbial community structure [52]. However, there was no change in the fungal:bacterial ratio when cover crops were interseeded at this location [52]. Additionally, a study that was conducted at two locations in Michigan found no effect of interseeded cover crops on β-glucosidase, N-acetyl-glycosaminidase, peptidase and phosphatase enzymes (Table 4 [44]). These enzymes are measured as an indication of how effective the soil is at decomposing organic matter and making nitrogen and phosphorous contained in organic forms available to plants [107]. The results from previous studies show that interseeding cover crops may affect soil biological properties in some cases, but more studies are needed to see if this is true.

7. Summary and Conclusions

Based upon the findings of this review, interseeded cover crops that were terminated by frost produced less biomass than cover crop species that overwintered into the following spring. Planting a diverse mixture of cover crops tended to produce a larger amount of biomass in the fall, but mixtures did not produce greater spring biomass compared to single species. Most cover crop species that overwintered produced similar biomass to cereal rye, which is one of the most commonly planted cover crop species in the northern portion of the United States where most of these studies were conducted. This finding suggests that interseeding cover crops into corn and soybean during V stages may not produce greater biomass than planting winter-hardy cover crops either during late R stages or postharvest. Planting cover crops during the R stages produced the most spring biomass whether planted into corn or soybeans, but the most fall biomass was produced when cover crops were planted into corn at the planting and V4 growth stage. Factors that affected cover crop biomass production included cover crop species, tillage practice, planting date, planting method, row spacing and average annual precipitation. Broadcast cover crops performed better under conventional tillage than with no tillage. Also, as precipitation increased, spring cover crop biomass increased. In corn, row spacing wider than 76 cm increased biomass production.

Interseeded cover crops did not improve grain yields but also did not lead to a large reduction in grain yields. However, interseeding cover crops into corn or soybeans at the V4 stage or before may negatively affect crop grain yield. Cereal rye, berseem clover and winter camelina tended to reduce corn grain yield, and alfalfa, cereal rye and red clover reduced soybean grain yields. In the year following the planting of the interseeded cover crop, annual ryegrass reduced both corn and soybean grain yields, and cereal rye, alfalfa and red clover cover crops affected corn yields. Also, in semi-arid climates (500–750 mm of precipitation), interseeded cover crops had the largest negative effects on grain yield. The findings of this review suggest that cover crops should be interseeded after the V4 stage into corn and the R4 stage in soybeans to minimize the negative effects of the cover crop on grain yields. Also, cover crops reduced weed biomass compared to the weedy control with no cover crop. However, chemically weeded control or cultivation generally had less weed biomass compared to the cover crop plots. Additionally, interseeded cover crops generally did not benefit most soil properties, apart from reducing soil nitrate in the soil. Although there were limited benefits to soil properties from interseeded cover crops, these studies were conducted for four years or less and produced limited biomass, which may be why limited improvement to soil properties occurred.

Many studies measured biomass production and crop grain yields when cover crops were interseeded into corn and soybeans after a short period of time. More studies are needed that measure soil properties after cover crops are interseeded into corn and soybeans for a longer duration. It may take a long period of time to see improvement in many of the soil properties that are indicators of soil health. Also, studies are needed to study the effects of row spacing and seeding rate on cover crop biomass production and grain yields. Almost all of the previous studies planted cover crops into corn or soybeans planted at one seeding rate and row spacing. Some producers are interested in planting corn at wider row spacing and incorporating cover crops. This allows more sunlight to enter the canopy for the cover crop to grow. The increased biomass production from the cover crop may enhance benefits to the soil provided by the cover crop and provide additional forage for grazing. Additionally, more studies are needed to see how effective different cover crop species are at suppressing weeds. Cover crops may be able to serve as an alternative to chemical weed control while improving soil health.

Interseeding cover crops into corn and soybeans at conventional row spacing generally did not produce a large amount of cover crop biomass. Interseeded cover crops did not perform better than planting cover crops using traditional methods, based upon the findings of this review. Grain yields generally were not affected by interseeded cover crops. The relatively low biomass production likely is why interseeded cover crops provided minimal benefits to soil health.