2.2. Experimental Design and Management
Two cover crop experiments were conducted during each of the 2016 and 2017 growing seasons with data collection on spring wheat in 2017 and 2018. Each of the experiments were considered a separate environment. The method of establishment was not an objective in this trial. Therefore, in one experiment in each year, the cover crop was direct-planted. In the second experiment, simulated air seeding was used to represent possible cover crop establishment methods.
The experimental design was a randomized complete block with a factorial arrangement. There were four replicates per experiment and each replicate consisted of 20 experimental units. The experimental unit size was 1.52 × 7.62 m. Treatments included soybean relative maturity (cultivar), cover crop species, and cover crop seeding rate. Soybean relative maturities included 0.4, 0.5, 0.8, and 0.9. The 0.4 is the earliest maturing cultivar. Soybean cultivars are listed in Table 1
. All soybean cultivars were glyphosate-tolerant (Roundup Ready 2 Yield), carried resistance to soybean cyst nematode (Heterodera glycines
Ichinohe) (except AG0434), had Phytophthora
resistance, and were pre-treated by the seed company (Asgrow; Bayer, Monheim, Germany) with Acceleron (a.i. pyraclostrobin and metalaxyl) seed treatment. Acceleron seed treatment is a fungicide combination providing protection from seed and soil borne diseases such as but not limited to; Pythium irregulare
, Phytophthora sojae
, Fusarium solani
, and Rhizoctonia solani
. The cultivars were inoculated with Vault SP (Bradyrhizobium japonicum
) inoculum (BASF, Ludwigshafen, Germany) at a rate of 1.8 g kg-1
soybean seed on the day of planting to encourage nodulation. The same cultivars were used in both growing seasons.
Soybean was planted as soon as field conditions were favorable in early to mid-May, with four soybean rows spaced 30.5-cm apart and using a seeding rate of 469,300 live seeds ha−1. The plots were planted with a Hege 1000 no-till planter (Hege Company, Waldenberg, Germany). Seeds were planted to a depth of approximately 3 cm.
Cover crop treatments were none (control), winter camelina, and rye. Cover crop seeding rate treatments were 100% of seeding rate and 75% of seeding rate. Winter camelina cultivar “Joelle” was planted at 6.72 kg ha−1
live seeds for the 100% seeding rate treatments and 5.04 kg ha−1
for 75% rate treatments to a depth of 1.3 cm. The quantity of winter camelina seeds per kg can be upwards of 770,000 seeds kg−1
compared with 39,000 seeds kg−1
for rye [12
]. The rye cultivar “Rymin” was planted at 67.2 kg ha−1
for the 100% seeding rate and 50.4 kg ha−1
for 75% rate, to the depth of 2.5 cm. Germination testing was conducted before planting. For both 2016 and 2017 growing seasons, a 95% germination rate was determined for rye and 90% for camelina. Seeding rate was adjusted based on germination.
All cover crops were interseeded into established soybean at the R7 growth stage of the 0.4 maturity cultivar. Staging of soybean was based on NDSU Soybean Production Field Guide, which defines R7 as beginning maturity—one normal pod on the main stem that has reached its mature pod color [37
]. In one experiment, the cover crops were planted in a single furrow in the center of all soybean rows, 15.3-cm from each corresponding row, resulting in three cover crop rows per experimental unit. Furrows were made to the depth of 1.3 cm for camelina and 2.5 cm for rye using a standard garden hoe. No furrows were made in the control plot (without cover crops). In the other experiment, the cover crop seed was broadcasted, to simulate seeding by airplane.
Weeds in soybean plots were controlled twice in 2016 and once in 2017, prior to the planting of the cover crops using (a.i. 48.7% glyphosate, N-(phosphonomethyl) glycine, in potassium salt form) Roundup PowerMAX (Monsanto Co., St. Louis, MO, USA) and (12.6% (E)-2-[1-[[(3-chloro-2-propenyl)oxy]imino]propyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one) and SelectMax (Valent U.S.A. Corporation, Walnut Creek, CA, USA). The herbicides were applied using TeeJet 8001 XR nozzle at a rate of 1.6 in 94 L ha−1 water and a spray pressure of 200 kPa. Cover crops were terminated in the spring using Roundup WeatherMAX.
In 2017 and 2018, (a.i. 9.15% S-cyano(3-phenoxyphenyl)methyl (+/−)-cis/trans-3-(2,2-dichloethenyl)-2,2-dimethylcyclopropanecarboxylate) Mustang Maxx (FMC Corporation, Philadelphia, PA, USA) was applied at a rate of 1.75 L ha−1
to both soybean and HRSW as soybean aphid (Aphis glycines
Matsumura) levels in soybean and grasshopper (Orthoptera: Acrididae) thresholds in HRSW surpassed thresholds as described by NDSU [37
]. Important field operation and measuring dates are provided in Table 2
Wheat was planted after cover crops were terminated. Fertilizer was broadcast-applied during the spring before the HRSW at a rate 112 kg per ha−1 of N using urea (46-0-0). In both years, the HRSW cultivar “Glenn” was used. All HRSW plots were planted as soon as field conditions were favorable in early May, with a Great Plains 3P605NT no-till planter (Great Plains Ag, Salina, KS, USA). Experimental units had seven rows spaced 18.3-cm apart. The seeding rate was 2,739,000 live seeds ha−1 and seeding depth approximately 2 cm. Weeds were controlled using Wolverine Advanced (4.56% fenoxaprop-p-ethyl, 1.5% pyrasulfotole, 6.13% bromoxynil octanoate, 5.93% bromoxynil heptanoate) (Bayer CropScience LP, Research Triangle Park, NC, USA) to control selective postemergent grassy and broadleaf weeds.
Soybean plant density was determined shortly after emergence (VE) by randomly selecting one linear m near the center of the plot. Then, counting all plants within the linear m in both inner two rows.
Cover crop canopy coverage, defined as a percentage of green plant matter, which covers the soil, was measured using the mobile phone application “Canopeo” developed by the Oklahoma State University Department of Plant and Soil Sciences, following cover crop emergence, before the first killing frost, and before termination in the spring. Canopeo measures the fractional green canopy cover through an image processed through the Canopeo application providing a green canopy coverage percentage [39
]. Canopy coverage data was collected from pictures used taken in the center of each plot at a height of 1 m, allowing 15 cm from the outside of last soybean row. Picture data were then processed using Canopeo application, which resulted in a percentage of green tissue within the area of the picture.
Cover crop biomass was collected in the spring preceding termination and subsequent HRSW planting. Biomass was sampled from an area within a 30.5 × 50 cm plastic square (0.1525 m2). The square was randomly tossed into each half of the lengthwise portion of each experimental unit, creating two samples per plot. An average of the two samples was used for the biomass calculation. Biomass samples were created by cutting all cover crop plants within the square at the soil level. Samples were then place in a dryer at a temperature of 40 °C until biomass sample showed no difference in weight during 24 h. Samples were then individually placed on a tray where foreign material was removed before weighing the sample using a Mettler Toledo XS6001S scale (Mettler-Toledo, LLC, Columbus, OH, USA).
The soybean and HRSW plots were harvested, after physiological maturity [40
], at harvestable moisture content using a Wintersteiger Classic plot combine (Wintersteiger Ag. Ried, Austria). Seed samples were cleaned using a Clipper seed cleaner (Ferrell-Ross, Bluffton, IN, USA), and seed samples were then weighed for yield. Moisture and test weight were determined using a GAC 2100 moisture tester (DICKEY-John Corp., Minneapolis, MN, USA), and observations were corrected to 13% and 13.5% moisture content for soybean and HRSW, respectively. Soybean oil and protein contents were not significantly different between treatments and are not reported in this paper.