Seeding Rate Effects on Smooth Bromegrass (Bromus inermis Leyss.) Interseeded with Annual Warm-Season Grasses

Abstract

Interseeding pastures with annual warm-season grasses may increase forage accumulation and nutritive value. Our objective was to evaluate the effects of seeding rates of crabgrass [Digitaria ischaemum (Schreb.) Schreb. Ex Muhl], sorghum–sudangrass (Sorghum bicolor × S. bicolor var. sudanense), and teff [Eragrostis tef (Zuccagni) Trotter] on the forage accumulation and nutritive value of pastures of smooth bromegrass (Bromus inermis Leyss.), an introduced perennial cool-season grass cultivated for pasture and hay production in the U.S. Western Corn Belt. In spring, before interseeding, forage accumulation averaged 4.03 and 6.39 Mg ha⁻¹ in 2020 and 2021, respectively. In summer, after interseeding, forage accumulation averaged 3.52 Mg ha⁻¹ in 2020 but was not affected by treatment. In 2021, forage accumulation averaged 6.22 Mg ha⁻¹ in sorghum–sudangrass interseeded stands compared to 4.08 Mg ha⁻¹ in non-seeded smooth bromegrass. Interseeding crabgrass and teff had limited effects on forage accumulation and nutritive value. Increasing the seeding rate of sorghum–sudangrass linearly increased yield of crude protein, total digestible nutrients, and dry matter. In the next spring, forage accumulation averaged 8.01 Mg ha⁻¹, and the stands showed no residual effects of the one-time interseedings. Sorghum–sudangrass proved to be the optimum annual warm-season grass for interseeding.

1. Introduction

In the U.S. Western Corn Belt, smooth bromegrass (Bromus inermis Leyss.), an introduced species cultivated for pasture and hay production, has some of the highest forage yields among perennial cool-season grasses [1,2,3]. As temperatures rise and growing degree-days accumulate throughout spring, smooth bromegrass exhibits rapid rates of dry matter accumulation, increasing stem growth, and a decline in forage quality [3]. Grazing during early spring allows producers to get an early start on pasturing with limited impact on sward yield and growth [4]. During summer and fall, though, smooth bromegrass shows a reduced growth, typically one-third of that in spring [1]. Nitrogen (N) fertilizer application is a big driver of growth, particularly in years with favorable precipitation [1,2,3]. In frequently clipped swards of smooth bromegrass in eastern Nebraska, USA, in a year with slightly above average precipitation, forage accumulation increased linearly with applications up to 180 kg N ha⁻¹ [5]. During a drought year, though, forage accumulation from smooth bromegrass showed no response to N fertilizer application [5]. Interseeding legumes into smooth bromegrass pastures offers an opportunity to reduce costs associated with N fertilization while producing similar amounts of forage [6]. Legume-interseeded pastures yield more forage later in the growing season, contributing to greater daily weight gains of cattle later in the growing season, more grazing days, and greater beef production [6].

In recent years, the amount of land in natural grasslands and cultivated pastures has seen declines throughout the Western Corn Belt [7], threatening the supply of forage for livestock producers. A strategy for increasing the forage supply may be to interseed pastures of smooth bromegrass or other perennial cool-season grass species with annual warm-season grasses [8]. The rapid growth and dry matter (DM) accumulation of annual warm-season grasses in summer complements the favorable growth profile of cool-season grasses in spring and offer a strategy for increasing forage accumulation and nutritive value [9,10]. In a recent study, perennial cool-season grasses interseeded with sudangrass [Sorghum bicolor (L.) Moench ssp. drummondii (Neus ex Steud.) de Wet and Harlan] and sorghum–sudangrass (S. bicolor × S. bicolor var. sudanense) were shown to have 100–214% greater forage accumulation than unseeded pastures [9] and greater mid- and late-summer yields of crude protein (CP) and digestible organic matter [10]. Planting forage sorghum [S. bicolor (L.) Moench] also was shown to be an option if the annual warm-season grass is allowed to accumulate until maturity or a killing frost as it will not regrow after grazing or mechanical harvest [11].

Data on the success of using other annual warm-season grasses, like crabgrass [Digitaria ischaemum (Schreb.) Schreb. Ex Muhl] and teff [Eragrostis tef (Zuccagni) Trotter], to interseed pastures remains absent. In a West Virginia, USA, study, tall fescue [Schedonorus arundinaceus (Schreb.) Dumort., nom. cons.] pastures interseeded with teff were evaluated, and the results showed that the teff established rapidly whether broadcast or no-till seeded [12]. The contribution teff made to the swards was greater in a year with warmer seasons than years with cooler seasons, and its sowing increased dry matter yields relative to non-interseeded plots [12]. Other studies have evaluated the forage accumulation and nutritive value of crabgrass interseeded into pastures of Kentucky bluegrass [Poa pratensis (L.)], orchardgrass [Dactylis glomerata (L.)], and tall fescue [13,14]. Although often considered a weedy grass, interseeding a newer variety of crabgrass into cool-season grasses may be advantageous due to its selection for rapid germination, improved growth, and uniformity regarding phenology compared to weedy types [15]. Crabgrass offers lower nonstructural carbohydrates compared to cool-season grasses, which can be beneficial when nonstructural carbohydrate intake is an animal health concern [16]. Crabgrass also has been shown to be highly digestible, and its CP concentration increases with the N fertilizer rate [17].

A factor to consider when interseeding pastures is the seeding rate of the annual warm-season grass. In a seeding rate study of sorghum–sudangrass interseeded into cool-season grass sod, the forage mass of sorghum–sudangrass showed positive linear and cubic responses to the seeding rate in harvests at 45 and 90 d after interseeding, and it was recommended that producers interseed sorghum–sudangrass with at least 28 kg ha⁻¹ [18]. A study in the northeastern USA found that the teff forage yield increased by 224 kg ha⁻¹ for each 2.24 kg ha⁻¹ increase in seeding rate from 3.36 to 10.08 kg ha⁻¹ [19]. A recent study evaluated seeding rates of 10 to 25 kg ha⁻¹ for teff managed as a grain crop in Ethiopia and found that the lowest seeding rate (i.e., 10 kg ha⁻¹) provided the greatest grain yield, aboveground biomass yield, and straw yield [20]. Seeding rates of 3.36 to 5.60 kg ha⁻¹ have been recommended for the establishment of crabgrass for forage in open land situations [21], but it remains unclear how the seeding rate for crabgrass and teff may change when interseeding pastures. The objective of this study was to evaluate the seeding rates of crabgrass, sorghum–sudangrass, and teff interseeded into smooth bromegrass, a perennial cool-season grass broadly adapted in the U.S. Western Corn Belt, the northern U.S., and Canada [2].

2. Materials and Methods

2.1. Experiment Design

We conducted replicated interseeding experiments from 2020 to 2021 and 2021–2022 in different areas of a smooth bromegrass (B. inermis Leyss.) stand at the University of Nebraska–Lincoln Eastern Nebraska Research, Extension and Education Center (40°49′40″ N 96°39′26″ W, and 358 m ASL) near Mead, Nebraska, USA. The site had a Tomek silt loam (fine-silty, mixed, superactive, and mesic Pachic Ardiudoll) and a Yutan silty clay loam (fine-silty, mixed, superactive, and mesic Mollic Hapludalf) soil [22] with a 5.9 pH, 4.38% organic matter, 28.1 ppm P, and 368 ppm K at a 0–15 cm soil depth [9]. Across a 30-year period from 1981 to 2010, annual precipitation and temperature averaged 792 mm and 10.6 °C at the nearby Mead 6S weather station [23]. In 2020, 2021, and 2022, annual precipitation summed to 427, 867, and 452 mm while temperature averaged 10.4, 10.7, and 9.8 °C, respectively [23].

The experiments had completely randomized designs with three replicates of ten treatments including three crabgrass [D. ischaemum (Schreb.) Schreb. Ex Muhl] interseeding rates (3.8, 7.5, and 11.3 kg ha⁻¹), three sorghum–sudangrass (S. bicolor × bicolor var. sudanense cv. Super Sugar) interseeding rates (22, 44, and 66 kg ha⁻¹), three teff [E. tef (Zuccagni) Trotter cv. Haymaker] interseeding rates (6.2, 12.3, and 18.5 kg ha⁻¹), and one non-seeded smooth bromegrass control. The crabgrass was a blend of Impact and Red River cultivars with a 50% Yellow Jacket seed coating (Barenbrug USA, Tangent, OR) designed to reduce water needed for establishment and improve seedling establishment and root development. Seeding rates represented 0.5, 1.0, and 1.5 times the rate for conventionally seeding each of the grasses as sole crops based on seed source recommendations and were in the range of rates used in previous studies for crabgrass [13,14,16], sorghum–sudangrass [9,10,18], and teff [12,20]. The taller-statured sorghum–sudangrass was used in this study because of its success when used in previous interseeding studies, while crabgrass and teff were used in this study based on expected rapid germination and quick growth [12,13,15] and potentially high digestibility [17].

We interseeded the grasses on 12 June 2020 and 18 June 2021 with a Great Plains 3P600 drill (Kincade Equipment Manufacturing, Haven, KS, USA) equipped with a cone seeder to accurately meter the seed on a pure live seed (PLS) basis in plots that were 1.68 m wide by 4.57 m long. Each plot contained nine rows with 15.25 cm row spacing. We seeded crabgrass and teff at a 0.64–1.27 cm depth and sorghum sudangrass at a 1.27–2.54 cm depth. Before interseeding, we allowed the existing smooth bromegrass to flower in spring before harvesting and removing all forage from the plots with a Carter Harvester (Carter Manufacturing, Brookston, IN, USA) on 9 June 2020 and 7 June 2021. We applied granular urea (46-0-0) at 70 kg N ha⁻¹ to all plots 2–3 weeks after interseeding and at 90 kg N ha⁻¹ in late April in the next spring. The site was not fertilized before the initiation of the experiments during the springs of 2020 and 2021.

2.2. Forage Accumulation and Nutritive Value

In the first experiment, plots were harvested and DM was determined before interseeding on 9 June 2020, about 90 d after interseeding on 16 September 2020, and in the following spring on 7 June 2021. In the second experiment, plots were harvested and DM was determined before interseeding on 7 June 2021, about 45 d after interseeding on 3 August 2021, about 90 d after interseeding on 20 September 2021, and in the following spring on 13 June 2022. Plots were cut at a 2.5 cm stubble height in the initial smooth bromegrass harvest and at a 15 cm stubble height in the post-interseeding and subsequent spring harvests. Before the post-interseeding and subsequent spring harvests, we hand clipped the forage in two 0.1 m² quadrats in each plot and sorted the material to determine functional group composition: cool-season perennial graminoids (i.e., mainly smooth bromegrass), seeded annual warm-season grasses, other annual grasses, and forbs. The hand-clipped and Carter-harvested samples were dried at 60 °C for at least 48 h, weighed, and scaled up to determine DM accumulation for the plot. The Carter-harvested samples from the 45 and 90 d post-interseeding harvest in the second experiment were mixed by hand, ground to a 1 mm particle size with a Wiley mill (Thomas Scientific, Philadelphia, PA, USA), and analyzed for CP, neutral detergent fiber (NDF), acid detergent fiber (ADF), and total digestible nutrients (TDN) at Ward Laboratories (Kearney, NE, USA). The CP percentage was determined by multiplying 6.25 × N concentration, which was measured by combustion. The NDF and ADF percentages were determined by refluxing with neutral and acid detergent solution [24]. The TDN percentage was computed from the equation TDN = 88.3 − (0.777 × ADF).

2.3. Statistical Analyses

We conducted two sets of analyses for each response variable using mixed model procedures in SAS version 9.4 (SAS Institute, Cary, NC, USA). First, we conducted a combined analysis across years without the data from the non-seeded plots to examine fixed effects of interseeded species, seeding rate, and their interaction. These analyses showed that interseeded species, seeding rate, and their interaction did not affect spring, summer, and annual forage accumulation. Thus, in a second set of analyses, we pooled data across seeding rates to compare fixed effects of interseeded species versus non-seeded stands and random effects of year [25]. To perform both sets of analyses, DM accumulation in the 45 and 90 d harvests in the second experiment was added together to match the single 90 d harvest in the first experiment. Estimate statements computed best linear unbiased predictions of intercepts and intercept differences as related to random effects of year [26]. Contrast statements evaluated the significance of linear and quadratic relationships with seeding rate as related to forage composition and nutritive value differences in the summer harvest. Lastly, we conducted a third set of analyses focusing in on the 2021 data only from stands interseeded with sorghum–sudangrass to evaluate forage DM accumulation and nutritive value differences due to non-significant forage accumulation responses in other plots. These analyses included data from non-seeded plots for evaluation of quantitative relationships with seeding rate.

3. Results

3.1. Forage Accumulation and Composition

In the spring, before interseeding, the forage accumulation averaged 4.03 and 6.39 Mg ha⁻¹ in 2020 and 2021, respectively. It did not differ between the interseeded and non-seeded smooth bromegrass stands either year (

Table 1. Spring, summer, and annual forage accumulation (Mg ha⁻¹) and perennial cool-season (CS) graminoids (%) in summer after interseeding crabgrass, sorghum-sudangrass, and teff in smooth bromegrass stands near Mead, NE, USA. Values represent best linear unbiased prediction of intercepts relative to random effects of year. Those without shared uppercase letters within rows and lowercase letters within columns differed at p < 0.05.

Response	Year	Crabgrass	Sorghum–Sudangrass	Teff	Non-Seeded
Spring forage	2020	4.45 Ab	4.18 Ab	4.01 Ab	3.97 Ab
	2021	6.71 Aa	6.40 Aa	6.15 Aa	5.71 Aa
Summer forage	2020	3.56 Aa	3.89 Ab	3.49 Ab	3.14 Ab
	2021	3.90 Ba	6.25 Aa	4.23 Ba	4.09 Ba
Annual forage	2020	7.80 Ab	8.19 Ab	7.42 Ab	7.25 Ab
	2021	10.84 ABa	12.50 Aa	10.55 ABa	9.66 Ba
CS graminoids	2020	81 Aa	75 Aa	83 Aa	87 Aa
	2021	19 Bb	17 Bb	24 ABb	36 Ab

). In the summer, after interseeding, the forage accumulation averaged 3.52 Mg ha⁻¹ in 2020, contributing to an annual forage accumulation of 7.67 Mg ha⁻¹, both of which showed no differences between the interseeded and non-seeded stands (Table 1). In stands interseeded with sorghum–sudangrass, though, the summer and annual forage accumulation, on average, exceeded that in non-seeded stands by 53% and 29%, respectively, in 2021 (Table 1).

Perennial cool-season graminoids (i.e., smooth bromegrass) consisted of 100% of the forage in the spring, before interseeding, in both establishment years. In the summer, after interseeding, they consisted of 82% of the forage on average in the establishment year 2020 but just 17–36% of the forage in the establishment year 2021 (Table 1). Stands interseeded with crabgrass (p = 0.06) and sorghum–sudangrass (p = 0.05), though, had a smaller percentage of perennial cool-season graminoids than non-seeded stands in 2021. The annual warm-season grass seeding rate did not affect the percentage of perennial cool-season graminoids in the forage accumulated after interseeding in 2020 (p = 0.494) or 2021 (p = 0.635).

The percentage of weedy and seeded annual grasses in the forage accumulated in the summer, after interseeding, depended on interseeded species × seeding rate interactions (p < 0.017 and 0.002, respectively), as well as the establishment year (

Table 2. Interseeded species × seeding rate effects on percentages of weedy and seeded annual grasses in stands of smooth bromegrass in 2020 and 2021 near Mead, NE, USA. The 1.0× rate was based on seed source recommendations for seeding each species as sole crops. Values represent best linear unbiased prediction of intercepts relative to random effects of year. Those without shared lowercase letters in year comparisons differed at p < 0.05 within each seeded species and functional group for cells containing lowercase letters. ** p < 0.01; NS, no significant difference.

Interseeded Species	Functional Group	Establishment Year	Seeding Rate			Polynomial Contrast
			0.5×	1.0×	1.5×	Linear	Quadratic
Crabgrass	Weedy annual	2020	16 b	13 b	19 b	NS	NS
		2021	49 a	45 a	52 a	NS	NS
	Seeded annual	2020	0 b	3 b	0 b	NS	NS
		2021	34 a	37 a	33 a	NS	NS
Sorghum–sudangrass	Weedy annual	2020	27	15	10	NS	NS
		2021	29	16	11	NS	NS
	Seeded annual	2020	6 b	10 b	14 b	NS	NS
		2021	48 a	65 a	72 a	**	NS
Teff	Weedy annual	2020	12 b	14 b	0 b	NS	NS
		2021	73 a	74 a	81 a	NS	NS
	Seeded annual	2020	1	0	0	NS	NS
		2021	1	0	0	NS	NS

). In 2020, the summer forage consisted of similar percentages of weedy (15 ± 10%) and seeded annual grasses (3 ± 6%) among treatments (mean ± SE). Stands interseeded with crabgrass and teff, though, had 33 and 67 percentage point increases in weedy annual grasses from 2020 to 2021. Seeded annual grasses, meanwhile, increased by 35 and 52 percentage points from 2020 to 2021 in stands interseeded with crabgrass and sorghum–sudangrass. The percentages of weedy and seeded annual grasses did not show quantitative relationships to the seeding rate in stands interseeded with crabgrass and teff either year. The percentage of weedy annual grasses in stands interseeded with sorghum–sudangrass did not show a strong relationship to the seeding rate when examined by year (p = 0.076 in 2020 and p = 0.124 in 2021), but it did show a negative linear relationship to the seeding rate when examined across years (p < 0.007). The percentage of the forage consisting of seeded annual grasses, meanwhile, increased linearly with the seeding rate in stands interseeded with sorghum–sudangrass in 2021 (Table 2).

In non-seeded stands, weedy annual grasses consisted of 15 ± 3.6% of the forage in 2020 and 60 ± 5.8% of the forage in 2021 (mean ± SE). The 2020 percentage did not differ from the average across the seeding rates of any of the interseeded stands in 2020 (p > 0.90 for each species comparison) nor did the 2021 percentage differ from the percentage in stands interseeded with crabgrass in 2021 (p = 0.18). Weedy annual grasses, though, were 17 percentage points greater on average across seeding rates in stands interseeded with teff (p = 0.08) and 39 percentage points less in stands interseeded with sorghum-sudangrass (p < 0.01) than non-seeded stands in the establishment year 2021.

In the spring the year after interseeding, smooth bromegrass accounted for 99% of the forage while weedy annual forbs accounted for the rest. The interseeded species, seeding rate, and establishment year did not affect forage accumulation, which averaged 8.01 Mg ha⁻¹, or its composition in the spring the year after interseeding. The stands were fertilized in spring, the year after interseeding, and thus, they had relatively greater forage accumulation than the non-fertilized stands in the spring, before interseeding (i.e., 4.03–6.39 Mg ha⁻¹).

3.2. Nutritive Value of Stands Interseeded with Sorghum–Sudangrass

The seeding rate affected the CP percentage in the forage harvested from stands interseeded with sorghum–sudangrass in the summer of 2021 (p = 0.010), which showed a negative quadratic relationship from the non-seeded stands (i.e., 0 kg PLS ha⁻¹) to stands interseeded with 66 kg PLS ha⁻¹ (Figure 1A; p = 0.022). The ADF, NDF, and TDN percentages, meanwhile, showed no relationship to the seeding rate, averaging 38.6%, 61.0%, and 58.5% across stands (p = 0.229, 0.282, and 0.231, respectively). The seeding rate, though, showed a potential to affect the yield of CP (Figure 1A; p = 0.129), TDN (Figure 1B; p = 0.065), and forage DM (i.e., accumulation) in summer (Figure 1B; p = 0.079), all of which increased in linear relationships with the seeding rate (p = 0.034, 0.013, and 0.016, respectively). The seeding rate of sorghum–sudangrass did not affect the annual forage accumulation in 2021 (p = 0.258).

4. Discussion

As observed in an earlier study [9,10], sorghum–sudangrass proved to be the optimum choice among annual warm-season grasses for interseeding perennial cool-season grass pastures. In 2021, the year with the highest precipitation, the forage in harvests at 45 and 90 days after interseeding summed to 6.22 Mg ha⁻¹ in stands with sorghum–sudangrass. This amount falls toward the lower end of the range of 4.85–10.54 Mg ha⁻¹ [9] and nearly 12.00 Mg ha⁻¹ [18] as reported in previous studies, where stands interseeded with sorghum–sudangrass were harvested twice at 45 and 90 d after interseeding. Harvesting only once at 90 days after interseeding allows for greater forage accumulation [9], but the forage CP and digestibility would diminish in the one harvest system [10]. Forage accumulation in the non-seeded stands in 2021 summed to 4.08 Mg ha⁻¹ in harvests at 45 and 90 days after interseeding, which compares with the range of 3.45–5.16 Mg ha⁻¹ reported for non-seeded smooth bromegrass [9] and nearly 5.00 Mg ha⁻¹ for other non-seeded perennial cool-season grasses [18]. In 2020, drought limited the forage accumulation in the sorghum–sudangrass-interseeded and non-seeded stands to 3.52 Mg ha⁻¹.

Interseeding crabgrass and teff did not improve forage accumulation compared to the non-seeded stand of smooth bromegrass. The occurrence of a summer drought likely limited the success of these species in the establishment year 2020. In the establishment year 2021, weedy annual grasses accounted for most of the forage and likely outcompeted crabgrass and teff for available space, water, and nutrients. A West Virginia study found success with teff interseeded into tall fescue, but its abundance and yield contributions varied from year to year, performing best during a hot summer [12]. Factors which may have improved the teff establishment in that study included later harvesting dates for the tall fescue (i.e., late June), later planting dates for the teff (i.e., early July), and later fertilization dates (i.e., August). A study in New Jersey, meanwhile, found similar forage accumulation in pastures interseeded with crabgrass as traditional cool-season grass pastures and discussed the seeding rate as a factor that could be increased to improve crabgrass success [13]. However, we did not observe a seeding rate response for crabgrass despite the highest rate, 11.3 kg ha⁻¹, being 1.5 times the rate used in the New Jersey study.

The seeding rate did affect the yield of CP, TDN, and forage DM (i.e., accumulation) in the summer of 2021 in stands interseeded with sorghum–sudangrass. In previous research with seeding rates of sorghum–sudangrass interseeded into a perennial cool-season grass sod, the forage mass of sorghum–sudangrass showed positive linear and cubic responses to the seeding rate in harvests at 45 and 90 d after interseeding, and it was recommended that producers interseed sorghum–sudangrass with at least 28 kg PLS ha⁻¹ [18]. In our study, the percentage of sorghum–sudangrass increased linearly as the seeding rate increased from 22 to 66 kg ha⁻¹ (Table 2), and increasing the seeding rate showed a potential to affect CP, TDN, and forage DM yields in summer (Figure 1). Each of these parameters increased in a linear relationship with the seeding rate, thus, demonstrating that the more sorghum–sudangrass, the better the yield. Previous research showing that interseeding sorghum–sudangrass increases CP and digestible organic matter yields compared to non-seeded pastures supports this observation [10]. The CP percentage, on the other hand, did not improve with sorghum–sudangrass interseeding, and indeed declined at the highest seeding rate. Nutritive value parameters related to fiber and energy (i.e., NDF, ADF, and TDN percentages) also showed no response to the seeding rate of sorghum–sudangrass. Others have observed similar nutrient percentages (i.e., digestible energy and CP) among interseeded and non-seeded cool-season pastures [16].

As in previous research, we observed years where there was no impact of interseeding. Factors that likely limited interseeding success in 2020 included inadequate soil moisture due to drought. Steps to reduce competition, such as the use of herbicides [27], may weaken existing cool-season grasses and therefore improve the establishment of interseeded annual warm-season grasses. However, the use of herbicides to suppress smooth bromegrass has a carryover effect, reducing the growth of smooth bromegrass the following season proportional to the rate of the herbicide applied [28]. Like previous research [18], we observed no residual effects of one-time seedings on subsequent forage mass and vegetation dynamics, indicating the resilience of smooth bromegrass to the one-time interseeding of annual warm-season grasses. In the spring the year after interseeding, smooth bromegrass accounted for 99% of the biomass, and the forage accumulation averaged 8.01 Mg ha⁻¹, ranking a little lower than the 10.5 Mg ha⁻¹ of the forage accumulation by another cool-season grass-dominated sod [18] the year after interseeding annual warm-season grasses. Future research directions include evaluating how repeated interseedings year after year affect the stand composition and resilience of perennial cool-season grasses, as well as refinements of seeding methods to improve the annual warm-season grass establishment success.

5. Conclusions

Smooth bromegrass, a perennial rhizomatous cool-season grass used for pasture and hay production in the U.S. Western Corn Belt, shows good forage accumulation without N fertilization during spring in eastern Nebraska. During summer, smooth bromegrass growth slows, and weedy annual grasses contribute a significant amount of forage that accumulates in pastures. After harvesting smooth bromegrass in spring for hay or silage, producers have an option to increase the summer forage accumulation by interseeding improved varieties of annual warm-season grasses. In this study, sorghum–sudangrass proved to be an optimum choice among annual warm-season grasses for interseeding smooth bromegrass, and increasing the seeding rate of sorghum–sudangrass improved its stand percentage and yield of CP, TDN, and forage DM in summer. Year, though, plays a significant role in the success of sorghum–sudangrass interseeding, and producers may find limited responses from interseeding during years like 2020, where a summer drought occurred after the initial spring smooth bromegrass harvest and subsequent interseeding. Despite periodic dry conditions and possible disturbances caused by a one-time interseeding of annual warm-season grasses, smooth bromegrass shows resilience, yielding a significant forage the next spring.