Warm-Season Grasses in Cover Crop Polyculture Mixes: Enhancing Resilience in Forage Cropping Systems
Funded by Results Driven Agriculture Research
Introduction and Objective
A cover crop polyculture that includes warm-season grasses seeded in the same field can sustainably contribute to cattle production systems by offering stability in forage yields and high nutritive values to improve animal performance. In Alberta, while cover crop blends are mostly cool-season crops, the inclusion of a warm-season crop such as sorghum sudangrass can provide effective competition and complementarity to the cropping ecosystem. With the effects of climate change severely affecting cropping systems, warm-season cover crop polyculture mixes could enhance cropping systems' resilience. Therefore, the objectives of this study were to:
evaluate forage yield, nutritional quality, and anti-nutritional factors
assess water-use efficiency
determine botanical composition
monitor short-term improvement in soil health (soil water characteristics, soil nutrients, and microbial diversity)
What We Did
The project was implemented at the Fairview Research Farm (RR 35 Fairview, AB) and the University of Alberta Research Farm, Breton site (NE-25-47-04-W5). The research design was a 2x5x4 factorial design with four replications in plots measuring 10 m x 2m and 8 m x 2 m. Seeding was carried out as same-row (SR) and alternate-row seeding (AR, side by side rows) using a 6-row Fabro Plot Drill with disc-type openers on 23 cm row spacing. The cropping history of both sites was a 3-year fallow.
Site soil information (0-6” and 6-12”) was:
Fairview: pH = 6.2, organic matter = 5.2%, electrical conductivity = 0.24 dS/m.
Breton: pH = 5.7, organic matter = 6.3%, electrical conductivity = 0.28 dS/m.
A mixture of cover crop (CC) and Sorghum Sudangrass (hybrid obtained by crossing Sudangrass and Sorghum), each seeded at 50%, while CDC Haymaker Oats monoculture control, and CC treatment alone were each seeded at full rates (Table 1). Table 1 provides treatments, field operations, and data collected for this trial.
Field Observations
Fairview Research Farm:
Seeding was done in relatively dry soil conditions. This slowed plant emergence. Although the site received precipitation about 2.5 weeks after seeding, some plants struggled to emerge, with only a few plants observed at establishment in some rows.
The monoculture CDC Haymaker Oats (control) emerged earlier compared to Sorghum Sudangrass or cover crops.
Among Sorghum Sudangrass, the MS 9000 varieties were slower to emerge relative to the WMR hybrid and Medium BMR. Additionally, the WMR hybrid and MS 9000 were slow to reach the head maturity stage.
Overall, the site was severely affected by drought conditions, impacting the plant growth
Breton site:
Seeding was also done in very dry soil conditions.
Plant emergence was relatively better due to adequate soil moisture for 2 weeks following seeding.
Similar to the Fairview site, the monoculture CDC Haymaker Oats (control) had a better emergence compared to the other varieties.
The site received substantial precipitation throughout the season, translating into higher yields.
CDC Haymaker Oats plot, Fairview
Sorghum + cover crop plots, Fairview
Findings
DM Yield
Fairview site: Monoculture CDC Haymaker Oats produced more DM yield (3,482 lb/ac), whereas WMR hybrid Sorghum Sudangrass + CC (AR) had the lowest yield (1,272 lb/ac). Among the mixtures, MS 9000 Sorghum Sudangrass + CC consistently yielded more (Table 1). Overall, same-row seeded treatments produced higher DM yield compared to alternate seeding.
Breton site: Monoculture CDC Haymaker Oats produced the highest DM yield (6,152 lb/ac). The alternate row seeding (AR) of WMR hybrid Sorghum Sudangrass + CC was the highest forage DM producing treatment among the mixtures, while the same-row seeding (SR) of WMR hybrid Sorghum Sudangrass + CC (SR) produced the lowest forage DM yield (Table 2).
Nutritive Value
The nutritive value of forage treatments at both sites was significantly different among treatments, except for acid detergent fiber at the Fairview site (Tables 1 and 2).
Fairview site: The CP in forages ranged between 9.3% for monoculture CDC Haymaker Oats (SR) and 22.2% for WMR hybrid Sorghum Sudangrass + CC (SR). Treatments which included a cover crop blend produced CP content of between 18.8 and 22.2%.
Acid Detergent Fiber (ADF) of forages was between 26.9-29.9%, whereas Neutral Detergent Fiber (NDF) ranged between 33.9 – 54.7%. The lowest ADF and NDF were reported for cover crops seeded in the same row. The Haymaker Oats had the highest ADF and NDF. Similarly, the highest Total Digestible Nutrients (TDN, 71.9%) was found in cover crops, same-row seeded, while monoculture CDC Haymaker Oats recorded the lowest (63 -66%). All treatment mixtures had >69% TDN (Table 1).
Neutral Detergent Fiber digestibility (NDFD48-hr) ranged between 49.9 and 61.1%. All Sorghum Sudangrass + CC treatments recorded NDFD48-hr ranging between 57.4-61.1%, with the highest found in WMR Sorghum Sudangrass + CC (AR).
High Ca and K contents were found in cover crops (SR), while P was higher in forages from a mixture of WMR hybrid Sorghum Sudangrass + CC (SR). Overall, Ca, P, and K were lower in monoculture CDC Haymaker Oats. Apart from cover crops (SR), all mixtures had similar macronutrients.
The Relative Feed Value (RFV) ranged between 117.7 and 177.4. The MS 9000 Sorghum Sudangrass + CC (SR), cover crops (SR), and WMR hybrid Sorghum Sudangrass + CC (SR) had the highest RFV, 174.4, 177.4, and 174, respectively. Conversely, monoculture CDC Haymaker Oats had the lowest RFV (Table 1).
The amount of nitrates in forages varied between 406 and 2000 ppm. The highest nitrates were found in forages from MS 9000 Sorghum Sudangrass + CC (AR). Among all treatments tested in the study, the monoculture CDC Haymaker Oats had the lowest nitrate accumulation (400 -500 ppm). Sole cover crops had nitrates between 500-600 ppm, while Sorghum Sudangrass + CC treatments recorded nitrates above 800 ppm (Table 1). All measured nitrate values fell within the safe range for forage utilization (0–2100 ppm), indicating no immediate risk of nitrate toxicity under proper feeding management.
Nutritive Value
Breton Site: CP in forages ranged between 8.4% to 14.1%. The highest CP was found in cover crops seeded as alternate-row, and a mixture of WMR hybrid Sorghum Sudangrass + CC (SR) (13.1-14.1%). The monoculture CDC Haymaker Oats had the lowest CP (8.4 -8.8%).
ADF in forages was between 36.4 to 41.9%, whereas 47.8-61.3% for NDF. While a mixture of WMR hybrid Sorghum Sudangrass + CC (AR) recorded the highest ADF (41.9%) and NDF (61.3%), cover crops (AR) had the lowest ADF and NDF (36.4% and 47.8%, respectively).
The highest TDN was recorded in a mixture of Medium BMR Sorghum Sudangrass + CC (SR), 66.7%. Among treatments, TDN ranged between 60.7 and 66.7%. The lowest TDN was found in a mixture of WMR hybrid Sorghum Sudangrass + CC (AR), 60.7%. Monoculture CDC Haymaker Oats and cover crops (AR) had similar TDN; likewise, MS 9000 Sorghum Sudangrass + CC (SR) and cover crops (SR) (Table 2).
The NDFD-48hr among treatments was above 54. The mixture of Medium BMR Sorghum Sudangrass + CC (SR) had the highest (63.3), and the mixture of WMR hybrid Sorghum Sudangrass + CC (AR), the lowest, 54.4. Cover crops either same or alternate row seeded produced similar NDFD48hr (Table 2).
The nitrates in forages ranged between 502 and 2,074 ppm. The highest amounts were found in a mixture of WMR hybrid Sorghum Sudangrass + CC (SR), while the lowest in a mixture of MS 9000 Sorghum Sudangrass + CC (AR) and monoculture CDC Haymaker Oat (AR), ~ 500 ppm. Among the treatments, mixtures of WMR hybrid Sorghum Sudangrass + CC, and cover crops, whether same or alternate-row seeded, had nitrate levels exceeding 1100 ppm. These nitrate levels are safe when present in forages (ideal range = 0 -2100 ppm).
Soil Health Indicators
Fairview Site: Significant differences were found in Organic Matter (OM), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and soil health index among treatments. However, no variations were found in nitrate (NO3), estimated nitrogen release (ENR), mineralizable nitrogen (MN), carbon dioxide (CO2), and active carbon (AC).
Soils of cover crops (SR or AR) had the highest OM, P, K, Ca, and health index, whereas all the other treatments, particularly the mixture of MS 9000 Sorghum Sudangrass + CC (AR or SR) had the lowest P, K, Ca, Mg, and soil health index (Table 5). By comparing soil health indicators post-harvest to pre-establishment, there was a slight increase in OM for the majority of treatments, particularly for cover crops (SR) by 28% (Tables 3 and 4). The P, K, Ca, and Mg increased by 58, 84, 10, and 7%, respectively. Though no significant differences were found among treatments, the MN, AC, and CO2 also increased substantially.
Breton Site: At the Breton site, significant differences were found in Ca, Mg, and AC among treatments. However, OM, P, K, NO3, ENR, MN, CO2, and soil health index were indifferent. Overall, P ranged between 13.3 – 15.0 ppm, 93- 105 ppm for K, 2827.5 – 3312.5 ppm for Ca, 366.8 – 424.5 ppm for Mg, and 1160 -1362 ppm for AC. Similar to the Fairview site, soils of cover crops had the highest OM, Mg, ENR, and CO2 after forage harvest (Tables 5 and 6).
Water Use Efficiency
There were wide differences in WUE among treatments across the two sites.
At the Fairview site, monoculture CDC Haymaker Oats had the highest WUE (56-72.8 lb DM per inch of water) while a mixture of WMR hybrid Sorghum Sudangrass + CC (AR) had the lowest (24.8 lb DM per inch of water). Overall, cover crop treatments (AR or SR), and mixtures of Sorghum Sudangrass + CC recorded WUE between 35.8 – 45.6 lb DM per inch of water (Table 7).
At the Breton site, WUE ranged between 35.5 lb DM per inch of water for cover crops (SR) and 59.7 lb DM per inch of water for monoculture CDC Haymaker Oats. Except for WMR hybrid Sorghum Sudangrass + CC (SR), all mixtures which had cover crops within stands had higher WUE than cover crops (AR or SR) and monoculture CDC Haymaker Oats (AR). Among mixtures, the mixture of WMR hybrid Sorghum Sudangrass + CC (AR) reported the highest WUE (Table 8).
Overall, though the highest WUE was recorded at the Fairview site, the general trend of WUE for forage crops at the Breton site was consistently superior.
Conclusion and Implications
Yield responses were site and moisture dependent. Monoculture CDC Haymaker Oats consistently produced high DM yields across both sites, while Sorghum Sudangrass + cover crop mixtures achieved the highest yields under favourable conditions, with MS 9000 performing best at the Fairview site and WMR hybrid at Breton. Overall DM yield at Fairview was 50% lower than the Breton site due to drought stress.
All forages met beef cattle nutritional requirements, with cover crops excelling in quality. Crude Protein exceeded minimum beef cattle requirements (7% CP) at both sites, with cover crops providing the highest CP (>19% at Fairview), lower ADF and NDF, and improved digestibility, particularly when harvested at optimal maturity.
Energy value and nitrate levels posed minimal nutritional risk. Total digestible nutrients (60–71%) across treatments were sufficient for all beef production stages, while nitrate (analyzed by wet chemistry method) concentrations remained within safe limits (<2,100 ppm).
Cover crops enhanced water use efficiency and soil health resilience. Monoculture CDC Haymaker Oats showed the highest WUE, but forage mixtures and cover crops provided balanced productivity under drought, while consistently improving soil health indicators (OM, nutrients, active C, CO₂, and health index), confirming their role in building resilient forage systems.
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