Evaluating Cow/Calf and Stocker Performance when Grazing Sudangrass With and Without Sunnhemp

Authors: Shelby L. Davies-Jenkins, Graduate Student, Animal Science; Daren D. Redfearn, Professor, Agronomy; Mary E. Drewnoski, Professor, Animal Science, Lincoln.

Summary with Implications

Sudangrass can provide high-quality summer forage, supporting cow/calf pairs and growing cattle. In this study, sudangrass or a sudangrass–sunnhemp mix was grazed over two years by cow/calf pairs and heavy stockers (~880 lb). There was no difference in carrying capacity between forage types. Sunnhemp inclusion had no effect on cow body condition or nursing calf gain. Cows maintained condition through peak lactation, and nursing calves gained 2.27 lb/day. Stockers gained 1.56 lb/day on sudangrass alone and 1.73 lb/day on the mix. However, the added seed cost of sunnhemp increased the cost of gain in year 1 and resulted in only a breakeven return in year 2. From a forage perspective, adding sunnhemp increased seed cost without improving forage production or grazing days. Based on this study, sunnhemp inclusion into sudangrass does not provide an economic advantage for cow/calf or stocker systems.

Introduction

The growth of cool-season perennial pastures in the Midwest slows during the summer months creating a potential gap in high-quality forage availability. Warm-season annual forages have the potential to fill this summer forage gap due to their ability to produce high yields of forage with high nutritive value. Sudangrass (Sorghum × drummondii) is a tillering annual warm-season forage that can produce substantial yields. Performance of cows and growing calves grazing sudangrass within a system have shown promising results for either maintaining or increasing performance compared to just grazing cool-season perennial pasture. Including a legume in summer annual forage systems may increase the nutrients available to grazing cattle. Sunnhemp (Crotalaria juncea) is a warm-season annual legume that has been known to grow to a similar height as sudangrass. Sunnhemp branches when cut indicating that it might do well in a rotational grazing system paired with sudangrass.  The objectives for this study were understand the difference between a sudangrass monoculture or sudangrass/sunnhemp mixture on 1) animal performance of beef cow/calf pairs or growing steers and 2) carrying capacity and forage productivity throughout the grazing season.

Procedure

This study was conducted over 2 years (year 1 - 2023 and year 2 - 2024) at the Eastern Nebraska Research, Extension, and Education Center near Mead, Nebraska. Two adjacent 49 ac fields were split into 4 pastures for a total of 8 pastures. Class of cattle (cow/calf pairs or growing steers) and forage treatment (monoculture of Piper sudangrass or mixture of Piper sudangrass and sunnhemp) were blocked by field with 1 replicate of each combination being assigned randomly to a location within the field. Thus, there were 2 replicates of each cattle class per forage treatment per year.  In year 1, pasture size was 15.8 ± 1.5 ac for cow/calf pairs and 8.9 ± 0.6 ac for growing steers. Pasture treatment assignments remained the same in years 1 (2023) and 2 (2024). However, due to flooding of low laying areas in year 2, pasture size was less, at an average of 12.1 ± 2.2 ac for cow/calf pairs and 8.2 ± 0.8 ac for growing steers. 

When drilling the sudangrass monoculture and sudangrass/sunnhemp mixture, the goal was to plant 25 lb of seed/ac. However, based on actual seed use, in year 1, sudangrass was drilled at 26 lb/ac and sudangrass/sunnhemp (50:50 mix by weight) was drilled at 29 lb/ac on May 26th, 30th, and 31st. This was a 12% increase in seeding rate for the mixture compared to the monoculture. In year 2, the sudangrass monoculture was drilled 22 lb/ac while the sudangrass/sunnhemp mixture was drilled at 31 lb/ac which is a 40% increase compared to the monoculture seeding rate. Seeding took place on June 5th, 6th, and 7th in year 2.  Fertilizer was applied to all pastures regardless of treatment. In year 1, soil nitrogen tests at planting revealed an average of 40 lb/ac of residual available nitrogen in the soil after the previous cereal rye crop. Therefore, there was no nitrogen applied prior to the start of grazing (July 13, 2023). Nitrogen was applied in the form of urea (46, 0, 0) at a target of 40 lb/ac on August 25, 2023. In year 2, soil tests at planting showed a residual nitrogen balance of 23.5 lb/ac. Nitrogen was applied as urea (46, 0, 0) at 50 lb/ac on July 8, 2024, two days prior to the start of grazing. Nitrogen was again applied on August 25th at the rate of 40 lb/ac using urea ammonium nitrate solution (32, 0, 0). Total nitrogen applied in both years was intentionally conservative (below the typical 100–150 lb N/ac recommendation for summer annuals) to allow sunnhemp, a legume, better opportunity to express any potential benefits.

Animal Management

Growing steers. In both years, growing steers were stocked heavier at the beginning of grazing and then stocking rates were reduced once forage growth slowed. Prior to experiment initiation, steers were limit fed a diet of 50% Sweet Bran®, and 50% alfalfa hay (DM basis) for 5 days at 2% of their body weight (BW) on a DM basis to equalize gut fill. To establish initial weight, steers were weighed for 3 days consecutively (d -1, 0, and d 1). On d 1 of both years, all growing steers were implanted with a Revalor-G®. Steers were then stratified by weight and assigned randomly to treatment groups. 

In year 1, a total of 80 growing steers (864 ± 2.6 lb BW average) were divided between 4 pastures for an initial stocking rate of 2.2 ± 0.1 steers/ac (1,970 ± 70 lb BW/ac). Grazing was initiated on July 13, 2023. On July 26, 2023 (d 13 of grazing) 27 steers were pulled off to reduce the stocking rate to 1.5 steers/ac. On September 8th when grazing ended, groups contained 14, 14, 13 and 12 steers, resulting in an ending stocking rate of 1.5 ± 0.1 steer/ac (1,305 ± 30 lb BW/ac). The weighted average stocking rates over the grazing season was 1.6 ± 0.1 steers/ac (1,445 ± 35 lb BW/ac).

In year 2, 53 steers were used (884 ± 33 lb BW). Initial stocking rate was 1.7 ± 0.2 steers/ac (1,470 ± 102 lb BW/ac). After 37 days of grazing, forage was limited. This resulted in the removal of all steers for 7 (1 group) or 10 days (3 groups) and during which time they were limit fed.  Following the limit feeding period, stocking rate was reduced to an average of 1.5 ± 0.1 steers/ac (1,326 ± 32 lb BW/ac). The weighted average stocking rate over the entire grazing season was 1.6 ± 0.1 steers/ac (1,432 ± 73 lb BW/ac).

Following grazing termination, steers were limit fed for 5 days. Steers were then weighed for 3 consecutive days. Estimated gains during the limit feeding periods were subtracted from the ending BW in all years. Additionally, in year 2, estimated gains during the 7- or 10-day limit feeding period were subtracted from ending BW. Gain estimations during these periods were based on the NASEM (2016) model using the nutritive values and intakes of the hay and Sweet Bran mix. Model predictions for the final limit feeding period were 2.5 lb/d in year 1 and 2.9 lb/d in year 2. Additionally for year 2, during the 7 to 10-day pull-off period, gains were predicted at 2.7 lb/d. 

To account for the difference in seed cost between the forage types the cost of gain of steers grazing either MIX or MONO treatments, was conducted using a partial budget analysis. Seeding costs used were the actual costs of purchasing the seed. The MIX treatment cost $1.55 per lb of seed while the MONO treatment cost $1.27 per lb of seed. Differences in seeding rate between MIX and MONO treatments were unintentional and thus the actual seeding rates used each year were not used in the final presented analysis. Instead, the average seeding rate between years for both MIX and MONO treatments (27 lb/ac) was used. Therefore, the final seeding cost for establishment of the was $41.77 per ac for MIX and $34.40 per ac for MONO. This seed cost was divided by the total gain per ac to get the cost of gain ($/lb BW) for the steers.

Cow/calf pairs. In year 1, cows (Angus cross bred) were stratified by previous treatment, body condition score (BCS), cow age, calf age and calf sex and then assigned randomly to a group which was then assigned randomly to a pasture. Prior to study initiation cows were assigned a BCS (1 = emancipated and 9 = obese) by 2 trained technicians over a 2-day period based on visual assessment and physical palpation. The 4 scores were then averaged together for the BCS value reported. Cow and calf BW was determined using a 2-day average. The average initial cow BW was 1,440 ± 13 lb and average initial cow BCS was 5.5 ± 0.1. Bull calves were all banded at birth, so all nursing male calves were steers during the trial. 

In year 1, a total of 68 cow/calf pairs were used. At turn out on July 14, 2023, the average initial stocking rate was 1.1 ± 0.1 cows/ac (1,745 ± 113 lb BW/ac). On August 11, 2023 (d 30 of trial) forage was limited for all cow groups, thus the pairs were moved to smooth bromegrass perennial pastures where they grazed for 7 days after which grazing of the warm season annual pastures resumed. Grazing was terminated on September 6, 2023 (trial d 56). This resulted in an average final stocking rate of 0.8 ± 0.1 cow/ac (1,312 ± 134 lb BW/ac). Overall, the weighted average stocking rate over the entire grazing period was 1.0 pair/ac (1,625 ± 121 lb BW/ac). At grazing termination pairs were brought into dry lot pens and were shrunk for 16-hour. Single day weights were then collected on the nursing calves. The BCS of the cows were collected over a two day period using the same methodology as outlined above. 

The same weighing and BCS methods were used in year 2. In the second year, cows were stratified by previous treatment, BCS, cow age, and calf fetal age then assigned randomly to a treatment group. In year 2, 49 pairs were used. Cows weighed 1,519 ± 121 lb and had a BCS of 6.5 ± 0.1 at trial initiation. Initial cow stocking rate was less than year 1, at 1.0 ± 0.03 cow/ac (1,670 ± 54 lb BW/ac). On d 33 (August 12, 2024), forage biomass became limiting in both treatments in half the cow groups. Pairs were pulled from the annual fields and placed on perennial pasture (predominantly smooth bromegrass) for 14 days. Similarly, forage became limiting on d 37 of the trial (August 16, 2024) and the other half of cow groups were placed on similar perennial pasture for 10 days. On August 26, 2024 (d 47) all pair groups were returned to the annual fields and grazing resumed. Grazing continued until September 17, 2024 (d 69) when forage became limiting for all pairs and the trial ended. The final stocking rate was 0.9 ± 0.1 cows/ha (1,623 ± 114 lb BW/ac). The weighted average stocking rate over the entering grazing period was 1.0 cow/ha (1,623 ± 46 lb BW/ac). At the end of the grazing period, weights of one pair (monoculture) were not included in the final analysis due to chronic cow lameness. 

Breeding occurred during grazing. A single shot of prostaglandin F2α (Lutalyse) was utilized to synchronize estrus and was given following 5 days of bull exposure in each year. The breeding season lasted for 53 d in year 1 and 73 d in year 2. Due to low number of cows in the groups, breeding success was not evaluated in this experiment. However, in year 1 the average pregnancy rate was 81 ± 9% and in year 2 it was 95 ± 9%. 

Grazing Management

Each of the 8 pastures were divided into 4 paddocks which were used to implement rotational stocking. Each year the paddocks in each pasture were visually evaluated for the most forage mass and selected to be the first paddock grazed. The first paddock was designated as the sampling paddock in which pre and post grazing forage mass was collected each time cattle grazed the paddock. Forage mass was measured by clipping four randomly selected 5.3 ft2 areas to ground level. Samples were then dried to a constant weight at 140°F in a forced air oven and the simple average of the mass in the 4 areas was used to calculate forage mass. Twenty standing heights were also collected at the time of forage mass sampling.

In year 1, steers were turned out on d 1 of the trial on July 13, 2023 (42 to 47 d after planting). Cow/calf pairs were turned out on July 14, 2023 (d 2 of trial).  Groups were rotated based on visual estimates of forage availability in the paddock, with a target of 10 to 14 inch post grazing height. This resulted in steers being rotated among paddocks every 3.2 ± 0.3 d and pairs rotated every 2.7 ± 0.1 d. Forage height measured prior to grazing in the sampling paddocks was 27.6 ± 1.8 inch for pairs and 28.6 ± 1.6 inch for steers. The average post grazing height was 13.5 ± 2.2 inch for steers and 12.8 ± 1.4 inch for cow/calf pairs.

In year 2, steers were turned out for grazing 32 to 34 d after planting on July 10, 2024 (d 1 of the trial). Pairs were turned out on July 12, 2024 (d 3 of trial). Steers were rotated among paddocks every 4.4 ± 0.1 d and pairs were rotated every 4.2 ± 0.3 d. Forage height measured prior to grazing in the sampling paddocks during this time was 30.9 ± 2.2 inch for steers and 34.5 ± 3.5 inch for pairs. The average post grazing height was 17.4 ± 2.5 inch for steers and 21.0 ± 4.1 inch for cow/calf pairs. 

Statistical Analysis

Forage evaluation. For forage mass and carrying capacity, treatments were arranged in a randomized complete block with a 2 by 2 factorial design. Experimental unit was pasture (n = 16). Treatments included forage type (monoculture vs mixture; n = 8) and class of cattle (cows vs growing steers; n = 8). The field was considered a block with 4 pastures per block containing one replicate of each forage type and cattle class combination per year. Data were analyzed using the MIXED procedure of SAS. All models included the fixed effects of forage type, year, class of cattle and their interactions. Block was considered random. Pre grazing mass was analyzed as a repeated measures. 

Animal performance. Treatments were arranged in a randomized complete block design. All data were analyzed using the MIXED procedure of SAS except nursing calf sex. All animal performance models included forage type, year and their interactions as fixed effects. Calf age at trial initiation was used as a covariate for nursing calf initial and final BW (P <0.01) but was not included in average daily gain (ADG) as it was not significant (P = 0.29). For nursing calf BW, ADG and day of age (DOA) at trial initiation, calf sex, dam age and their interactions were also included as fixed effects. Nursing calf sex was analyzed using the GLIMMIX procedure of SAS with binomial distribution of the data. 

For all models, interactions were dropped from the model when P > 0.20. Means were separated using the pdiff statement when the F-test was significant. Treatment differences were considered significant when P ≤ 0.05 and considered a tendency between P > 0.05 and P ≤ 0.10.

Results

Forage Measures

For pre-graze forage mass over the grazing season, there were no significant interactions with forage type (P > 0.28). There was a significant effect of forage type (P ≤ 0.01) on pre-graze forage mass with mass of MONO pastures being 15% greater (P ≤ 0.01) compared to MIX pastures (Table 1). However, the overall carrying capacity (AUM/ac) achieved was not affected (P = 0.58) by forage type (Table 1). 

Animal Performance

For all cow/calf performance measures there were no forage type by year interactions (P ≥ 0.29).  

Initial cow BCS was not different (P = 0.26) between forage type (Table 2). Cow final BCS tended to be greater (P = 0.10) for cows grazing mixtures compared to cows grazing monocultures. This is likely not a biologically relevant change and likely has more to do with the relative differences between the two treatment groups at trial initiation. The numerical difference between the initial BCS of cows grazing the mixture was 0.09 units compared to the monoculture. For the final BCS, the mixture was numerically 0.11 units greater compared to the monoculture. Therefore, it would be reasonable to assume that the differences that occurred in BCS were primarily the result of initial BCS. This is further emphasized when looking at change in BCS. Change in BCS was not different (P = 0.84) between forage type. This indicates that BCS of dams were not affected by the addition of sunnhemp into sudangrass pastures during the summer growing season. 

For all BCS metrics there was a significant effect of year (P < 0.01; Table 3). Initial cow BCS was 1.06 ± 0.08 units lesser in year 1 compared to year 2. Final BCS was 1.31 ± 0.07 units lesser in year 1 compared to year 2.  In year 1, cows lost 0.07 ± 0.06 units of BCS whereas in year 2 cows gained 0.19 ± 0.06 units of BCS. The difference in cow BCS in years 1 and 2 was likely due to a difference in type of forage grazed prior to treatment initiation. In year 1, pairs received hay before grazing smooth bromegrass beginning in mid-May. In year 2, pairs grazed cereal rye in April and May before switching to smooth bromegrass in June. 

At trial initiation, calf age was not different between forage types (P = 0.27; Table 2), with calves averaging 74 days of age at turn out. However, calf age was different (P < 0.01) between years with calves being slightly older (P < 0.01) in year 1 than year 2 (Table 3). Nursing calf initial BW, final BW and ADG were not different (P ≥ 0.45) between forage type (Table 2). There was an effect of year (Table 3) with nursing calves having lesser (P < 0.01) ADG in year 1 than in year 2. Overall, for cow/calf pairs, the inclusion of sunnhemp in sudangrass stands will increase forage establishment costs without changing animal performance. 

Growing steer performance. There were no forage type by year interactions (P ≥ 0.28) for the growing steer performance measures of initial BW, final BW or ADG. Steer BW was not different at either trial initiation or termination due to forage type (P ≥ 0.18; Table 4) or year (P ≥ 0.47). However, ADG tended (P = 0.09) to be greater for steers grazing the MIX (1.74 lb/d) than the MONO (1.56 lb/d). There was no effect (P = 0.17) of year on ADG of stocker steers. For total gain per acre of the growing steers, a forage type by year interaction (P = 0.05) was observed. There was no difference (P = 0.69) in the gain per acre achieved between MIX (175 ± 12 lb/ac) and MONO (173 ± 12 lb/ac) in year 1. However, in year 2, MIX (140 ± 12 lb/ac) gained an additional (P = 0.02) 23 lb/ac compared to MONO (117 ± 12 lb/ac) treatments. 

To account for the difference in seed costs and gain per acre observed, a partial budget analysis of cost of gain was evaluated. There was a significant (P = 0.03) interaction of forage type and year for cost of gain. Cost of gain was $0.03/lb greater (P = 0.01) when grazing MIX ($0.21 ± 0.017 /lb) compared to MONO ($0.18 ± 0.017 /lb) in year 1. However, in year 2 cost of gain was not different (P = 0.50) between steers grazing MIX and MONO at $0.27 and $0.26 ± 0.017/lb, respectively. For growing steers, the inclusion of sunnhemp in sudangrass may increase animal performance.

Conclusion

Sudangrass provided high-quality summer forage that supported acceptable gains in both cow/calf pairs and growing steers. Including sunnhemp in the mixture did not increase carrying capacity or improve cow/calf performance. While sunnhemp inclusion slightly improved stocker average daily gain, the increased seed cost was not offset by improved performance. These findings suggest that while sudangrass is a valuable summer forage option, the addition of sunnhemp does not offer a performance or economic benefit in either cow/calf or stocker systems.

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