Utilizing summer annual grass-legume forage mixtures has the potential to improve forage yield and nutritive characteristics, and/or animal performance during times when cool-season pasture growth is limited by high temperatures. Legumes can utilize atmospheric nitrogen, which can increase crude protein and forage digestibility in mixtures. As nitrogen application generally improves both the yield and nutritive characteristics of summer annual forages, but can have a negative effect on legume competitiveness, nitrogen fertilizer recommendations for legume-containing summer annual mixtures are not well established.
Two experiments were conducted to determine the feasibility of utilizing summer annual mixtures in Kentucky, USA. The first experiment was a small plot study. The objective was to evaluate the effects of increasing botanical diversity and N application rates on the yield, botanical composition, and nutritive characteristics of summer annual forage mixtures. The second experiment was a grazing study that evaluated the effects of increasing summer annual species diversity on forage yield and nutritive value, and animal performance.
In the first experiment, N rates of 0, 56, 112, 168, and 224 kg N ha-1 were applied to a sudangrass monoculture, a three-species mixture, and an 11-species mixture. Sward biomass in three out of four environments increased as N application increased (average of 14 kg DM ha-1 per kg N ha-1; p \u3c 0.05). As all treatments were dominated by grass species, mixture complexity had no effect on forage DM accumulation for three out of four environments (4000, 5830, and 7280 kg DM ha-1 averaged over N rates for three environments; p \u3e 0.05). Swards were dominated by sudangrass and pearl millet (73 and 24% in simple mixtures, and 62 and 22% in complex mixtures, respectively), resulting in low functional diversity, likely due to high grass seeding rates. Mixture complexity also did not affect most nutritive characteristics (p \u3e 0.05). Although N application up to 224 kg N ha-1 often had a positive impact on forage quality parameters, forages in three out of four environments would not support the nutritional demands of growing or lactating cattle when averaged across harvests. A sensitivity analyses showed that applying N resulted in positive net returns only when hay prices were very high and N prices were low. When pasture utilization rates and hay feeding/storage losses are accounted for, enterprise budgets determined grazing to have 10% greater expenses than haying.
In the second experiment, yearling angus-cross beef calves were assigned to graze one of three summer annual forage treatments, a sorghum-sudangrass monoculture, a simple three-species mixture, or a complex 12-species mixture. Animals grazed for an average of 40 days per year without supplementation. Forage yield was not different between treatments (P \u3e 0.85). Although several forage quality parameters were affected by mixture, none provided useful insight into differences observed in average daily gain (ADG). In 2017 and 2019, calves grazing the monoculture and simple mixture had higher ADG than calves grazing the complex mixture (2017: 0.79 vs. 0.66 kg/day, P \u3c 0.03; 2019: 0.59 vs. 0.43 kg/day, P \u3c 0.03). In 2018, there were no differences in ADG (P \u3e 0.3); however, calves only gained 0.01 kg/day, possibly due to lower nutritive value of more mature forages. Forages in 2018 were abnormally tall and calves were observed to be flightier and more agitated. The added stress of a low-visibility environment may have contributed to poor gains. Taller forages may also have limited dry matter intake and/or sward utilization since calves could not reach the top of the plants.
In these studies, increasing species diversity did not improve forage yield, nutritive characteristics, or animal performance. This was likely due to heavy grass competition and poor legume establishment. If sward diversity is of interest, care must be taken to select compatible species, utilize appropriate seeding rates, and implement management that will promote less well-adapted species. Under the constraints of these experiments, utilizing summer annuals in forage systems in Kentucky would only be economical when hay costs were high, when production costs were low, and when animal performance was enhanced
