Species richness and functional diversity enhance winter annual forage productivity and nutritive value

Abstract

Winter annual forages can extend grazing into the cool-season in the US southeast, however uncertainty regarding potential tradeoffs between winter annual mixtures versus monocultures exists. We examined increasingly species-rich and functionally diverse winter annual forage mixtures at two Florida locations to evaluate impacts of including more plant species with distinct traits (e.g., different maturities and nitrogen [N] scavenging vs. N₂-fixing capabilities) on forage accumulation (FA), nutritive value, and evenness of forage distribution across the cool-season compared with a rye (Secale cereale L.) monoculture. Furthermore, we quantified the N₂-fixing potential of winter annual legumes to measure their contribution to soil fertility and ecosystem N cycling. We found that greater species richness enhanced total cool-season FA at both sites, ranging from 0.92 to 4.38 Mg DM ha⁻¹ in Gainesville and 6.48 to 9.11 Mg DM ha⁻¹ in Marianna. In Marianna, where legumes accounted for approximately 25% or more of the biomass accumulated by winter annual forage mixtures, legume inclusion enhanced forage shoot N concentration. Furthermore, >50% of the N in legume tissues derived from the atmosphere, supplying 0.18–2 kg N ha⁻¹ in Gainesville and 30–52 kg N ha⁻¹ in Marianna. Altogether, our data suggest that species richness enhanced winter annual forage production and legume inclusion improved forage nutritive value although these responses were site-specific. Thus, our multi-site study provides information that can guide decision-making regarding appropriate winter annual forage selections to increase cool-season forage production in the southeastern United States.