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.