Plant Community Structure and Nitrogen Dynamics Underlie Sustained Productivity of Grass-Legume Forage Mixtures in Wyoming.

Grass-legume mixtures enhance forage productivity and nutritive value. These benefits diminish due to poor legume persistence. An experiment was conducted during 2015 to 2017 at Sheridan Research and Extension Center in Wyoming to determine biodiversity and nitrogen (N) use indices influencing yield and nutritive value of grass-legume mixtures. Treatments included alfalfa (Medicago sativa L.), sainfoin (Onobrychis viciifolia Scop.), and birdsfoot trefoil (Lotus corniculatus L.) monocultures or mixtures with meadow bromegrass (Bromus riparius Rehm.). There were 50-50% and 70-30% mixtures of meadow bromegrass with each legume, 50-25-25% mixture of meadow bromegrass with two legumes and 50-16.7-16.7-16.7% mixture of meadow bromegrass with all legumes. Grass monocultures received 0, 56, and 112 kg N ha^-1. Dry matter (DM) was measured three times annually to determine overyielding, species evenness, temporal stability, and synchrony. Crude protein (CP), acid detergent fiber (ADF), neutral detergent fiber (NDF), in vitro dry matter digestibility (IVDMD), and relative feed value (RFV) were measured for nutritive value and N nutrition index (NNI) was calculated for grass. The 50-50% mixture of meadow bromegrass with alfalfa and 50-25-25% mixture of meadow bromegrass, alfalfa, and birdsfoot trefoil had highest three-year total yields of 43 Mg ha^-1. Mixtures improved from underyielding in 2015 to transgressive overyielding in 2017. Overyielding was correlated with temporal stability (R = 0.5; p = 0.003) and NNI (R = 0.6; p = 0.01). Mixtures had higher CP (161 to 177 g kg^-1) compared to grass monoculture (129 to 144 g kg^-1). Crude protein (R² = 0.17 p = 0.04) and digestibility (R² = 0.2, p = 0.04) of different mixtures were maximum at moderate species biomass evenness of ~0.75. Tri-species and quaternary mixtures with alfalfa had desynchronized species growth and improved crop N status. This offset the effects of adverse weather on legume persistence, yield, and nutritive value of mixtures.