Profiling Canopy Light Interception and Growth Forms to Predict Forage Yield and Nutritive Value for Meadow Bromegrass-Alfalfa Mixtures.

Alfalfa (Medicago sativa L.) changes light interception and growth forms in acclimation to shading by grass which can be less expensive estimators of forage yield and nutritive value unlike the Near Infrared Spectrophotometry. In a 2016-2017 study at the University of Wyoming Sheridan Research and Extension Center, intercepted photosynthetic photon flux density (PPFD), specific leaf area (SLA), and total leaf area (TLA) were evaluated at 5 cm interval heights (HT) to predict dry matter (DM) and nutritive value for meadow bromegrass (Bromus riparius Rehm.)-alfalfa crop mixtures. There were monocultures, 50:50, and 70:30 grass-legume seeding ratios (SR). The SR and HT interacted (p<0.0001) to influence DM, crude protein (CP), acid detergent fiber (ADF), and neutral detergent fiber (NDF). The DM for grass-alfalfa 70:30 mixture (0.96 to 0.51 Mg ha^-1) surpassed that for 50:50 formulation (0.8 to 0.23 Mg ha^-1) at 0 to 45 cm. Mixed crops entirely resembled in their CP (143 to 326 g kg^-1) but exceeded that for alfalfa monoculture (128 to 157 g kg^-1) up to 20 cm height. Mixed crops had perpetually similar ADF (410 to 214 g kg^-1) but lower values (p<0.05) than alfalfa monoculture (457 to 405 g kg^-1) up to 20 cm. Mixed crops had similar NDF (426 to 308 mg kg^-1) from 5 to 45 cm but lower values (p<0.05) than alfalfa monoculture (535 to 413 g kg^-1) up to 20 cm. Multiple linear regression revealed that alfalfa SR, HT, PPFD and TLA jointly explained 84% of variation in forage DM (R²=0.84, p<0.05) and 99% of CP (R²=0.99, p<0.05). The alfalfa SR, HT and TLA together contributed 74% of variation in NDF (R²=0.74, p<0.05) while SR, HT and PPFD represented 85% of change in NDF. The results demonstrate that alfalfa growth forms and light interception can reliably predict forage yield and nutritive value.