Seasonal and diurnal variation in forage nutritive value of cool-season grasses as predicted by near-infrared reflectance spectroscopy (NIRS)

A better understanding of seasonal and diurnal variation in forage nutritive value may help to maximize protein intake and digestibility for grazing livestock. Forage nutritive value was evaluated in four cool-season grasses common to Kentucky pastures: orchardgrass (Dactylis glomerata L.; OG), perennial ryegrass (Lolium perenne L. ssp. perenne; PRG), Kentucky bluegrass (Poa pratensis L.; KBG), and tall fescue [Schedonorus arundinaceus (Schreb.) Dumort.; TF]. In 2015 and 2017, 10 cool-season grass cultivars representing four species, planted in quadruplicate (40 plots) in central Kentucky, were maintained vegetatively with 2–4 week mowing and sampled in the morning and the afternoon at the same times every 2–4 weeks, from May to November. Near-infrared reflectance spectroscopy (NIRS) was used to predict dry matter (DM) and in vitro true dry matter digestibility (IVTDMD), as well as concentrations of acid detergent fiber (ADF), neutral detergent fiber (NDF), and crude protein (CP). On most harvest dates, ADF and NDF were higher in the morning than in the afternoon, and highest in KBG and TF species. Concentrations of CP did not have a consistent diurnal pattern across harvest dates. On most harvest dates, IVTDMD increased in the afternoon and was highest in PRG. The results could inform grazing management for weight gain in beef cattle or weight loss in overweight horses.