Predictions of the nutrient composition of the diets of supplemented versus unsupplemented grazing beef cows based on near-infrared reflectance spectroscopy of feces.

Near-infrared reflectance spectroscopy of fecal samples from crossbred beef cows grazing native range forage was performed to determine whether supplemental feeding would alter the prediction of forage quality. In one trial, 2.27 kg.cow-1 x d-1 of supplemental feed with 20% CP and 3.3 Mcal of DE/kg had a detectable, but unimportant, effect on the predicted forage digestibility of OM, whereas the predicted forage content of CP was increased from 5.6 to 6.4% (P < .01). In a second trial at another location, supplemental feeding of isonitrogenous (700 g.cow-1 x feeding-1) feeds that provided low, medium, or high levels of DE three times weekly caused detectable, but unimportant, changes in the predicted digestibility of forage OM, whereas important changes were noted in the predicted CP content of grazed forages. Although forage quality could not be evaluated from spectra developed with unsupplemented cows, a change in the plane of nutrition was detectable. In the first trial, apparent effects of supplemental feeding on predicted diet quality were not detected if fecal sampling occurred 36 or 56 h after the supplemental feeding ceased for CP and OM digestibility, respectively. Whether supplemental feeding altered the grazing behavior of cows and quality of forage grazed, or merely altered composition of fecal samples, was not determined.