Transforming nutritional monitoring of free-grazing herbivores: four decades of progress in fecal near-infrared reflectance spectroscopy

Fecal near-infrared reflectance spectroscopy (fecal-NIRS) has been used for more than four decades to estimate the nutritional characteristics of herbivore diets, including crude protein, digestibility, and feed intake. Despite its widespread use, a comprehensive synthesis of advances in this technology remains lacking. This review summarizes the development and application of fecal-NIRS over the past 40 years and evaluates its utility for monitoring forage diet quality in free-grazing herbivores. Over this period, advances in instrumentation, calibration methodologies, spectral preprocessing, and data analytics have expanded the applicability of fecal-NIRS across diverse herbivore species and production systems. Across studies, fecal-NIRS consistently provided accurate predictions of dietary CP (R2 = 0.85–0.99). Prediction accuracy for digestibility (R2 = 0.60-0.93) and intake (R2 = 0.41–0.89) was more variable, reflecting differences in species, diets, and calibration datasets. Although lower-accuracy models (with low R2) may be insufficient for precise quantification, they can still provide useful insights into nutritional trends and identify periods of nutritional deficiency or improvement under grazing conditions. Prediction performance was influenced by diet composition, species differences, environmental conditions, and methodological approaches. Overall, fecal-NIRS has transformed from an experimental technique into a reliable, cost-effective, and noninvasive technology for assessing diet quality in free-grazing herbivores. Continued advances in calibration development, portable spectrometers, and data analytics are expected to further expand their role in precision nutritional management, livestock production, and wildlife nutrition research.