Comparison of methods to estimate crude protein and digestible organic matter content of diets ingested by free-ranging sheep

Nutritional assessment of grazing ruminants, particularly on natural heterogeneous rangelands, remains a challenge for researchers and technicians. The aims of this study were: i) to compare methodologies of herbage sampling to determine crude protein (CP) and digestible organic matter (DOM) contents of ingested herbage and, ii) to evaluate the use of faecal near-infrared reflectance spectroscopy (fNIRS) to estimate those herbage attributes using either sampling procedure. Herbage samples were collected through hand-clipping (HC) or directly from rumen extrusa at the beginning (ExBM) and at the end (ExEM) of the longest daily sheep meal. In addition, DOM content of ingested herbage was also calculated from the amount of N excreted in faeces (FN). The study was conducted throughout thirteen consecutive months with six rumen-cannulated male sheep kept free on a rangeland plot in southern Brazil. Monthly, data on diet CP and DOM contents were collected by either method and related to near-infrared spectra of faeces collected from each respective sheep using the partial least squares method. Crude protein content in herbage ingested by sheep ranged from 108 to 172 g/kg DM and was, on average, lower (P < 0.05) in ExEM than in either ExBM or HC samples. Digestible organic matter content of ingested herbage, estimated through rumen in situ incubations, ranged from 546 to 717 g/kg DM and was, on average, similar for ExBM, ExEM, and HC samples. However, when this variable was estimated through the FN approach, values ranged from 446 to 601 g/kg DM and were, on average, lower (P < 0.05) than those found through other methods. The fNIRS models based on data obtained through extrusa (EX) and HC procedures produced similar and reliable results for estimating CP of herbage ingested by sheep, whereas the FN approach yielded the best fNIRS model for estimating DOM content of herbage.