Can a combined cattle and sheep model based on fecal crude protein estimate the feed intake and digestibility?

The present study aimed to evaluate a unified model using fecal crude protein (CP) as a marker to estimate feed intake and digestibility in both cattle and sheep. The experiment involved five experimental periods with cattle fed with fresh Italian ryegrass ( Lolium multiflorum Lam.) in a two-year metabolism trial, evaluating three forage allowances: 15 and 22.5 g DM/kg body weight or ad libitum . Measured variables involved organic matter digestibility (OMD), organic matter intake (OMI), dietary crude protein (dCP), total fecal output, and fecal CP. In addition, a sheep dataset (n = 107) of Italian ryegrass intake at varying phenological stages was analyzed. Regression models (hyperbolic for OMD, linear for OMI, and dCP) were developed separately for each species and combined across species. Predicted values were compared with observed data using parallelism tests and relative predictive error (RPE). The combined cattle-sheep models for OMD, OMI, and dCP, showed R 2 of 0.72, 0.74, and 0.69 and RPEs of 6.4 %, 14.5 %, and 15.2 %, respectively, with no significant species effect. The fecal CP can effectively predict OMD, OMI, and dCP in both species grazing Italian ryegrass. Thus, the species-independent combined model provided reliable estimates. • Fecal crude protein (CP) can be used as a nutritional marker in sheep and cattle • Data from experiments with cattle and sheep feed Italian ryegrass were evaluated • A single mixed model based on fecal CP was tested to estimate digestibility and intake • A single mixed model can estimate the digestibility and intake in cattle and sheep