The evaluation of dietary protein levels and the comparative application of the NASEM model for ration formulation in finishing Nellore cattle

Typically, true protein sources are the most expensive ingredients in feed formulations for feedlot cattle. However, low-protein diets can limit animal performance. This study aimed to use the mechanistic NASEM model to formulate diets with similar metabolizable protein levels while varying crude protein content and evaluating the impact of these adjustments on performance and nutrient digestibility in feedlot Nellore bulls. A total of 120 Nellore bulls (initial body weight = 386.18 ± 23 kg; 24 ± 1 months of age) were allocated to 15 pens (8 bulls/pen). A randomized complete block design was used, with bulls stratified into five blocks according to their initial body weight. Within each block, pens were randomly assigned to one of three dietary treatments differing in crude protein (CP) concentration: High CP (HCP), Medium CP (MCP), and Low CP (LCP), while maintaining the same metabolizable protein (MP) level across all diets. The CP and MP concentrations were as follows: HCP: 140 g CP/ kg dry matter (DM) and 86 g MP/kg DM; MCP: 125 g CP/kg DM and 86 g MP/kg DM; LCP: 110 g CP/kg DM and 86 g MP/kg DM. Final BW ( P < 0.001) and ADG ( P = 0.003) as well as dry matter intake (DMI) ( P < 0.001) increased linearly with higher dietary CP concentrations. Similarly, both hot and cold carcass weights ( P = 0.002) and carcass gain ( P < 0.001) increased linearly with increasing CP levels in the diets. Comparing the NASEM model predictions with observed data, the predicted DMI from the model was approximately 15% and 10% lower than the observed values in the 14% and 12.5% CP treatments, respectively. Additionally, the estimated DMI by NASEM was 3% higher than the observed intake in 11% of CP. In summary, increasing dietary CP from 11% to 14% by replacing starch with true protein increased DMI and improved the performance of feedlot Nellore bulls. The mechanistic NASEM model underestimated DMI in diets containing 12.5% and 14% CP, which led to lower predictions of MP, energy balance, and ADG in feedlot Nellore bulls.