The objective was to propose and validate a bicompartimental sub model to estimate the apparently digestible crude protein fraction (CPad) in feed offered to cattle in tropical conditions. The two theoretical compartments used as basis for the sub model were crude protein from cell content (CPCC) and crude protein from cell wall (CPCW). Feed content of neutral detergent insoluble protein (NDIP) was used as an analytical approach for CPCW. Evaluations of true digestible fractions of CPCC and CPCW were based on nutritional entity concept and on potential availability from undegradable neutral detergent insoluble protein (UNDIP), respectively. The basic structure of the sub model was: CPad (% DM) = 0.98 × (CP - NDIP) + 0.835 × (NDIP - UNDIP) - 1.61, and CPad (% DM) = 0.98 × (CP - NDIP) + 0.67 × (NDIP - UNDIP) - 0.97 for growing and finishing cattle and lactating dairy cows, respectively. The validation of sub model was made from results of two trials on growing and finishing cattle (eight feeds). The sub model showed estimates similar to those obtained in vivo. Estimates of CPad were also obtained from a previously proposed unicompartimental sub model. Estimates obtained by the bicompartimental sub model were more accurate and precise as compared to unicompartimental sub model.
apparent digestibility; cell content protein; cell wall protein; total digestible nutrient
