Tissue protein and energy deposition in steers fed isocaloric diets with different levels of nitrogen

Metabolism and comparative slaughter feedlot trials were conducted with 85 Angus-Hereford crossbred steers to evaluate effects of isocaloric diets with five levels of N on protein and energy deposition in tissues. The diets had Urea Fermentation Potentials (UFP) of 3.8, 1.2, -1.4, -3.9 and -6.9 and metabolizable protein (MP) levels of 69.2, 76.6, 80.1, 80.8 and 80.0 g/kg dry matter, respectively, with increasing dietary N levels. Crude protein digestibility increased (P less than .005) with increasing N levels, but total digestible nutrients (TDN), digestible energy (DE) and metabolizable energy (ME) showed no significant relationship to variation in N level. Metabolism trial N and energy utilization indicates that N to energy balance occurred at zero to slightly negative UFP. In the feedlot trial, average daily gain, ME intake, dry matter intake and energy gain increased with increasing dietary MP level. Daily gains in empty body fat and energy and carcass characteristics indicating increased finish also increased with increasing MP level. However, empty body protein gain increased with increased N level and indicated that in isocaloric diets, urea-N could replace a portion of the plant protein without decreasing tissue protein gains. The results support the UFP system for determining N to energy balance in feedlot diet.