Investigating the effects of sex of growing Nellore cattle and crude protein intake on the utilization of recycled N for microbial protein synthesis in the rumen by using intravenous 15N15N-urea infusion

The aim of this study was to investigate the effect of sex and levels of dietary crude protein (CP) on the recovery of 15N in microbial protein as well as to evaluate nitrogen balance compounds and variables related to urea renal handling in Nellore cattle. Four Nellore bulls and four Nellore heifers, fitted with rumen cannulas, were distributed in a two 4 × 4 Latin square design. The four experimental diets differed in levels of CP: 90.0, 110.0, 130.0, and 150.0 g/kg on a dry matter basis. Continuous intravenous infusion of 15N15N-urea solution was provided at a rate of 0.30 mmol urea/h followed by determination of 15N in ruminal bacteria, ammonia (NH3), and feces to determine the incorporation of recycled N in rumen bacteria. The increased dietary CP revealed a linear increase (P < 0.05) in total digestible nutrients (TDN), total digestibility of CP and neutral detergent fiber (NDF), retained nitrogen, urinary excretion of urea-N, amount of filtered and absorbed urea (g/day), and plasmatic concentrations of urea-N. Fecal excretion of N presented a linear increase (P < 0.05) in relation to dietary CP. Rumen NH3 concentration increased in a linear fashion (P < 0.05), and rumen 15N-NH3 contents and fecal 15N contents decreased in a linear fashion (P < 0.05) with increasing dietary CP. The regressions NH3 = 23.6183 × e(0.0104×NI) and 15N-NH3 = −0.00062 × NI + 0.1219 were obtained in order to calculate rumen NH3 concentrations (mg/l) and 15N-NH3 (atom percent excess), respectively, as a function of N intake (NI, g/day). The contents of total-N and 15N did not differ between liquid-associated bacteria (LAB) and particle-associated bacteria (PAB). There was no effect of sex (P > 0.05) on the percentage of infused 15N that was recovered from feces and bacteria. There was no effect of dietary CP level on the percentage of 15N infused in plasma that was recovered from feces. However, there was a decreasing linear effect of the level of dietary CP (P < 0.05) on the percentage of 15N infused in the jugular vein recovered from bacteria. We concluded that microbial protein synthesis was not affected by sex. The LAB and PAB had the same total-N content and recovery of infused 15N-urea. The greater recovery of infused intravenous 15N-urea on microbial protein synthesis in a low CP diet suggests greater urea recycling in association with this diet.