Effects of ruminally degradable dietary protein level on nitrogen metabolism in wethers

Abstract

The objective of this study was to investigate the effects of ruminally degradable dietary protein (RDP) level on N kinetics in growing goats. Nine growing wethers, weighed 17.4±2.4kg, were randomly assigned to three diets containing 0.53 (high RDP, HRDP), 0.47 (medium RDP, MRDP), or 0.43 (low RDP, LRDP) of RDP/CP. The ration consisted of 60% concentrate and 40% forage, offered in an equal portion to goats at 3-h intervals during the period of measuring nitrogen (N) kinetics. Single doses of 15N-urea and 15N-ammonium sulfate were injected into the jugular vein and rumen, respectively, to measure N flux in the body, and transfer of N between the body and ruminal bacterial pools. Nitrogen intake was about 12g/day, with no differences among treatments. The dietary RDP level did not affect (P>0.05) excretions of fecal N, urinary N and urinary urea-N, and N retention, although N retention tended to increase (P=0.11) with the decrement of dietary RDP levels. The ruminal ammonia N (RAN) concentration (P<0.001), RAN pool size (P<0.05) and the irreversible loss rate of RAN pool (P<0.05) decreased with the decrement of dietary RDP level. Dietary treatment had no effect (P>0.05) on flux and recycling rate of RAN pool. The plasma urea N (PUN) concentration (P<0.001), PUN pool size (P<0.001), total flux (P=0.02) and recycling rate (P<0.001) were all decreased with the decrement of dietary RDP level. Dietary RDP level affected the transfer of urea from plasma to rumen, and the amount (P=0.03) and percentage (P=0.10) of RAN derived from PUN were all decreased with the decrement of dietary RDP level. The current results indicate that there is an alternative way to improve N utilization efficiency through adjusting dietary protein composition in ruminants.