Feeding Oscillating Dietary Crude Protein Concentrations Increases Nitrogen Utilization in Growing Lambs and This Response Is Partly Attributable to Increased Urea Transfer to the Rumen

摘要

Our objective was to determine the effects of feeding oscillating compared with static dietary crude protein (CP) concentrations on nitrogen (N) retention and urea flux across ruminal epithelia. Twenty-seven Suffolk wether lambs (n = 9) were assigned to a medium-CP diet [MEDIUM; 127 g CP?kg dry matter (DM)?1] or to diets with oscillating CP content (OSC) fed in 2 different sequences, i.e. 2 d of low CP (103 g CP?kg DM?1) followed by 2 d of high CP (161 g CP?kg DM?1; OSC-HIGH) or vice versa (OSC-LOW). Diet adaptation was for 24 d, followed by 8 d of total urine and feces collection. On d 33, lambs were slaughtered 4 h after the morning feeding, such that those receiving OSC-LOW and OSC-HIGH diets were slaughtered on d 3 of receiving the low- or high-CP diets, respectively. Ruminal epithelia were collected and mounted in Ussing chambers and the serosal-to-mucosal urea flux (Jsm-urea) was measured using 14C-urea. Ruminal NH3-N concentration was lower (P = 0.001) in lambs fed OSC-LOW compared with those fed OSC-HIGH. Although N intake was similar, retained N (P = 0.001) and microbial N supply (P = 0.001) were greater in lambs fed OSC compared with those fed MEDIUM. The total Jsm-urea was higher (P = 0.001) in lambs fed OSC-LOW compared with those fed OSC-HIGH. Across diets, the addition of phloretin [a known specific inhibitor of facilitative urea transporter (UT)-B] reduced Jsm-urea by 19.5–22.3% (P = 0.001); however, phloretin-insensitive Jsm-urea was the predominant route for transepithelial urea transfer. Taken together, these data indicate that feeding oscillating dietary CP concentrations improves N retention partly by increasing urea recycling to the rumen when animals are fed low-CP diets, but the greater rates of urea transfer cannot be attributable to upregulation of UT-B.