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

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 sequerces, i.e. 2 d of low CP (103 g CP.kg DM l) followed by 2 d of high CP (161 g CP.kg DM I; OSC-HIGH) or vice versa (OSC-LOW). Diet adaptation was for 24 d, followed by 8 d of total urine and feces collection. Ond 33, lambs were slaughtered 4 n after the morning feeding, such That those receiving OSC-LOW and OSC-HIGH diets were saughtered on a 3 of receving the low- or high-CP diets, respectively. Ruminal epithelia were collected and mounted in Ussing chambers and the serosal-to-mucosal urea flux (J(sin-urea)) was measured using C-14-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 = 0.011 were greater in lambs fed OSC compared with those fed MEDIUM. The total was J(sin-urea) (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 by J(sin-urea) 19.5-22.3% (P = 0.001); however, phloretin-insensitive was the predominant route for transepithelia. urea transfer. Taken together, these cam indicate that feeding oscillating dietary CP concentrations in:proves N retention partly by increasing urea recycling to the rumen when anima s are fed low-CP diets, out the greater rates of urea transfer cannot be attributaple to upregulation of UT-B. J. Nutr. 141: 560-567, 2011