title: 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.
abstract: 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 (J(sm-urea)) was measured using (14)C-urea. Ruminal NH(3)-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 J(sm-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 J(sm-urea) by 19.5-22.3% (P = 0.001); however, phloretin-insensitive J(sm-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.
authors.affiliation: Department of Animal and Poultry Science, University of Saskatchewan, Saskatoon, Canada S7N 5A8.
authors.forename: Kiran
authors.initials: K
authors.lastname: Doranalli
authors.forename: Gregory B
authors.initials: GB
authors.lastname: Penner
authors.forename: Timothy
authors.initials: T
authors.lastname: Mutsvangwa
mesh_terms: D000818:Animals; D001692:Biological Transport; D004044:Dietary Proteins / Q000008:administration & dosage*; D006863:Hydrogen-Ion Concentration; D009584:Nitrogen / Q000378:metabolism*; D012417:Rumen / Q000378:metabolism*; D012756:Sheep; D014508:Urea / Q000378:metabolism*
chemical_list: D004044:Dietary Proteins; D014508:Urea; D009584:Nitrogen