title: A comparison of nitrogen utilization and urea metabolism between Tibetan and fine-wool sheep.
abstract: To study metabolic adaptation to harsh foraging conditions, an experiment was conducted to characterize and quantify N utilization efficiency and urea metabolism in Tibetan and fine-wool sheep fed 4 levels of dietary N (11.0, 16.7, 23.1, and 29.2 g N/kg DM) in 2 concurrent 4 × 4 Latin square designs. Urea kinetics were determined using continuous intrajugular infusions of 15N15N-urea. Urinary excretions of total N and urea N increased linearly (P < 0.001) with dietary N and were not different between breeds (P ≥ 0.37). Fecal N excretion increased with dietary N for Tibetan sheep but not for fine-wool sheep (linear dietary N × breed; P < 0.05). Nitrogen retention (both amount per day and percentage of N intake) increased with increasing dietary N concentration (P < 0.001), and the rates of increase were greater in fine-wool than in Tibetan sheep (linear dietary N × breed and cubic dietary N × breed; P < 0.05). In Tibetan sheep, N retention as a percentage of intake was greatest for diets containing 16.7 g N/kg DM, whereas it was maximal for fine-wool sheep when the diet contained 23.1 g N/kg DM. Urea N entry rate, urea N recycled to the gastrointestinal tract (GIT), and urea N returned to the ornithine cycle all increased with dietary N (P < 0.05), and all were greater in Tibetan than fine-wool sheep for the 11.0 g N/kg DM diet but were greater in fine-wool than Tibetan sheep for the diet with 29.2 g N/kg DM (linear dietary N × breed; P < 0.05). Urea N excreted in feces, both amount and fraction of GIT entry rate, was less in Tibetan than fine-wool sheep for the 11.0 and 16.7 g N/kg DM diets but similar for diets with 23.1 or 29.2 g N/kg DM (linear dietary N × breed; P < 0.01). For the lowest-protein diet, the fraction of urea N production recycled to the GIT was greater in the Tibetan than fine-wool sheep (88% vs. 82%), but for the diet with 29.2 g N/kg DM it was greater for fine-wool than Tibetan sheep (46% vs. 39%; linear dietary N × breed; P < 0.05). Plasma urea N increased more rapidly in response to increasing dietary N concentration for fine-wool sheep than for Tibetan sheep (linear dietary N × breed; P < 0.05). Urea tubular load and the amount and percentage of urea reabsorbed by the kidney were greater in Tibetan than fine-wool sheep (P < 0.05). These results suggest that Tibetan sheep have mechanisms that allow them to utilize N more efficiently than the fine-wool sheep when dietary N is inadequate.
authors.forename: J W
authors.initials: JW
authors.lastname: Zhou
authors.forename: J D
authors.initials: JD
authors.lastname: Mi
authors.forename: E C
authors.initials: EC
authors.lastname: Titgemeyer
authors.forename: X S
authors.initials: XS
authors.lastname: Guo
authors.forename: L M
authors.initials: LM
authors.lastname: Ding
authors.forename: H C
authors.initials: HC
authors.lastname: Wang
authors.forename: Q
authors.initials: Q
authors.lastname: Qiu
authors.forename: Z P
authors.initials: ZP
authors.lastname: Li
authors.forename: R J
authors.initials: RJ
authors.lastname: Long
mesh_terms: D000821:Animal Feed / Q000032:analysis*; D000822:Animal Husbandry; D000824:Animal Nutritional Physiological Phenomena; D000818:Animals; D001806:Blood Urea Nitrogen; D001947:Breeding; D004032:Diet / Q000662:veterinary; D004777:Environment; D005243:Feces; D009584:Nitrogen / Q000378:metabolism*; D012756:Sheep / Q000235:genetics* / Q000378:metabolism*; D018609:Tibet; D014508:Urea / Q000097:blood / Q000378:metabolism*; D014935:Wool
chemical_list: D014508:Urea; D009584:Nitrogen