title: Nitrogen metabolism and recycling in Holstein heifers.
abstract: To study the effect of dietary N level on urea kinetics and recycling, four Holstein heifers (267 +/- 3.6 kg) were used in a Youden square design. Isocaloric diets with a N content of 1.44, 1.89, 2.50, 2.97, and 3.40% were fed at approximately 1.8 times maintenance intake. Increasing the N content of the diet increased urinary N excretion (P < 0.001) and N balance (P < 0.01), but did not affect the fecal N excretion (P = 0.21). Increasing the level of dietary N, increased urea production (P < 0.001) and excretion (P < 0.001), but no effect (P = 0.24) could be detected in the amount of N recycled to the gut. Urea recycled with the saliva, however, increased (P < 0.001) both in absolute and relative terms, with increasing dietary N. No difference could be detected on the amount of recycled N that was used for anabolism or returned to the ornithine cycle, but less (P = 0.001) N originating from urea was excreted in feces as dietary N increased. Ruminal ammonia concentration increased (P < 0.001) with increasing N intake, but total tract neutral detergent fiber digestibility was depressed only on the lowest N intake diet. No difference (P = 0.30) was detected in ruminal microbial yield among diets, but more (P < 0.003) N was derived from blood urea at low N intakes, and the efficiency of use of the recycled N decreased (P < 0.001) with increasing levels of dietary N. Adaptive changes to low-N diets were a decrease (P < 0.003) in the renal clearance of urea and an increase (P < 0.001) in the gastrointestinal clearance of urea. Urea transporters were present in the rumen wall of the heifers and differentially expressed depending on dietary N content, but their role in the transfer of urea into the rumen remains uncertain. Different mechanisms of N salvage and recycling were involved when animals were fed low-N diets that ensured a supply of endogenous N to the gastrointestinal tract and, due to the reduced contribution of dietary N, an increased efficiency of the N recycled was observed.
authors.affiliation: Department of Animal Science, Cornell University, Ithaca, NY 14853, USA. jcm41@cornell.edu
authors.forename: J C
authors.initials: JC
authors.lastname: Marini
authors.forename: M E
authors.initials: ME
authors.lastname: Van Amburgh
mesh_terms: D000641:Ammonia / Q000378:metabolism; D000818:Animals; D001806:Blood Urea Nitrogen; D002352:Carrier Proteins / Q000302:isolation & purification* / Q000378:metabolism; D002417:Cattle / Q000378:metabolism*; D004043:Dietary Fiber / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D005243:Feces / Q000737:chemistry; D005260:Female; D007700:Kinetics; D008562:Membrane Glycoproteins / Q000302:isolation & purification* / Q000378:metabolism; D026901:Membrane Transport Proteins; D009584:Nitrogen / Q000008:administration & dosage / Q000378:metabolism* / Q000652:urine; D011897:Random Allocation; D012417:Rumen / Q000378:metabolism*; D014508:Urea / Q000378:metabolism*; D000096982:Urea Transporters
chemical_list: D000641:Ammonia; D002352:Carrier Proteins; D004043:Dietary Fiber; D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D009584:Nitrogen; D014508:Urea; D000096982:Urea Transporters