Urea kinetics and nitrogen balance and requirements for maintenance in Tibetan sheep when fed oat hay

J. W. Zhou, X. S. Guo, A. A. Degen, Y. Zhang, H. Liu, J. D. Mi, L. M. Ding, H. C. Wang, Q. Qiu, R. J. Long

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Abstract

Tibetan sheep inhabit the Qinghai-Tibetan Plateau, an area characterized by sparse vegetation of low protein content much of the year. Consequently, we predicted that their N requirements for maintenance would be low. To test our prediction, we determined urea kinetics, N balance and requirements for maintenance in five growing wethers (43.0±2.3kg live weight). A 4×4 Latin square design was used with 4 levels of oat hay intakes (0.3, 0.5, 0.7, and 0.9×voluntary intake), in which each period lasted for 21 days. Urea kinetics was determined using a continuous intrajugular infusion of 15N15N-urea. There was a linear increase in faecal N excretion, urinary N elimination and N retention (P<0.01) with an increase in feed intake. From the regression equation generated between daily N retention and daily N intake, the estimated N requirements for maintenance were 0.50g/kg BW0.75 per day, that is, only 66% of the amount recommended by NRC for growing sheep of its size. However, it should be noted that the NRC values were for sheep without marked feed restriction. Urea-N entry rate (UER), gastrointestinal tract (GIT) entry rate (GER), return to ornithine cycle (ROC) and faecal urea-N excretion all increased linearly (P<0.01) with an increase in DMI. The ratio of UER to apparent digestible N intake increased linearly from 1.53 to 2.99 with a decrease in feed intake. The proportion of UER that entered the gut did not differ (P>0.05) among intakes and ranged between 0.52 and 0.61. GER used for anabolism decreased (P<0.01) from 0.45 to 0.28, whereas GER to ROC increased linearly (P<0.01) from 0.51 to 0.68 with increasing DMI. Glomerular filtration rate increased (P<0.05) with increasing DMI, but urinary creatinine excretion and plasma urea-N concentrations remained constant (P>0.05). The proportion of renal urea-N reabsorbed increased linearly with a decrease in DMI (P<0.05) while GIT urea-N clearance was always higher than kidney urea-N clearance. The low N requirements demonstrated by the Tibetan sheep supported our prediction. The increase in renal urea-N absorption rate with a decrease in nitrogen and feed intake, the greater ratios of UER to apparent digestible N intake and the greater GIT urea-N clearance to kidney urea-N clearance, regardless of N intake explain, at least in part, how Tibetan sheep cope with low nitrogen intake.

Original languageEnglish
Pages (from-to)60-68
Number of pages9
JournalSmall Ruminant Research
Volume129
DOIs
StatePublished - 1 Aug 2015

Keywords

  • Feeding level
  • Nitrogen balance
  • Nitrogen requirements
  • Tibetan sheep
  • Urea kinetics

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