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

- Tibetan sheep were offered oat hay at 0.3-0.9 times voluntary intake.
- Estimated N requirements for maintenance were 0.50 g/kg BW0.75 per day.
- This is 66% of the amount of N recommended by NRC for growing sheep of their size.
- It was concluded that Tibetan sheep can cope with low N intake.

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 ± 2.3 kg 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 0.50 g/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.