Tibetan sheep require less energy intake than small-tailed Han sheep for N balance when offered a low protein diet

Tibetan sheep are indigenous to the Qinghai-Tibetan Plateau and graze natural pasture all year round; small-tailed Han sheep were introduced to the plateau, are raised mainly in feedlots and graze natural pasture only in summer. A low energy intake could have a negative effect on N utilization and N balance in ruminants. We hypothesized that Tibetan sheep require less energy for N balance and utilize N more efficiently than Han sheep when offered a low protein and low energy diet. To test this hypothesis, we examined the effect of dietary energy on whole-body N retention and urea kinetics in Tibetan (47.7 ± 2.46 kg BW) and Han (46.2 ± 3.42 kg BW) sheep. Diets were formulated to be low in crude protein (˜7%) with different metabolizable energy levels (6.73, 7.65, 8.57 and 9.49 MJ/kg). Urea kinetics were determined using continuous intrajugular infusion of ¹⁵ N ¹⁵ N-urea for 72 h. N retention was greater (P < 0.01) in Tibetan than in Han sheep and increased linearly with increasing energy intake (P < 0.001). Tibetan sheep maintained a positive N balance on all dietary energy levels, whereas Han sheep were in a negative N balance at the lowest energy intake. To achieve a N balance of zero, Tibetan sheep required 0.28 MJ/kg ^0.75 , whereas Han sheep required 0.33 MJ/kg ^0.75 per day. Fecal N excretion was less (P < 0.05) for Tibetan than for Han sheep. Plasma urea concentrations tended to be greater in Tibetan than in Han sheep (P < 0.10), but urinary urea excretion was lower in Tibetan sheep at the lowest energy diet (linear dietary energy × breed; P < 0.05). As energy intake increased, urinary N decreased linearly (P < 0.001) and microbial N production increased linearly (P < 0.001). Urea entry rate and gastrointestinal tract entry rate were greater in Tibetan than in Han sheep (P < 0.01). Urinary purine derivatives excretion, microbial N production, and the portion of microbial N that originated from recycled urea were greater in Tibetan than in Han sheep (P < 0.01). Renal urea reabsorption rate was greater in Tibetan than in Han sheep (P < 0.05) and increased linearly with dietary energy level (P < 0.05). We concluded that: 1) Tibetan sheep were better able to cope with low N diets when energy intake was low; and 2) when N intakes are low, N utilization is improved by supplemental energy.