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

Zhou, J.W., Guo, Y.M., Kang, J.P., Degen, A.A., Titgemeyer, E.C., Jing, X.P., Wang, W.J., Shang, Z.H., Li, Z.P., Yang, G., Long, R.J.
Animal feed science and technology 2019 v.248 pp. 85-94
absorption, crude protein, energy intake, excretion, feedlots, gastrointestinal system, grazing, low calorie diet, low protein diet, metabolizable energy, pastures, sheep, summer, urea, China
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 15N15N-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/kg0.75, whereas Han sheep required 0.33 MJ/kg0.75 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.