Apparent digestibility, rumen fermentation, digestive enzymes and urinary purine derivatives in yaks and Qaidam cattle offered forage-concentrate diets differing in nitrogen concentration

Abstract Yaks ( Bos grunniens ) and Qaidam yellow cattle ( Bos taurus ) are indigenous to the Qinghai-Tibetan Plateau and graze natural pasture all year. Yaks are raised at higher elevations than cattle and are not offered supplementary feed whereas cattle receive supplementary feed and are sheltered at night during winters. We hypothesized that the species differ in metabolism of energy and nitrogen (N) and predicted that the differences would favour yaks to the harsh conditions of the plateau. To test our prediction, we examined nutrient digestibility, rumen fermentation, digestive enzymes and urinary purine derivatives (PD) in yaks and cattle offered forage-concentrate diets differing in N concentration. Four castrated yaks and 4 castrated cattle of similar ages (2.5 years) and body weights (200 kg) were used in two concurrent 4×4 Latin square designs. There was no difference ( P > 0.05) between yaks and cattle in apparent digestibilities of dietary nutrients, concentrations of ruminal N components and activities of digestive enzymes ( P > 0.05). Crude protein and acid detergent fiber digestibilities increased linearly ( P P P P > 0.05), and both variables were not affected by dietary N content ( P > 0.05). As dietary N increased, ruminal concentrations of ammonia, urea and amino acids increased linearly (all P P P > 0.10) for both species. The activity of carboxymethylcellulase decreased linearly ( P P P > 0.10); however, the PD nitrogen index (PNI) was greater in yaks than in cattle for the lowest N diets (linear dietary N × species, P P P P P