Effects of level of feed intake and season on digestibility of dietary components, efficiency of microbial protein synthesis, rumen fermentation and ruminal microbiota in yaks

Yaks (Poephagus grunniens) are raised under the harsh conditions of the Qinghai-Tibetan Plateau (QTP) and, traditionally, they graze only natural grasslands all year round without receiving any supplements. The pasture peaks in biomass and quality during the summer when yaks usually gain body weight; however, it is sparse and of poor quality in winter when yaks generally lose body weight. Yaks are well adapted to the extreme conditions and are able to cope with low energy and nitrogen intakes. We hypothesized that the level of feed intake would affect the digestibility of nutrients and the efficiency of microbial protein synthesis (EMPS, as g microbial N/kg organic matter intake) and predicted that at very low dietary intakes in yaks, dietary components would be digested at a higher rate but that there would be little, if any effect on EMPS. We also predicted that the EMPS in yaks would be higher than values reported for cattle. To test these predictions, we offered six yaks a dry matter intake of 0.010 to 0.011 BW (low intake: LI) and six yaks a dry matter intake of 0.020 to 0.021 BW (high intake: HI) both in summer and in winter. Apparent digestibilities of dry matter (DM, P = 0.04), organic matter (OM, P = 0.048) and crude protein (CP, P = 0.04) were greater in the LI yaks than in the HI yaks in winter. The HI group was in positive N balance in summer and in winter, whereas the LI group was in positive N balance in summer, but not in winter. The total volatile fatty acids (TVFA) tended or was greater (P = 0.09 in summer; 0.053 in winter) and estimated microbial N yield was greater (P < 0.001) in the HI than in the LI yaks in both seasons. The EMPS did not differ between seasons (P = 0.51) or between feed levels (P = 0.26) and, in general, was higher in yaks than in cattle. The proportion of Fibrobacter succinogenes, a major fibrolytic ruminal bacterium, was significantly greater in LI than in HI yaks (P =0.007). We concluded that in yaks: 1) the apparent digestibilities of the dietary components increase at low level dietary intakes in winter when the forage is of poor quality; 2) EMPS does not differ between feed levels and is generally higher in yaks than for reported values in cattle; and 3) the higher digestion of NDF and ADF in the LI yaks than in the HI yaks in winter when the pasture is of poor quality, was due, at least in part, to a greater proportion of ruminal Fibrobacter succinogenes bacterium.