Comparison of Rumen Fermentation, Bacterial Communities and Microbial Protein Synthesis Between Yaks and Qaidam Cattle Fattened in A Small-holder Feedlot

Yaks (<italic>Poephagus grunniens</italic>), native to the Qinghai-Tibetan Plateau, are well adapted to the harsh alpine environment. It was reported that yaks digest low-quality forage more efficiently and require less protein and energy for maintenance than Qaidam cattle (<italic>Bos taurus</italic>). Given the central role of rumen bacteria in nutrient degradation, we hypothesized that interspecies differences in ruminal bacterial composition may underlie the yak's adaptation to low-nutrient diets. To test this hypothesis, we compared feed conversion efficiency, rumen fermentation characteristics and bacterial communities in the two species fattened on a low concentrate diet in a small-holder feedlot. Six yaks (211 ± 6.0 kg) and 6 cattle (210 ± 7.0 kg), all castrated males, were offered a diet consisting of 2.0 kg/day of concentrate and <italic>ad libitum </italic>oat hay pellets for 105 days, which included 10 days for dietary adaptation and 95 days for data recording. Dry matter intake (<italic>P</italic> < 0.01) and feed conversion ratio (<italic>P</italic> < 0.05) were greater in cattle than in yaks. Urinary purine derivative excretion and microbial nitrogen production were greater (<italic>P</italic> < 0.05) in cattle than yaks, but the purine nitrogen index and microbial protein synthesis efficiency were greater (<italic>P</italic> < 0.01) in yaks. Additionally, the ruminal concentrations of ammonia-N and free amino nitrogen were greater (<italic>P</italic> < 0.05) in yaks than cattle. The relative abundances of the fibrolytic bacteria <italic>norank_f_UCG-011</italic> and <italic>Romboutsia</italic>, were greater (<italic>P</italic> < 0.05) in yaks than cattle. These results suggest that interspecies differences in rumen microbial composition and N utilization confer a microbial protein synthesis advantage to yaks when fattened on a low concentrate diet in a small-holder feedlot. The advantages for the yaks over cattle in the present study, however, were less evident than reported for these species when grazing. Grazing yaks have a wide choice of dietary intake, which can differ substantially from co-grazing cattle, but under feedlot conditions they are offered the same feed, and their diets are similar. Dietary intake is a major determinant of rumen bacteria, which could explain why rumen bacteria and responses of yaks and cattle in feedlots would be closer than when grazing.