A comparison between yaks and Qaidam cattle in in vitro rumen fermentation, methane emission, and bacterial community composition with poor quality substrate

Yaks (Bos grunniens) and Qaidam cattle (Bos taurus) graze the Qinghai-Tibetan Plateau (QTP) grassland all year, but yaks graze at altitudes higher than cattle. Yaks are raised by vertical transhumance, grazing at higher altitudes in summer and lower altitudes in winter. It is mainly in winter, when forage is of poor quality, that the two species co-graze. At this time, yaks do not receive supplements or shelter while cattle require supplements and night shelter. Previous studies reported that both energy and protein utilization were greater in yaks than cattle, and the rumen microbiota differed between the species. Based on the differences between species, we hypothesized that the rumen microbiota would enable yaks to digest a poor quality diet to a greater extent than cattle. This could explain why yaks are able to overcome the harsh winter without supplements while grazing poor quality pasture, but cattle require supplements. To test the hypothesis, fermentation parameters were studied in vitro using rumen inocula from yaks and cattle. Four isonitrogenous substrates of different energy levels (6.62, 8.02, 9.42, 10.80 MJ ME/kg) were used. The neutral and acid fiber detergent digestibilities were greater (P < 0.05) in yaks than in cattle and both increased linearly (P < 0.05) with increasing energy level. Methane production was lesser (P < 0.05) in yaks than cattle, and decreased linearly (P < 0.05) with increasing energy level. Concentrations of total volatile fatty acids (VFAs) and acetate, the ratio of acetate to propionate, and ammonia-N (except at 48 h) were greater (P < 0.05) in yaks than cattle; whereas concentrations of propionate (except at 6 h), butyrate, and iso-VFAs were lesser (P < 0.05) in yaks than cattle. In addition, total VFAs, propionate, butyrate, and ammonia-N increased linearly (P < 0.05), whereas, acetate and the ratio of acetate to propionate decreased linearly (P < 0.05) with increasing energy level. The relative abundance of Bacteroidetes was greater (P < 0.05), whereas of Firmicutes and the ratio of Firmicutes to Bacteroidetes were lesser (P < 0.01) in yaks than cattle, and Bacteroidetes decreased linearly (P < 0.001), whereas Firmicutes and the ratio of Firmicutes to Bacteroidetes increased linearly (P < 0.01) with increasing energy levels. The relative abundances of fibrolytic bacteria (e.g. Rikenellaceae_RC9_gut_group and Butyrivibrio) and of H₂-incorporating bacteria (e.g. Quinella) were greater (P < 0.05), whereas, amylolytic bacteria (e.g. Selenomonas and Succiniclasticum) and of H₂-producing bacteria (e.g. Lachnospiraceae_NK3A20_group and Ruminococcus) were lesser (P < 0.05) in the rumen inoculum from yaks than cattle. It was concluded that the greater relative abundance of fibrolytic and H₂-incorporation bacteria and lesser H₂-producing bacteria enable yaks to better digest fibers and produce less CH₄ than cattle.