Effects of ratios of yak to cattle inocula on methane production and fiber digestion in rumen in vitro cultures1

Yaks are well-adapted to the harsh environment of the Tibetan plateau, and they emit less enteric methane (CH4) and digest poor-quality forage better than cattle. To examine the potential of yak rumen inoculum to mitigate CH4 production and improve digestibility in cattle, we incubated substrate with rumen inoculum from yak (YRI) and cattle (CRI) in vitro in five ratios (YRI: CRI): 0:100 (control), (2) 25:75, (3) 50:50, (4) 75:25 and (5) 100:0 for 72 h. The YRI: CRI ratios of 50:50, 75:25 and 100:0 produced less total gas and CH4 and accumulated less hydrogen (H2) than 0: 100 (control) at most time points. From 12 h onwards, there was a linear decrease (P < 0.05) in carbon dioxide (CO2) production with increasing YRI: CRI ratio. At 72 h, the ratios of 50:50 and 75:25 had higher dry matter (+7.71% and +4.11%, respectively), as well as higher acid detergent fiber digestibility (+15.5% and +7.61%, respectively), when compared to the 0:100 ratio (P < 0.05). Increasing the proportion of YRI generally increased total VFA concentrations, and, concomitantly, decreased the proportion of metabolic hydrogen ([2H]) incorporated into CH4, and decreased the recovery of [2H]. The lower [2H] recovery indicates unknown [2H] sinks in the culture. Estimated Gibbs free energy changes (∆G) for reductive acetogenesis were negative, indicating the thermodynamic feasibility of this process. It would be beneficial to identify: 1) the alternative [2H] sinks, which could help mitigate CH4 emission, and 2) core microbes involved in fiber digestion. This experiment supported lower CH4 emission and greater nutrient digestibility of yaks compared to cattle. Multi-omics combined with microbial culture technologies developed in recent years could help to better understand fermentation differences among species.