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

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): (1) 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 (ΔG) 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.