Effects of Increasing Corn Grain Inclusion and Reducing Hay Proportion on Growth Performance, Methane Emissions, Rumen Fermentation, and Microbial Diversity in Winter-Housed Yaks

The expansion of ruminant production has increased methane (CH4) emissions, highlighting the need for nutritional strategies that improve productivity while mitigating environmental impacts. Yaks, generally considered low CH4 producers, are increasingly raised under intensive winter-housed systems on the Qinghai–Xizang Plateau, highlighting the need to assess how dietary concentrate-to-forage (C:F) ratios affect both CH4 emissions and growth performance. This study investigated the effects of three dietary C:F ratios [L-C (48:52), M-C (60:40), H-C (72:28)] on growth performance, ruminal fermentation, microbial diversity (n = 6 per group) and CH4 emission (n = 3 per group) in winter-housed yaks. The results indicated that average daily gain (ADG) was significantly higher in M-C and H-C, while the feed-to-gain ratio (F/G) was significantly lower in M-C and H-C than in L-C (p < 0.05). Total CH4 production (g/day) did not differ among treatments (p > 0.05), while CH4 yield per unit body weight gain (CH4/BWG) was significantly reduced in M-C and H-C (p < 0.05). The protozoal count was significantly lower in H-C, and the proportions of isobutyrate and isovalerate were significantly higher in H-C and M-C compared with L-C (p < 0.05). 16S rRNA gene sequencing revealed that increasing the C:F ratio reduced the relative abundance of the archaeal genus Methanobrevibacter, while Thermogymnomonas exhibited a significant increase (p < 0.05). Collectively, these findings indicate that increasing the C:F ratio in winter-housed yaks improves growth efficiency and lowers CH4/kg BWG, with the M-C group showing the most favorable balance between productivity and environmental sustainability.