Effect of Unsaturated Fatty Acid Ratio In Vitro on Rumen Fermentation, Methane Concentration, and Microbial Profile

It is well known that dairy cows are fed diets with high fat content, which can adversely affect rumen fermentation. However, whether the effects of high fat content on rumen fermentation are related to the composition of fatty acids (FA) is for further study. We explored the effects of unsaturated fatty acid (UFA) ratios in vitro on rumen, methane concentration and microbial composition under the same fat levels. The experiment included a low-unsaturated group (LU, UFA proportion: 42.8%), a medium-unsaturated group (MU, UFA proportion: 56.9%), and a high-unsaturated group (HU, UFA proportion: 70.9%). The incubation fluid pH and NH₃-N levels were not significantly different in the three groups. Total volatile fatty acid (TVFA), acetate, propionate, butyrate, and valerate in the MU group had a decreased trend compared to the LU group (0.05 < <i>p</i> < 0.1), and no difference was found in other volatile fatty acids (VFAs) among the three groups. Furthermore, gas production kinetic parameters among the three groups did not differ significantly. The LU group’s CH₄ concentration was significantly higher than the HU group (<i>p</i> < 0.05). The CO₂ concentration in the LU group was also significantly higher than the MU and LU groups (<i>p</i> < 0.05). Additionally, 16S rRNA microbial sequencing results showed that the Shannon diversity value significantly increased in the MU group (<i>p</i> < 0.05) compared to the LU group. Other alpha diversity indices (Chao 1, observed species, and ACE) did not differ among the three groups. The increased proportion of UFA significantly decreased the relative abundance of <i>Succinivibrionaceae_UCG_001</i> and <i>Fibrobacter</i> (<i>p</i> < 0.05). Meanwhile, the multiple <i>Lachnospiraceae</i> bacteria significantly increased in the MU group (<i>p</i> < 0.05). Overall, our findings indicated that the microbial community in the incubation system could be affected by elevating proportions of UFA, affecting the yield of VFA, whereas the CH₄ concentration was reduced.