Effect of 2-hydroxy-4-(methylthio) butanoate (HMTBa) supplementation on rumen bacterial populations in dairy cows when exposed to diets with risk for milk fat depression

Diet-induced milk fat depression (MFD) is a condition marked by a reduction in milk fat yield experimentally achieved by increasing dietary unsaturated fatty acids and fermentable carbohydrates. 2-hydroxy-4-(methylthio) butanoate (HMTBa) is a methionine analog observed to reduce diet-induced MFD in dairy cows. We hypothesize that the reduction in diet-induced MFD by HMTBa is due to changes in the rumen microbiota. To test this, 22 high-producing cannulated Holstein dairy cows were placed into two groups using a randomized block design and assigned to either control or HMTBa supplementation (0.1% of diet DM). All cows were then exposed to three different diets with either a low risk (32% NDF, no added oil; fed d 1 to 7), a moderate risk (29% NDF and 0.75% soybean oil; fed d 8 to 24), or a high risk (29% NDF and 1.5% soybean oil; fed d 25 to 28) for diet-induced MFD. Rumen samples were collected on d 0, 14, 24, and 28, extracted for DNA, PCR-amplified for the V1-V2 region of the 16S rRNA gene, sequenced on an Illumina MiSeq, and subjected to bacterial diversity analysis using the QIIME pipeline. The alpha diversity estimates (species richness and Shannon diversity) were decreased in the control group compared to the HMTBa group. Bacterial community composition also differed between control and HMTBa groups based on both weighted uniFrac (relative abundance of commonly detected bacteria) and unweighted uniFrac (presence/absence) distances. Within the HMTBa group, there were no differences in bacterial community composition between d 0 and d 14, 24, and 28; however, in the control group, d 0 samples were different from d 14, 24, and 28. Certain bacterial genera including Dialister, Megasphaera, Lachnospira, and Sharpea were increased in the control group compared to the HMTBa group. Interestingly, these genera were positively correlated with milk fat trans-10, cis-12 conjugated linoleic acid and trans-10 C18:1, fatty acid isomers associated with biohydrogenation-induced MFD. It can be concluded that diet-induced MFD is accompanied by significant alterations in the rumen bacterial community and that HMTBa supplementation reduces these microbial perturbations.