Objective  Changes on the fat content of milk from and rumen microbial flora in dairy cows fed with corn meal and flaxseed oil supplemented forage were studied.

  Method  In a self-control experimentation, 4 Chinese Holstein cows of similar physical conditions and normal lactation were fed on a daily diet added with 1.5 kg of corn meal and 150 mL per animal of flaxseed oil for 7 pre-trial days followed by 15 days for the test.

  Result  In the feed containing approximately 27% starch, the dietary supplements significantly lowered the fat content (P < 0.01) with a significantly higher lactose content (P < 0.05) in the milk produced by the cows. There were 29 microbial phyla found in the rumen. The flora of Tenericutes significantly decreased (P < 0.05), Actinobacteria and Planctomycetes significantly increased (P < 0.01), and Spirochaetae significantly increased (P < 0.05) over control. Among the 389 genera of microbes in the rumen, 110 were significantly different (P < 0.05). Comparing to control, Lactobacillus increased significantly (P < 0.01), Enterococcus decreased significantly (P < 0.01), Christensenellaceae R-7-group decreased significantly (P < 0.05), Prevotellaceae-UCG-001 increased significantly (P < 0.05), Lachnospiraceae-UCG-006 decreased significantly (P < 0.01), Lachnospiraceae-NA increased significantly, Butyrivibrio-2 decreased significantly (P < 0.05), and Ruminococcus-1 increased significantly.

  Conclusion  Under the experimental conditions, the dietary supplementation of corn meal and flaxseed oil significantly reduced the fat content in milk of the dairy cows. It did not significantly alter the diversity but significantly changed the structure and abundance of the microbial flora in the cow rumen. The acidogenic as well as starch-utilizing bacteria significantly increased.