Effects of High-Grain Diet on Performance, Ruminal Fermentation, and Rumen Microbial Flora of Lactating Holstein Dairy Cows

The objectives of the current study were to evaluate the fluctuations in production performance, rumen fermentation, and microbial community in lactating dairy cows fed a high-grain diet (HG). In this study, 16 healthy Holstein lactating dairy cattle with similar milk yields of 16.80 ± 4.30 kg/d, days in milk 171.44 ± 23.25 days, and parity 2.2 ± 1.5 times were selected and randomly allocated into two groups. One group was fed a low-grain diet (LG; 40% concentrate, DM basis; n = 8), and the other group was fed a high-grain diet (HG; 60% concentrate, DM basis; n = 8). The experiment lasted 6 weeks, including 1 week for adaptation. The experimental results showed that the milk fat content in the milk of lactating cows in the HG group was significantly reduced (<i>p</i> < 0.05), and the milk urea nitrogen (MUN) content showed an increasing trend (0.05 < <i>p</i> < 0.10) compared with the LG group. Compared with the LG group, rumen fluid pH was significantly decreased after feeding a high-grain diet, and contents of total volatile fatty acids (TVFA), acetate, propionate, and butyrate were significantly increased (<i>p</i> < 0.05). The acetate/propionate significantly decreased (<i>p</i> < 0.05). HG group significantly increased the abundance of <i>Prevotella</i> and <i>Bacteroides</i> in rumen fluid while significantly reducing the abundance of <i>Methanobrevibacter</i> and <i>Lachnospiraceae ND3007_group</i> (<i>p</i> < 0.05). Microorganisms with LDA scores > 2 were defined as unique, with the bacterial genus <i>Anaerorhabdus_furcosa_group</i> identified as a biomarker for the LG group, and the unique bacterial genus in the HG group were <i>Prevotella</i>, <i>Stenotrophomonas</i>, and <i>Xanthomonadaceae</i>. The prediction results of microbial function showed that a total of 18 KEGG differential pathways were generated between the two treatment groups, mainly manifested in metabolic pathways, signal transduction, and the immune system. In conclusion, the HG group promoted rumen fermentation by altering the microbial composition of lactating cows. Our findings provide a theoretical basis for the rational use of high-grain diets to achieve high yields in intensive dairy farming.