Impact of soybean hulls on milk composition, nutrient digestibility, serum metabolites, rumen fermentation, and gut microbiota of postpartum dairy cows.

Soybean hulls, a by-product of industrial processing, are rich in polysaccharides and are an important biological resource. This study evaluated the effects of replacing traditional forage fiber with soybean hulls in the diets of postpartum dairy cows on milk yield, milk composition, rumen fermentation, and liver metabolism. Twenty multiparous Holstein cows were randomly assigned from calving to 21 d postpartum to 2 diets: a long-fiber diet (LF, 0% soybean hulls) and a short-fiber diet (SF, soybean hulls at the level of 7.8% of DM). The DMI, nutrient digestibility, milk yield, energy-corrected milk production, 4% fat-corrected milk production, and milk lactose were significantly higher in the SF group than in the LF group. In addition, the total volatile fatty acids, acetic acid, butyric acid, isobutyric acid, pentanoic acid, and isovaleric acid concentrations in the rumen were significantly higher in the SF group than in the LF group. Moreover, pancreatic glucagon concentrations were significantly higher in the serum of the SF group. Furthermore, the LF group showed higher abundance of Ruminococcus, Lachnospiraceae XPB1014_group, unclassified_f_Eggerthellaceae, and Lachnobacterium, whereas the SF group showed higher abundance of Eubacterium_nodatum_group, Roseburia, Pseudoramibacter, Coprococcus, Catenisphaera, Erysipelotrichaceae_UCG-006, Dlalster, and Coriobacteriaceae_UCG-003. The SF group showed upregulation of protein processing in the endoplasmic reticulum, lipid metabolism, atherosclerosis, and the PPAR signaling pathway. In conclusion, increasing the soybean hull content as a carbohydrate source for postpartum cows beneficially enhanced milk yield and milk quality, along with rumen fermentation and liver metabolism, supporting its use as an alternative feed for sustainable dairy production.