Saccharomyces cerevisiae and Bacillus subtilis Show Modest, Node-Dependent Effects on Insulin Sensitivity in Periparturient Cows.

Direct-fed microbials showed no significant benefit on periparturient cow insulin sensitivity.

This randomized controlled trial evaluated whether direct-fed microbials (DFM)—specifically Saccharomyces cerevisiae, Bacillus subtilis, or their combination—could improve insulin sensitivity and reduce endotoxemia in periparturient dairy cows. Twenty Holstein cattle were enrolled from 60 days before to 21 days after calving across four treatment arms. Blood and milk samples were collected at multiple timepoints to measure endotoxin levels and metabolic indices including QUICKI and RQUICKI. Using appropriate statistical methods with multiplicity control (BH-FDR) and prespecified equivalence testing (TOST/ROPE), researchers found no significant differences between DFM-supplemented groups and controls for endotoxin or metabolic parameters. While QUICKI demonstrated equivalence at several timepoints, endotoxin and RQUICKI did not. The findings suggest that DFM supplementation produces only modest, localized effects during early lactation in this population. The authors emphasize the importance of rigorous methodology, including prespecified equivalence testing and appropriate multiplicity correction, when evaluating interventions with expected small effect sizes.

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