A hierarchical 3-level meta-analysis of the effects of supplemental histidine on lactational performance in dairy cows and the associated influencing factors.

The effect of His supplementation on the lactational performance in dairy cows as well as the potential interfering factors have not been well characterized. Nineteen peer-reviewed papers from PubMed, Web of Science, and Google Scholar databases published by December 31, 2024, were pooled to calculate the weighted mean differences (WMD) and CI for continuous variables using a stratified 3-level meta-analysis with a random-effects model. A moderator analysis was used to evaluate the influences of cow breed, lactation stage, dietary MP supply, basal diet type, and other supplements as well as method and dosage of administration on the effects of supplemental His. Results showed that His supplementation increased DMI, milk yield, milk protein yield, and milk protein concentration, but decreased milk fat concentration in dairy cows. The effects of His on milk protein concentration was more prominent in dairy cows when they were fed an MP-deficient diet or a corn silage-based diet. More beneficial effects of His on DMI, milk protein concentration and milk protein yield, milk fat concentration, and lactose yield were found in early-lactation cows relative to mid-lactation cows. Histidine supplementation via abomasal infusion, rather than feeding a rumen-protected form, increased the milk protein concentration. Regression analysis showed that total digestible His should be supplied at 72.6 g/d to achieve the optimal milk protein yield in dairy cows. These findings provide guidelines for His supplementation to improve the lactational performance in dairy cows.