Meta-analysis of the effects of supplemental leucine alone or in combination with other branched-chain amino acids on lactational performance in dairy cows and the associated influencing factors.

Effects of Leu alone or in combination with Ile and Val (ILV) supplementation on lactational performance in dairy cows as well as the potential interfering factors need to be systematically investigated. PubMed, ScienceDirect, Web of Science, and Google Scholar databases were searched for peer-reviewed papers published by August 31, 2024, to explore the factors influencing the effects of branched-chain AA supplementation in dairy cows. Weighted mean difference with CI of continuous variables from 17 studies were pooled using a hierarchical 3-level meta-analysis with a random-effect model. Moderator analyses were performed to evaluate the effects of different cow breed, lactational stage, dietary MP supply, basal diet type, supplementation method, and supplementation dosage and duration on the heterogeneity of outcomes. Results showed that Leu supplementation increased the milk protein concentration, and ILV supplementation tended to increase milk protein yield in dairy cows. The positive effect of Leu supplementation on milk protein concentration was more prominent in dairy cows fed an MP-adequate or corn silage-based diet. Moreover, the milk protein concentration of cows had a tendency to increase when Leu was supplemented via infusion rather than feeding a rumen-protected Leu. Further analysis showed that total digestible Leu should be supplied at 266 g/d (or 11.2% of MP) or supplementation with a duration of 7 d, and total digestible ILV should be supplied at 620 g/d (or 24.2% of MP), to achieve the optimal performances in dairy cows. These findings indicated the beneficial effects of supplemental Leu and ILV on milk protein output in dairy cows.