An updated hierarchical three-level meta-analysis of the effects of supplemental lysine on lactational performance in dairy cows and the associated influencing factors

Although Lys has been widely used in dairy cows, its effects on lactational performance are inconsistent, and the potential interfering factors have not been systematically investigated. Thirty-three reviewed publications from PubMed, Web of Science, and Google Scholardatabases up to March 31, 2025were pooled to calculate the weighted mean differences (WMD) and confidence intervals 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, lactational stage, dietary MP supply, basal diet type, and other additions as well as method, dosage and duration of administration on the effects of supplemental Lys. Results showed that Lys supplementation increased the milk yield (WMD = 0.52 kg/d, [0.30, 0.74]), milk protein yield (WMD = 0.03 kg/d, [0.02, 0.04]), milk protein concentration (WMD = 0.05 %, [0.03, 0.07]), and milk fat yield (WMD = 0.02 kg/d, [0.01, 0.03]) in dairy cows. The positive effects of Lys supplementation on milk yield and milk fat yield were more prominent in dairy cows of Holstein breed, or in cows fed a MP-adequate diet, or in cows fed a corn silage-based diet, or administration through feeding rumen-protected Lys compared with the respective other groups. The response of milk protein yield in dairy cows was greater when Lys was supplemented in early-lactation (WMD = 0.04 kg/d, [0.02, 0.06]). Lysine supplementation along with Met significantly increased the milk yield (WMD = 1.09 kg/d, [0.53, 1.64]), while sole Lys addition had more greater effects on milk protein concentration (WMD = 0.06 %, [0.03, 0.09]). Regression analysis showed that the optimal dosage and duration of Lys supplementation for milk yield, milk protein yield and milk fat yield were 203, 208, 204 g/d (6.8, 6.9, 6.8% of MP), and 90, 85, 43 d, respectively. These findings collectively provide practical guidelines for Lys application in improving the lactational performance in dairy cows.