Meta-Analysis of Rumen-Protected Methionine in Milk Production and Composition of Dairy Cows

SIMPLE SUMMARY: In terms of amino acid nutrition of dairy cows, many scholars have shown that adding rumen-protected methionine to dairy cow feed can improve milk yield and milk components such as milk protein, lactose and milk fat, but the research of some scholars is inconsistent. This paper aims to summarize and analyze all the research contents through meta-analysis and comprehensively understand the impact of rumen-protected methionine on the milk yield and milk composition of dairy cows. The results show that adding rumen-protected methionine to cow feed did not significantly improve milk yield nor the lactose concentration in milk but did improve the fat and protein concentrations in milk, and the effects were better in the high-protein feed than that in the low-protein feed. ABSTRACT: This study aims to evaluate the influence of rumen-protected methionine (RPM) on the milk yield and milk compositions of dairy cows by employing a meta-analysis method. The articles in the publication databases between January 2010 and January 2022 which reported on various concentrations of RPM supplements in dairy cow diets and then monitored the milk yield and milk compositions were searched. A total of 14 studies were included, covering 27 treatments with a total of 623 dairy cows. Comprehensive Meta-Analysis V3 was used for statistical analysis, the forest map was drawn by the standard mean difference (SMD) with a 95% confidence interval (95% CI), and the SMD was calculated by a random effect model. The dose effect curve was drawn by fitting the SMD and RPM dose of each study to explore the optimal dosage of RPM. Compared with the basal diet, the RPM supplement significantly increased the percentages of milk fat (SMD (95% CI): 1.017% [0.388, 1.646]) and milk protein (SMD (95% CI): 0.884 [0.392, 1.377]). However, the milk yield (SMD (95% CI): 0.227 kg/d [−0.193, 0.647]) and lactose concentration (SMD (95% CI): 0.240% [−0.540, 1.020]) were not affected. The subgroup analysis found that the effect of the RPM supplement on the milk fat and milk protein was greater in the high-protein feed than in the low-protein feed. Multiple regression analysis showed that feeding RPM significantly improved the milk yield and milk protein percentage of dairy cows. The results of the dose–effect analysis show that the optimal range for the RPM was 7.5–12.5 g/d. RPM supplements in a dairy diet can improve the milk protein percentages and milk fat percentages of dairy cows.