Meta-analysis of 2-hydroxy-4-methylthio-butanoic acid supplementation on ruminal fermentation, milk production, and nutrient digestibility.

Methionine is considered one of the most important essential AA for milk protein synthesis in dairy cows. Supplementation of unprotected, free Met is nearly 100% degraded by ruminal microorganisms, which complicates supplementation. 2-Hydroxy-4-methylthio-butanoic acid (HMTBa) can be converted to Met in the body and is used as a Met source in dairy production. However, results of published studies assessing the effects of supplementing Met sources, including HMTBa, on performance variables are inconsistent. A meta-analysis was performed to quantitatively summarize the accumulated results of HMTBa supplementation on animal performance and nutrient digestibility. Data pertaining to HMTBa dose, dietary composition, and major performance variables (rumen volatile fatty acid composition, milk production, nutrient digestibility) were collected from 39 articles containing 169 treatment means. Publications were from scientific journals published from 1970 to 2018; 1 internal report from Novus International Inc. (St. Charles, MO) was also included. The HMTBa effects on response variables were analyzed using linear mixed models with random study effects. Other explanatory variables tested included neutral detergent fiber and crude protein percent as well as days in milk. Results showed that HMTBa supplementation increased blood Met concentration and milk fat yield but had no effect on nutrient digestibility.