Effect of different forms of methionine on lactational performance of dairy cows.

Abstract Methionine is one of the first limiting AA in dairy cows. The use of rumen-protected Met to correct deficient diets is limited by the lack of a product that could be incorporated into a pelleted concentrate. The main objective of this trial was to test, at practical doses (approximately 10g of absorbable Met), the efficacy of 2 forms of pelletable Met hydroxy analogs, d,l-2-hydroxy-4-(methylthio)-butanoic acid (HMB) and the isopropyl ester of HMB (HMBi), to provide Met to cows, especially for milk protein synthesis, compared with a negative control and to Smartamine M (SmM). These treatments were tested according to a 4×4 Latin square in 16 Holstein cows. Plasma Met concentrations were increased by 110 and 65% that of the control value after HMBi and SmM treatments, respectively. Milk protein yield increased by 32 and 41 g/d for HMBi and SmM, respectively. d,l-2-Hydroxy-4-(methylthio)-butanoic acid supplementation did not improve Met availability to the cows for milk protein synthesis. The HMBi treatment induced an increase in 15:0 in milk at the expense of a general reduction in even-numbered short-and medium-chain fatty acids. Moreover, HMBi and SmM supplements led to an increase in the saturation level of C 18 fatty acids consistent with the improvement of Met supply. It was concluded that HMBi is a new "rumen-protected" form of Met that can be supplied to cows integrated into pellets.