Methionine is one of the most limiting amino acids in lactating dairy cow diets. Methionine hydroxy analogues as 2-hydroxy-4-(methylthio)butanoate are synthetic amino acids in which the amino group has been replaced with a hydroxyl group in order to protect methionine from ruminal degradation. They are normally fed to improve milk and milk protein production of dairy cows. In addition, a recent meta-analysis concluded that supplementation of methionine hydroxy analogues can increase milk fat yield.
In the current dairy market, this parameter is economically important to dairy producers. Many dairy farms experience milk fat depression, which is a syndrome characterized by a reduction in milk fat content. The biohydrogenation theory suggests that this problem is the result of a direct inhibition of fat synthesis in the mammary gland by fatty acid intermediates produced during rumen biohydrogenation of polyunsaturated fatty acids when cows receive diets with high concentration of unsaturated fatty acid and low level of effective fiber.