The positive effect of Methionine (Met) on milk performance is well investigated in a lot of scientific and field studies. Garthwaite et al. (1998) summarized the effect of supplementing rations with metabolizable Lysine (Lys) and Met. Results indicated an improvement of the milk performance by balancing the diets for Lys and Met. Very important insight was, that the positive effect is much stronger by starting the feeding of rumen protected AA during close-up period. But the profitability doesn´t come exclusively from a larger milk production, also from a better milk composition and improved animal health (Table 1).

Another important benefit of balancing for amino acids is the effect on milk composition (Table 1). The milk protein content is very important not only because its repercussion on the milk price received but also because the effect on cheese production. Because the most important factor affecting the cheese yield (amount of cheese produced from one kg of milk) is milk casein. Cheese manufacturing is a dual process of concentration and biological transformation of milk components into gustatory and aromatic compounds that determine the unique characteristic of cheese. The quality and quantity of cheese depends on the amount of milk casein (KIR-18-00074, 19-20467, Storry et al., 1983). Cheese making properties are therefore improved by selecting high casein milk (Wedholm et al., 2006, Chitchyan et al., 2009). Casein content of milk protein is about 80 %. Balancing the diets for amino acids improves milk protein content (Table 1). Additionally, there is strong evidence that well balanced diets can improve the casein content of milk protein. As a result, double positive effect on cheese yield.

It is also remarkable that balancing for Methionine and Lysine requirements improves the antioxidant animal capacity, better health status and immunometabolism. Apart from participation in the very low-density lipoprotein (VLDL) synthesis, Met and Lys have another important role in fat metabolism. They are both required for the synthesis of carnitine, essential for the transport of non-esterified fatty acids (NEFA) from the cytosol into the mitochondria for subsequent fatty acid oxidation (Drackley, 1999). Moreover, it is well known that carnitine plays additional important roles in protecting organisms from oxidative stress, promoting substrate oxidation in brown adipose tissue, and regulating energy partitioning in the body (Wu, 2013).

Many amino acids have positive effects on physiological processes that are independent of their effects on the synthesis of proteins. This has been termed “functional effects” of amino acids. Met and Lys are the “functional amino acids” that have been linked to reproduction and other important animal health benefits. 

Table 1. Milk components and productivity expressed as percentage of increase after amino acid implementation related the non-amino acid use. TPAS-19-1075. (*) Return on investment.