The effect of dietary absorbable methionine and lysine concentrations on the milk production and milk composition of grazing dairy cows

Previous research has shown a response in milk yield, protein concentration and protein yield to supplementing diets based on grass silage with absorbable methionine and lysine. The objective of this experiment was to determine the effect on milk yield, protein concentration and yield, and protein composition of supplementing a grass-based diet with these absorbable amino acids. The experiment was a randomised block design with two treatments using 52 cows (including 12 heifers) in mid lactation for a 7-week experimental period, the last 4 of which were used to measure treatment effects. The treatments were (1) grass ad libitum plus 0.25 kg beet pulp/molasses per day (Control), and (2) grass ad libitum plus 0.25 kg beet pulp/molasses plus 6.2 g of absorbable methionine (MetDI) and 8.1 g of absorbable lysine (LysDI) per day. Both treatment groups grazed together as one herd. The amino acids were supplied in a rumen protected form (Smartamine(TM) RPAN Technology) and all figures relate to estimated intestinally absorbable methionine and lysine as a proportion of total digestible supply of amino acids. Estimated absorbable methionine and lysine concentrations were increased from sub-optimal proportions of 0.0193 and 0.0689 of PDIE (true protein digestible in the small intestine) to adequate proportions of 0.0230 and 0.0737 of PDIE by the supplementation. Optimum proportions for methionine and lysine are 0.025 and 0.073 of PDIE. There was no significant difference between the treatments in milk yield, milk composition, composition of milk protein, grass intake or blood metabolites. It is concluded that supplementing cows in mid-lactation on a grass diet with protected methionine and lysine has no effect on milk yield or constituents at the levels of production achieved in this experiment.