Methionine has been shown to be the first-limiting amino acid for milk protein synthesis in lactating dairy cows fed maize silage-based diets complemented with soyabean meal (Sloan et al., 1992) (Pisulewski et al., 1993). Thus, the aim of this trial was to investigate the hypothesis that methionine was first-limiting or if not at least colimiting with lysine for milk protein synthesis in dairy cows fed grass silage complemented with soyabean meal.

8 muciparous and 4 primiparous dairy cows six weeks into lactation were randomly allocated to three 4x4 Latin squares (cows and heifers separately), each period containing 2 weeks. The four treatments consisted of Tl = Control ; T2 = 3.7 g methionine ; T3 = 7.4 g methionine ; T4 = 7.4 g methionine and 22.2 lysine . All figures relate to estimated intestinally available amino acids, g/day (Smartamine™ technology Rhône-Poulenc Animal Nutrition). The basal ration was a second cut grass silage offered ad libitum plus 0.57 kg of hay plus an average 10.9 kg concentrate.