Response of Holstein and Brown Swiss cows fed alfalfa hay-based diets to supplemental methionine at two stages of lactation

In this study, we evaluated the effects of dietary supplementation at two s tages of lactation with various levels of Mepron85 (M85) and M85 plus DL-me thionine (DL-Met) on milk production and composition of Holstein and Brown Swiss cows fed an alfalfa-hay and corn grain-based diet. In experiment 1, c ontrol diets were formulated to supplement, in early lactation [days in mil k (DIM) = 73.2], concentrations of metabolizable methionine at 104% of the estimated requirements based on the Cornell Net Carbohydrate and Protein Sy stem. Treatment groups were fed the control diet plus 10, 20, or 30 g/d of M85 at 116, 128, or 139% of the estimated requirements for metabolizable me thionine. The supplementation with 10 g/d in Brown Swiss and 30 g/d of M85 in Holstein cows increased milk yields and fat percentage, but had no effec ts on protein percentage. These data suggested that the estimated postrumin al supply of metabolizable methionine in the control ration was limiting fo r maximum milk fat synthesis. Conversely, in experiment 2, the cosupplement ation with M85 (15 g/d) plus DL-Met (15 g/d) to cows in midlactation (DIM = 140.5) did not influence fat percentage, but increased protein yield and p ercentage (+0.1%) in both Holstein and Brown Swiss, and lactose percentage (+0.18%) in Holstein cows. The supplementation with 15 g/d of M85 reduced m ilk and protein yields, whereas 15 g/d of DL-Met reduced protein percentage in four of the five experimental weeks for Holstein cows. We conclude that supplementation with M85, alone or in combination with DL-Met, may be used to influence milk composition, but these effects are influenced by dosage and type of supplemental methionine, breed, and stage of lactation.