Lactational and systemic responses of dairy cows to postruminal infusions of increasing amounts of methionine.

Five multiparous Holstein cows were used in a study with a 5 x 5 Latin square design to measure the effects of postruminal infusion of Met on lactational performance and plasma metabolites. The treatments were duodenal infusions of 1) 10 g/d of Lys (control), 2) 10 g/d of Lys plus 6 g/d of Met, 3) 10 g/d of Lys pus 12 g/d of Met, 4) 10 g/d of Lys plus 18 g/d of Met, and 5) 10 g/d of Lys plus 24 g/d of Met. The cows were fed a diet of 61% maize silage, 31% concentrate, and 5% dehydrated alfalfa. The DMI were similar among treatments. Milk yield, 4% FCM, and milk fat yield and content were not affected by infusions. In contrast, milk protein yield and content were increased linearly as Met infusion increased, which was true also for plasma Met and Cys concentrations. Using measurements of AA flow to the duodenum and assumed intestinal digestibilities of 0.8 for digesta and 1.0 for infused AA, estimated concentrations of Lys and Met in total AA absorbed in the small intestine were 7.3% for Lys and 1.52, 1.73, 1.94, 2.15, and 2.36% for Met for diets 1 through 5, respectively. The substantial linear increases in milk protein yield and content indicated that postruminal Met supply was not adequate over the entire range of Met infusions. In conclusion, the extent of Met limitation in this study could be defined only as that exceeding 2.4% of total AA absorbed in the small intestine.