Lack of sufficient methionine and lysine delivered post-ruminally may limit milk production response to bovine somatotropin (bST). To test this hypothesis, four Holstein cows fitted with rumen and duodenal cannulas were used in a 4 × 4 Latin square design with 14 d periods. Treatments were: (1) control, (2) continuous duodenal infusion of 8 g methionine and 24 g lysine/d, (3) injection of 25 mg bST/d and (4) infusion of methionine and lysine plus injection of bST. Infusion of amino acids led to trends for small increases in milk (3%), fat (5.5%), and protein (3.7%) yield. Larger and significant increases (8.7, 14 and 6.9% for milk, fat and protein yield respectively) were achieved with bST administration which also increased milk fat content. Plasma levels of urea-N and essential amino acids were reduced with bST. Duodenal nutrient flow was generally unaffected by treatment. The production response to bST was not enhanced in cows producing an average of 34 kg milk when provided additional methionine and lysine post-ruminally in this short-term study.
