Influences of methionine hydroxy analog on milk and milk fat production, blood serum lipids, and plasma amino acids.

Feeding for 150 days of 25 g per day of methionine hydroxy analog was tested on 100 cows (52 treated and 48 controls) in the Brigham Young University herd. Effect of days after parturition of initiating methionine hydroxy analog feeding also was observed. Milk yields were not affected by methionine hydroxy analog, but fat percent and fat yields were increased 21 and 17% by the additive. Abnormally low milk fat by control cows (2.84%) magnified the response to methionine hydroxy analog feeding. Feeding methionine hydroxy analog beginning 0 to 12 days postpartum elicited much larger increases of milk fat than started later (17 to 102 days). Blood from the coccygeal vein of 20 cows from each treatment had 10% more triglycerides from cows fed methionine hydroxy analog than from control cows. Increases of arterio-venous differences across the mammary gland of triglycerides and lipoproteins of blood serum suggested that increases of milk fat could have resulted from greater uptake of performed fat by the udder. Feeding methionine hydroxy analog increased methionine, isoleucine, and leucine in serum of coccygeal vein, but methionine was the only amino acid with significantly higher arteriovenous differences across the mammary gland.