Effects of methionine hydroxy analog on milk secretion and ruminal and blood variables of dairy cows fed a low fiber diet.

Twenty-eight Holstein cows were in a reversal trial to determine effects of adding 34 g/day of methionine hydroxy analog to a low fiber ration. Cows were subjected to this feeding treatment sequence after parturition: days 1 through 28, normal fiber ration; days 29 through 56, low fiber ration; days 57 through 77, low fiber ration with or without 34 g/day of methionine hydroxy analog; days 78 through 98, low fiber ration; days 99 through 119, low fiber ration with or without 34 g/day of methionine hydroxy analog (days 57 through 77 treatments reversed). Milk, rumen fluid, and blood were collected at regular intervals during each period. Overall, supplementation with methionine hydroxy analog increased only milk fat 6%. After milk fat percentage was depressed by low fiber diet, supplementation with methionine hydroxy analog increased milk fat percentage only for those cows that initially had less than 20% decrease of milk fat percentage. This enhanced milk fat percentage was accompanied by increase of ratio of ruminal acetate to propionate in animals with less than 10% depression. Supplementation with methionine hydroxy analog had no effect on blood glucose, insulin, or acetate concentrations and did not affect uptake of acetate by the mammary gland. Initial milk fat production affects the ability of methionine hydroxy analog supplementation to enhance milk fat percentage of dairy cattle fed low fiber diets.