Effects of the ratio of ruminal propionate to butyrate on milk yield and blood metabolites in dairy cows.

Four Ayrshire cows (mean = 56 DIM) were used in a 4 x 4 Latin square design to study the effects of the ratio of propionate to butyrate in the rumen on milk yield, milk composition, and blood metabolites. The cows were fed a basal diet (16.2% CP, 43.4% NDF) consisting of 50% grass silage, 6% grass hay, and 44% concentrate (percentage of DM). The diet supplied 44 Mcal/d of metabolizable energy and was supplemented with isoenergetic infusions of VFA (4.5 Mcal/d). Propionate (900 g/d) was replaced gradually with 33, 67, and 100% of butyrate on an energy basis. Replacement of propionate with butyrate in the infusate decreased propionate and increased butyrate concentrations in ruminal fluid and in blood plasma. Yields of milk and lactose decreased, and yield of milk fat increased, as butyrate increased. Milk fat content increased, and lactose content decreased, as butyrate increased. Increased ruminal supply of butyrate decreased plasma glucose concentration and increased blood ketone body concentration. When only butyrate was infused (750 g/d), either liver metabolism was changed or tissue mobilization was increased, as indicated by the increased production of long-chain milk fatty acids and increased plasma concentrations of acetate, Gly, and branched-chain AA. An increase in ruminal butyrate supply at the expense of propionate adversely affected milk yield and the repartitioning of nutrients between milk components. At a high percentage, increased butyrate might also adversely affect the overall metabolism of the cow.