Interaction of dietary fat and acid detergent fiber diets of lactating dairy cows

Twenty multiparous and 4 primiparous cows averaging 32 DIM were used in an experiment with a 2 x 2 factorial design for 12 wk to determine whether the response to supplemental choice white grease in the diet was augmented with higher fiber. Treatments were 1) 21% ADF with no added fat; 2) 21% ADF plus 6% added fat; 3) 28% ADF with no added fat; and 4) 28% ADF plus 6% added fat. Addition of fat decreased milk fat percentage and tended to decrease milk protein percentage. An interaction of fat by fiber by week for milk fat percentage suggested that ruminal fermentation was disrupted as the trial progressed when fat was included in the lower fiber diet. This disruption did not occur with the higher fiber diets. Yields of milk and FCM were not affected by treatment. There was no fat by fiber interaction for milk yield, which suggested that the yield response to fat was unaffected by fiber concentration. There were no treatment interactions for results of ruminal fermentation. Fiber increased ruminal acetate and the ratio of acetate to propionate but lowered molar percentages of propionate. Supplemental fat decreased acetate and increased propionate. The ratio of acetate to propionate was significantly decreased when fat was fed.