EFFECTS OF RUMEN-INERT FAT ON LACTATION, REPRODUCTION, AND HEALTH OF HIGH PRODUCING HOLSTEIN HERDS

Two hundred twenty of 443 cows freshening between June 1989 and March 1990 in five commercial Holstein herds were fed .45 kg/d of rumen-iner t fat from calving until 200 DIM. Control diets were fed as TMR and co ntained, on average, 3.7 to 4.8% supplemental fat (DM basis). Test her ds had rolling herd averages of 9300 to 13,250 kg of milk. Production of 4% FCM and milk increased 1.01 (3.3%) and 1.50 kg/d (4.6%), respect ively, for primiparous cows fed additional fat. Multiparous cows from four herds demonstrated no response; multiparous cows in one herd incr eased production of 4% FCM by 2.88 kg/d (8.2%), milk by 2.45 kg/d (6.4 %), and milk fat by .14 kg/d (10.6%) in response to additional fat. An explanation of response differences among herds for multiparous cows was not possible. For primiparous and multiparous cows, increased gene tic potential increased treatment response. Increased body condition s core at calving influenced treatment response of multiparous cows. Thi nner cows produced more milk and less milk fat in response to addition al dietary fat than did fatter cows. Most reproductive indices were un affected by treatment. Cows receiving additional fat had lower, but no nsignificantly lower, incidences of most health disorders. Responses t o rumen-inert fat by cows receiving high concentrations of dietary fat were marginal and were affected by body condition score at calving an d by genetic potential.