ADMINISTRATION OF RECOMBINANT BOVINE SOMATOTROPIN TO DAIRY-COWS FOR 4CONSECUTIVE LACTATIONS

Effects of long-term administration of recombinant bovine somatotropin (bST) to dairy cows on complete lactational performance [60 (+/-3) to 284 (+/-3) d in milk (DIM)] were studied for four consecutive lactati ons. Beginning on d 60 (+/-3) postpartum, Holstein cows received biwee kly injections (500 mg) of bST (n = 39) or a placebo (control; n = 39) during the first lactation of the study. Cows either continued on the same treatment (n = 26) or were switched to the opposite treatment (n = 29) during the second lactation. Cows that changed treatments were injected for only 16 wk during the second lactation. Six cows per trea tment completed four consecutive lactations. Treatment with bST during the first lactation did not have a residual effect on milk yields dur ing the second lactation. Injections of bST during the second lactatio n increased milk yield 6.5 kg/d from 60 (+/-3) to 172 DIM. For the fou r lactations, cows receiving bST yielded 3.7 kg/d (14%) more milk and gained 52 kg (37%) more body weight than did controls. Pretreatment (f rom 0 to 56 DIM) milk yields in yr 2, 3, and 4 were not affected by pr evious bST treatment. Milk yield, efficiency of feed utilization, and body weights were enhanced in cows injected with bST for four consecut ive lactations. Previous bST treatment did not diminish milk yields in subsequent lactations.