The objectives of the trial were to Investigate the effects of long-term administration of bovine somatotropin (BST) from week 2 and 10 post-partumn on the yields of milk, milk components, body weight, body condition, health and reproductive status during the first lactation of dairy heifers.

19 Friesian dairy cows in their first lactation were allocated at calving to one of 3 treatments: Daily subcutaneous Injection of recomblnantly derived bovine somatotropin (25 mg/d) from either week 2 (BST2;n=6) or week 10 (BST10;n=6) of lactation, or injection of buffer solution from week 2 of lactation (C;n=7). Treatments continued until week 42 of lactation, or until 60 days prior to predicted calving date, if this occurred before week 42 of lactation. Initially cows were housed in cubicles and fed ad libitum individually through calan doors, on a diet of concentrate and grass silage (Table 1) on a 60:40 dry matter ratio. In approximately mid-lactation cows were turned out to grass but continued to receive concentrates according to milk yield and body condition. Milk yield was recorded daily and milk composition twice weekly as were live weight and body condition. Health and reproductive status were monitored over the lactation.