The objectives of the trial were to investigate the effects of bovine somatotrophin (BST) from either week 2 or week 10 post-partum on the yields of milk, milk components, body weight, body condition, feed intake, health and reproductive status of dairy cows over a second consecutive lactation. The results from the heifer lactation have been reported previously (Butterwick et al. 1988).

17 Friesian dairy cows in their second lactation were continued on the same treatment as in their heifer lactation: daily subcutaneous injection of recombinantly derived bovine somatotrophin (BST;25mg/d) from either week 2 (BST2;n=6) or week 10 (BSTl0;n=5) post-partum, or injection of a buffer solution from week 2 of lactation (C;n=6). 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 through Calan doors, on a diet consisting of concentrate and grass silage (Table 1) on a 60:40 dry matter ratio. After day 110 of lactation cows were turned out to pasture but continued to receive concentrates according to milk yield and body condition score. Milk yield and feed intake were recorded daily, milk composition, body weight and body condition score were recorded twice weekly. Health and reproductive status were monitored over the lactation.