Two experiments were conducted with British Friesian/Holstein cattle to evaluate the response in milk production to the long-term use of recombinantly derived bovine somatotropin in a slow release form (somidobove, Lilly Research Centre Ltd). The first compared three doses of BST and the second measured the effect of BST given to animals receiving two levels of concentrates.

Thirty-eight and forty autumn calving cattle were used in experiments 1 and 2 respectively. A randomised block design was used in the first experiment and a 2 x 2 factorial in the second. Animals were blocked on the basis of similarity in calving date and milk yield during the preliminary period. Treatments were allocated at random within blocks, animals remained on the same treatment throughout each experiment. In experiment 1 animals received seven successive subcutaneous injections of BST at intervals of four weeks, the initial injection was given about 11 weeks after calving. A control and three doses of BST were compared: 320, 640 and 960 mg. In experiment 2 animals were given given six injections of 640 mg BST at the same intervals.