The effect of bovine somatotropin on dairy cows under French farming conditions

Abstract

Twenty experiments were carried out on 360 cows receiving slow release somatotropin (BST) every 14 or 28 days, at doses equivalent to 24 or 35 mg/day. Data obtained from these experiments suggest that the effect of slow release BST on milk production varies significantly with the total dose injected per month, but only slightly with the frequency of injection (once or twice a month). Depending on dose and frequency of injection, the average change in milk yield varied between + 2.2 and + 3.7 kg milk/day with winter rations and between + 1.2 and + 2.3 kg milk/day on pasture. The lower yield on pasture could be due to the quality of the grass, to the pregnancy stage or to the length of BST treatment. Milk composition in cows in the middle of lactation was not modified on pasture. However, protein content was slightly lower in milk from cows on winter rations (- 0,5 g/l) which had been treated with BST 3 to 4 months after the peak of lactation. This data should be reevaluated using more trials and after detailed statistical analysis. Further, the production and composition of the milk fluctuated between successive injections, and these fluctuations increased as the interval between injections increased. Treated cows showed a decrease in their overall condition (- 0.3 point approx.) when they received winter rations at the peak of lactation (which may lead to a significant delay in fertilisation), or when they were out to pasture and given a limited amount of concentrate. There were large variations between experiments and between cows. The data gave no information on possible long term effects on the health and longevity of the BST-treated cows.