[Effects of long-term BST application in dairy cows]

The effects of administration of BST on milk production of dairy cows are summarised on the basis of a number of review papers. Then the results of a trial with forty control cows and thirty-seven cows, in which six doses of 640 mg of BST were subcutaneously injected at twenty-eight day intervals from the fourth month after calving in a third of fourth lactation are discussed. The response to administration of BST was a 3.2 kg of milk yield, 165 g for milk fat, 105 g of milk protein and 155 g lactose. Efficiency of milk production during the period of treatment was seven per cent higher in the treated cows. During the successive lactations, the response to BST in milk fat slightly declined, but similar responses were recorded for milk protein and lactose. Plasma levels of 3-hydroxybutyrate and NEFA were higher after treatment with BST. Fertility and health were not affected by this treatment. Calves from treated cows showed five per cent lower birth weight, which had no effects on viability and postnatal weight gain. Treatment with BST did not have any effect on the concentrations of calcium, magnesium, phosphorus and NEFA in milk. In the successive lactations, the somatic cell count in the milk of treated cows increased slightly more than that in controls. During the third and fourth lactation, in the period of treatment, the number and percentage of polymorphonuclear neutrophils and number of macrophages were higher in milk showing higher cell counts. More quarters of the udders of treated cows were bacteriologically infected compared to the controls before and after treatment. Small injection sites were detected on 50-60 per cent of the treated cows, which disappeared rapidly. In the third and fourth lactation, no marked differences between controls and treated cows were recorded in frequencies of diseases and culling.