Abstract
Lactating Holstein cows of mixed parity were randomly allocated to 4 levels of recombinant bovine somatotropin (rBST) (0, 5.2, 10.3, and 15.6 mg/day) administered into the tail fold area from early lactation (28-35 days in milk) to the end of lactation. On dairy A, 60 cows were studied for the first lactation and 43 of the same cows were studied for 2 complete lactations. On dairy B, 32 cows were studied for a single lactation, but all cows had been treated with the same dose of rBST during the previous lactation. Duplicate quarter milk samples were aseptically collected from all cows at trial entry, at 60-day intervals during the trial and at trial end or dry-off. All cases of clinical mastitis were recorded an milk samples were collected from affected quarters for microbiological analysis. Milk samples were analysed by standard microbiological precedures. Preliminary analysis of data indicated that treatment with rBST did not adversely affect incidence, prevalence and duration of subclinical intramammary infections or incidence of clinical mastitis.
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