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Virginia Cooperative Extension -
 Knowledge for the CommonWealth

BST and Mastitis

Dairy Pipeline: March 1998

by G. M. Jones
Extension Dairy Scientist, Milk Quality and Milking Management
Virginia Tech 540-231-4764

According to Michigan State University scientists, the approval of bST has caused considerable scientific and public controversy over potential effects that this product might have on the incidence of clinical mastitis and subsequent antibiotic use resulting from therapy. Of 9 trials in which bST was administered every 14 days, 7 studies concluded that the incidence of clinical mastitis did not differ between cows that were administered bST and untreated control cows. In 2 trials, a significant increase in the incidence of clinical mastitis was reported for bST treated cows. In a review of 15 full lactation studies, the rate of clinical mastitis was significantly higher for cows that had been treated with bST even when cows that became infected with mastitis during the pretreatment period were excluded. The "review concluded that bST does not alter the normal relationship (positive correlation) that exists between milk production and mastitis". These researchers reported their results in the Journal of Dairy Science of a trial using 3 commercial dairy herds and 1 university herd to determine the effect of bST on the incidence of clinical mastitis, including number of days milk was discarded for mastitis or cows culled for mastitis. The four herds had mean bulk tank somatic cell counts less than 300,000 for 12 months before trial. Herds practiced pre and post-milking teat dipping and dry cow therapy. All cows calving in one week received bST. All cows calving during the next week were untreated controls. BST was administered every 14 days starting nine weeks into lactation. Of 555 cows, 127 cases of clinical mastitis occurred during the test period before bST (50.4% in control cows and 49.6% in treated cows). During the trial, there were 85 clinical mastitis cases: 19 repeat cases in the same cows (8 cases in 7 control cows and 11 cases in 7 treated cows); 40 in control and 45 in treated cows. Rates of clinical mastitis (cases per 100 cow-months) were: 3.90 and 2.81 for control and treated cows during pretreatment, and 1.52 and 1.85 during treatment. Milk samples were cultured from 82% of the cows. Infections by major pathogens were 15 (5.5%) for control and 14 (4.9%) in treated cows. Cows treated with bST were 6% more likely to have an occurrence of clinical mastitis than were those that were not treated. Discarded milk did not differ between treatments. Culling for mastitis was 11.3% in control group cows and 15.2% in bST treated group. No differences were found for any measured feature of clinical mastitis. This study was conducted on well-managed farms that had controlled contagious mastitis and also had experienced relatively low rates of clinical mastitis caused by environmental organisms. The authors concluded that bST was not associated with an increase in the incidence of clinical mastitis, milk discarded because of therapy for clinical mastitis, or culling for mastitis.

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