RECOMBINANT BOVINE SOMATOTROPIN AND CLINICAL MASTITIS - INCIDENCE, DISCARDED MILK FOLLOWING THERAPY, AND CULLING

Holstein cows (n = 555) from four Michigan dairy farms were randomly a ssigned to receive bovine somatotropin (bST) or to serve as untreated controls. Bovine somatotropin (500 mg) was administered every 14 d beg inning at 63 to 69 d of lactation and continuing until approximately 2 1 d prior to dry-off or until the cow was removed from the herd. Trial objectives were to determine the effect of bST on the incidence of cl inical mastitis, number of days that milk was discarded because of the rapy for clinical mastitis, and culling for mastitis. A total of 127 ( 22.9%) cases of clinical mastitis occurred during lactation, In the pr etrial period (before 63 to 69 d of lactation), 42 (33.1%) cases occur red, and 85 (66.9%) cases occurred during the trial. Of the 42 pretria l cases, 57.1% occurred in control cows, and 42.9% occurred in treated cows. Of the 85 trial cases, 47.1% occurred in control cows, and 52.9 % occurred in treated cows. Using logistic regression, the odds ratio for the occurrence of clinical mastitis in treated cows was 1.06 (95% confidence interval = 0.62 to 1.81), The number of days that milk was discarded following therapy for clinical mastitis and the culling rate for mastitis did not differ between study groups.