EFFECT OF TUBERCULOSIS ON MILK PRODUCTION IN DAIRY COWS

OBJECTIVE: To evaluate differences in milk production between cows with positive and negative caudal fold tuberculin test results in a Mycobacterium bovis-infected dairy herd. DESIGN: Cross-sectional epidemiologic survey. ANIMALS: 369 Holstein cows with lactation duration between 200 and 360 days. PROCEDURE: The caudal fold tuberculin test was performed. Information on milk production data, parity, calving season, days of lactation, previous milk production, and whether cows had clinical mastitis was obtained from farm records. Composite milk samples were collected and submitted for bacterial culture. RESULTS: 170 cows had positive tuberculin test results, and 199 had negative results. Cows with positive test results produced less milk (mean, 347 kg [763 lb]) than did cows with negative test results after adjusting for variables biologically related to milk production. Calving season (spring, summer) was significantly associated with reduced milk production. Cows with clinical mastitis produced less milk than did cows without clinical mastitis, but the difference was not statistically significant in the multiple regression analysis. CLINICAL IMPLICATIONS: In this herd, tuberculosis was associated with a 4% decrease in milk production. Milk production losses per cow attributable to calving season (spring, summer) were 3 times those attributable to tuberculosis. However, because of the high prevalence of tuberculosis in this herd, the impact of tuberculosis on total herd milk production was an additional cause for concern.