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Milk yield and reproductive performance of Holstein cows seropositive for tuberculosis
Bovine tuberculosis is a critical, debilitating and chronic disease of dairy cattle in intensive systems. Studies done in the past have not completely characterized the impact of this disease on productivity of cows. Therefore, the objective of this study was to find out if seropositivity for tuberculosis impairs reproductive performance and milk yield in high milk-yielding Holstein cows. For this purpose 1,044 healthy cows and 105 Holstein cows seropositive for tuberculosis were used. Cows tested positive for bovine tuberculosis were from various large commercial dairy operations from the same region. Cows that reacted to an intradermal injection of tuberculin were transferred from their barns to an isolated new dairy facility. Cows free from this disease were placed in the same barn as the seropositive cows, but in an isolated division and served as control animals. The GENMOD, npar1way and LIFETEST procedures of SAS (SAS Inst., Inc., Cary, NC, USA) were used to assess the significance of treatment on reproductive variables. The reproductive performance of positive reactors was impaired; overall pregnancy per artificial insemination differed (P<0.05) between seropositive and healthy cows (16.9 vs. 20.7). Seropositive animals required 4.52 ± 2.94 services per pregnancy compared with 4.34 ± 2.72 for control cows. The intervals between calving and conception were similar between seropositive (154 ± 78 days) and seronegative animals (150 ± 77 days). Control cows tended (P =0.08) to produce more milk than seropositive cows over a 305-day lactation (10,684 ± 1,720 vs. 10,345 ± 1,736; 3 milkings per day, mean ± SD). It was concluded that cows tested positive for bovine tuberculosis exert a mild but significant negative effect on both reproductive performance and milk yield.
Keywords: tuberculosis, bovine and milk