Cull rates of dairy cattle with antibodies to bovine leukemia virus

The relationship of cull rate to age was investigated retrospectively in dairy cows with and without antibodies to bovine leukemia virus (BLV). Banked sera from eight annual herd tests on one 200-cow herd were tested for presence of BLV antibodies by agar-gel immunodiffusion using the Mr 51,000 glycoprotein antigen of BLV. Age-specific cull rates were computed for BLV-antibody-positive and antibody-negative cows yearly from 2 to 7 years of age. Cull rates, transformed by the Arc-sin square root, were analyzed by weighted regression. Transformed cull rates increased significantly as BLV-antibody-positive cows aged (one-tailed P = 0.023) but not as antibody-negative cows aged (one-tailed P = 0.59). A Mantel-Byar survival analysis showed significantly longer survival beyond 3.5 years of age among antibody-negative cows than among antibody-positive cows (P = 0.008).