Economic losses caused by enzootic bovine leukosis (EBL) have been of interest since World War II, when the neoplastic form of EBL increased dramatically in Europe. Olson (1974) and House et al. (1975) showed that animals with lymphosarcoma caused by the bovine leukosis virus (BLV) had reduced milk yields, a less efficient reproductive performance and high veterinary costs and mortality rates, while many carcasses were rejected at slaughter. However, the actual impact of BLV infection in cattle without lymphosarcoma is not clear. The purpose of the study reported here was to compare some productive and reproductive responses of cattle that were antibody-positive (BLV+) or negative (BLV−) for BLV.

Holstein dairy cows in commercial dairy farms were used in this study. Blood samples were collected and subjected to BLV serological examination by the agar gel immunodiffusion test of Miller & van der Maaten (1976). Animals were then grouped as BLV+ or BLV− according to their serological response to the BLV antigen. Productive and reproductive histories were obtained from individual animal records and the following factors were considered: milk production, calving interval and birth rate. For milk production, we had the daily milk yields of 547 animals, and for calving interval the time between two successive parturitions for 444 cows. These values were examined by ANOVA and when this was significant a Student's t test was carried out for each age group. Birth rates, the percentage of animals that calved in 1 year, were available for 557 animals and were examined with the Z-two proportion test. For all analyses, P<0·05 was considered significant.