PRODUCTIVE AND REPRODUCTIVE-PERFORMANCE IN CATTLE INFECTED WITH BOVINE LEUKOSIS VIRUS

Economic losses caused by enzootic bovine leukosis (EBL) have been of interest since World War II, when the neoplastic form of EEL increased dramatically in Europe. Olson (1974) and House et al. (1975) ed that animals with lymphosarcoma caused by the bovine leukosis virus (BLV) h ad reduced milk yields. a less efficient reproductive performance and high veterinary costs and mortality rates, while many carcasses were r ejected at slaughter. However, the actual impact of BLV infection in c attle without lymphosarcoma is not; clear. The purpose of the study re ported here was to compare some productive and reproductive responses of cattle that were antibody-positive (BLV+) or negative (BLV-) for BL V.Holstein dairy cows in commercial dairy farms were used in this stud y. Blood samples were collected and subjected to BLV serological exami nation 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 h istories were obtained from individual animal records and the followin g factors were considered: milk production, calving interval and birth rate. For milk production, we had the daily milk yields of 547 animal s, and for calving interval the time between two successive parturitio ns 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. Bi rth rates, the percentage of animals that calved in 1 gear, were avail able for 557 animals and were examined with the Z-two proportion test. For all analyses, P < 0.05 was considered significant.