Milk and fat production in dairy cattle influenced by advanced subclinical bovine leukemia virus infection.

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Abstract
Genetic potentials (pedigree-estimated breeding value) for milk and for fat were compared in cows grouped according to subclinical stage of bovine leukemia virus infection. Genetic potential for milk production was significantly greater in seropositive cows with persistent lymphocytosis (662 .+-. 72 kg) and in seropositive hematologically normal cows (554 .+-. 34 kg) than in seronegative herdmates (418 .+-. 53 kg). When 305-day twice-daily-milking mature equivalent milk production records for the current lactation were adjusted for genetic potential, bovine leukemia virus-infected cows that were hematologically normal had significantly greater milk production than did seronegative herdmates, suggesting that early bovine leukemia virus infection was positively associated with milk yield. Genetic potential for fat production was significantly greater for cows with persistent lymphocytosis (21 .+-. 2 kg) than for other seropositive (16 .+-. 1 kg) and seronegative herdmates (13 .+-. 2 kg); however, 305-day twice-daily-milking mature equivalent fat production for the current lactation was not significantly different between the groups. Thus, cows with persistent lymphocytosis did not produce fat according to their genetic potential. As an apparent consequence of tendencies for greater milk yield and less fat production, milk fat percentage was significantly reduced in cows with persistent lymphocytosis (3.33 .+-. 0.09%) and other seropositive cows (3.48 .+-. 0.05%) relative to seronegative herdmates (3.67 .+-. 0.07%). These results suggest a need to reevaluate the economic impact of bovine leukemia virus infection on the dairy industry.

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