Milk Production & BLV
The 1996 USDA National Animal Health Monitoring System dairy study32 determined that 95 kg of milk/cow/year was lost for each 10% increase in the within-herd BLV prevalence. A Michigan study5 reported similar herd-level production losses associated with BLV. Compared with young cows, older cows tend to produce more milk and are also more likely to be infected with BLV5,6,30,33,34. This complicates attempts to determine the true impact of BLV on milk production. Also, some studies5,34 have found that BLV-infected cows produce as much or more milk than do their uninfected herd mates until the lactation during which their immune systems become substantially compromised, often resulting in culling before their 305-day mature equivalent milk production decreases. Because BLV infection adversely affects cow longevity, dairy herds with a high BLV prevalence tend to have a low mean cow age because the cows do not remain in the herd very long5,6. This tangled web of associations may at least partially explain why the effect of BLV on dairy productivity has been so long unrecognized and underappreciated. Analysis of the Michigan database8 used a 2-level hierarchical model with lactation number and herd (random effect), and the results support the results of others in finding a significant negative association between BLV-infected cows and milk production.