Cow Longevity & BLV
In the previously discussed Michigan study5 herds with a lower proportion of their cows in their 3rd or greater lactation had increased within-herd BLV prevalence. This finding led to another study6 in which 3,849 dairy cows in 112 herds were followed for a mean of 597 days after being tested for antibodies against BLV with the BLV milk ELISA. It was found that cows with antibodies against BLV were 23% more likely to die or be culled during the 597-day follow-up observation period than were BLV-negative herdmates6. As milk BLV antibody titer increased, survival probability decreased. For example, cows with the highest BLV antibody titers (BLV milk ELISA optical density results >0.50) were at 40% greater risk of dying or being culled than were cows without antibodies against BLV6. Although results of several studies6,34–39 indicate that BLV infection impairs cow longevity, this was not found by all other researchers40,41. Tiwari et al.42 reported that BLV-infected cows tend to have reduced longevity compared with uninfected cows, but this association between BLV status and longevity was not statistically significant (P <0.30) when herd and lactation number were controlled in the analysis. In a study of dairy herds in Ontario, Canada43, the cull rate for cattle seropositive for antibodies against BLV was 27% higher than the cull rate for BLV-negative cattle, although that effect was only observed in older cows (lactation number ≥3). In that study43 and the study conducted in Michigan6, the association between a cow’s BLV status and longevity was stronger for older cows than for first lactation cows.