Crossbred beef heifers (N = 59) were vaccinated at the time of synchronization/breeding with either a commercially available bovine herpesvirus type 1 modified live virus (MLV) (one dose) or inactivated virus vaccine (one or two doses). The estrus cycle was synchronized at vaccination and heifers were artificially inseminated 8 days (one dose) or 36 days (two dose) after initial vaccination. Pregnancy rates were greater for control heifers (90%; P = 0.02) and heifers given the inactivated virus vaccine (one dose: 86%; P = 0.08; or two: 90%; P < 0.01) than those given the MLV vaccine (48%). No control heifers experienced an abnormal estrous cycle, whereas only two (two dose; 2/21) and one (one dose; 1/7) heifers in the inactive virus groups had abnormal estrous cycles and were similar to control (P > 0.10). Heifers given the MLV vaccine had a greater (P = 0.02) percentage of abnormal estrous cycles (38%; 8/21) compared with the control and inactivated groups. Of the heifers with an abnormal estrous cycle, 100% of heifers given the inactivated vaccine (one or two dose) conceived at their return estrus, whereas only 38% of heifers given the MLV vaccine conceived at their return estrus (P > 0.10). During the synchronization period, concentrations of estrogen were greater (P < 0.01) in the control and the two-dose inactivated group compared with the MLV group. After AI, progesterone concentrations were greater (P < 0.01) in control heifers compared with the inactivated and MLV groups, but were similar (P ≥ 0.18) between the inactivated and MLV groups. Therefore, naïve heifers vaccinated with the inactivated vaccine were less likely to have an abnormal estrous cycle and had significantly higher pregnancy rates compared with heifers vaccinated with the MLV vaccine. In summary, vaccination of naïve heifers with an MLV vaccine at the start of a fixed-time AI protocol had a negative effect on pregnancy success.
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