Influence of Vaccination with a Combined Chemically Altered/Inactivated Bhv-1/BVD Vaccine or a Modified Live Vaccine on Reproductive Performance in Beef Cows and Heifers

Previous research has indicated that vaccination with a modified live BHV-1/BVD vaccine (MLV) has negative effects on AI conception rates when performed 30 d prior to AI. To further investigate vaccine effects on reproductive performance, a field trial was conducted on 10 herds of well-vaccinated cows and heifers (n=1565; all of which had been on a MLV vaccination program) to evaluate whether a MLV vaccine administered prebreeding would have negative impacts on conception rates compared to a combined chemically altered/inactivated BHV-1/BVD vaccine (CA/IV). Within herd, cows were blocked by parity and calving date and randomly assigned to receive one of the two treatments (MLV or CA/IV vaccine). Animals received a single (cows) or two injections (heifers) of either the MLV or CA/IV vaccine with the final injection occurring between 27 and 89 d prebreeding. All females were synchronized with the 7-d CO-Synch + CIDR protocol and inseminated (AI) at the appropriate time after CIDR removal (cows 60 to 66 hrs; heifers 52 to 56 hrs) and remained separated from bulls for at least 10 d after AI. Pregnancy success and fetal age were determined between d 34 and 86 after AI, and > 30 d after the breeding season. Data were analyzed using the GLIMMIX procedure in SAS with herd as a random variable. Conception rates to AI were greater in the CA/IV vaccine group compared to the MLV vaccine group (P = 0.05; 60% vs 52%). Furthermore, interval from vaccination with either vaccine until AI also influenced conception rates (P = 0.02). Animals vaccinated 27 to 30 d prebreeding and animals vaccinated 30 to 37 d prebreeding had similar (P = 0.98; 52% and 52%) conception rates; however, both were decreased compared to animals vaccinated 38 to 89 d prebreeding (P < 0.03; 64%). There was no treatment by interval interaction (P = 0.79). Furthermore, there was no effect of treatment (P = 0.18) or treatment by interval interaction (P = 0.17) on breeding season pregnancy rates. Days postpartum tended (P = 0.07) to influence conception rates with short postpartum cows (≤ 60 d) having decreased conception rates compared to cows that were further postpartum. In summary, vaccination of well-vaccinated beef cows and heifers with a MLV vaccine pre-breeding (28 to 89 d) decreased AI conception rates compared to a CA/IV vaccine.