Diseases in beef cattle populations can have a negative influence on reproductive performance. Immunological vaccine science advancements have and are reducing the negative affect of disease on reproductive performance. Synchronizing a group of heifers and/or cows has a positive cost:benefit ratio. Therefore, management procedures that have the potential to negatively impact pregnancy rates are to be discouraged. Research has shown administration of infectious disease vaccines for Bovine Viral Diarrhea virus (BVD) and Infectious Bovine Rhinotracheitis (IBR) effectively reduces these diseases in beef cattle populations. BVD virus manifestation can cause significant reproductive harm, when bulls become infected, by reducing semen quality and as a reservoir of infection in the breeding herd. BVD virus also causes early embryonic death and abortion. Fetuses that become infected in utero may die in utero, be born weak, or are born normal, but persistently infected (PI) with the virus, and shed the virus infecting other animals in their contemporary group. PI born calves have high death rates during their first year of life. Calves born infected are generally small, lack vigor, and do not grow. Infections from IBR (Bovine Herpesvirus 1) are identified with illness connected to the upper respiratory tract including rhinitis, tracheitis, and reproductively associated with embryonic death, abortion, stillborn dead calves, and calves born weak with no vigor. Clearly, unvaccinated cattle infected with either of these infectious illnesses can have significant economic loss.
Research has shown timing of vaccinations for these diseases in close proximity to a synchronized fixed-time AI procedure can affect conception and pregnancy. Vaccination timing among naïve heifers (not previously vaccinated) is critical to ensure that ovarian corpus luteum development after ovulation is not impaired. Van der Maaten and Miller (1985) and Smith et al. (1990) hypothesized that when virus enters the ovarian dominant follicle the corpus luteum may not develop correctly resulting in reduced conception and pregnancy.
Artificial insemination protocols are labor intense; therefore, whenever a trip through the chute can be avoided labor can be focused elsewhere. Therefore, producers commonly vaccinated heifers at the same time intrauterine progesterone CIDR implants were being installed. The practice, while labor saving, sets some naïve heifers in a synchronized breeding group up to have abnormal estrous cycles, when vaccinated with a modified live virus vaccine (MLV). Perry et al. (2013), compared naïve unvaccinated control heifers to heifers vaccinated with inactivated virus vaccine (IVV) and those vaccinated with a MLV 36 and 8 days before fixed-time AI. There were 10 and 14% abnormal cycles among the control and IVV heifers, but 38% abnormal estrous cycles among heifers receiving the MLV vaccine. The second or return cycle of heifers that had abnormal estrous cycles in the control and IVV groups had 100% conception rates. Of the heifers that had received a MLV vaccine, the return conception rate was 38%.
Perry et al. (2016), in an effort to further evaluate vaccine administration timing, IVV and MLV vaccines were administered 30 days before the synchronized breeding season started. In this research study, 1436 females from 9 herds received either saline (unvaccinated control group), MLV, or IVV vaccine. AI conception rates for the saline, MLV, and IVV were 43.5, 40.0, and 46.5%, respectively, indicating that vaccination 30 days before the synchronized time AI breeding was not enough time to reduce the negative effect of MLV on AI conception.
Perry et al. (2017), in a follow-up study, used 1565 females from 10 herds to compare vaccination timing intervals before the synchronized fixed-time AI breedings. Comparing chemically altered/inactivated vaccine (CA/IV) to MLV administered 27 to 89 days before fixed-time AI, the inactivated vaccine had greater conception rates (60% vs. 52%). For the other time intervals compared to the MLV, 27 to 30 days and 30 to 37 days had similar conception rates of 52%. However, when vaccination occurred 38 to 89 days from the fixed-time AI breeding, the CA/IV conception rate increased to 64%.
Variable results have been reported when comparing investigations evaluating vaccination timing prior to the start of a synchronized fixed-time AI. Recently, Stewart et al. (2023), evaluated the effect on reproductive performance when commercially available inactivated (IV) and MLV vaccines were administered to lactating beef cows 10 days prior to breeding (7 day – CO-Synch + CIDR). The study used 2,138 previously vaccinated cows from 14 cooperating herds in Virginia. The cows used in this study were from fall and spring breeding herds. For the fall breeding, AI pregnancy rates among cows receiving the MLV were greater (54% vs. 46%), which is different from previously reported studies. Conversely, results for the spring breeding herds were similar and did not differ (48% vs. 49%).
Replacing bulls with AI requires strict attention to a wide variety of environmental and management factors and vaccine administration is just one of the many factors. Although reproductive performance results are conflicting between modified live and killed vaccines, there is a large body of data suggesting that MLV vaccines for BVD and IBR can negatively affect reproductive performance, when administered at the start of a fixed-time synchronized AI protocol. The current management recommendation is to avoid giving MLV vaccinations at the same time synchronization protocols are started and, since MLV are more effective, these should be given at a minimum of 45 days and preferably 90 days before the start of any synchronized fixed-time AI protocol. Citations in this article are available upon request.
