Preparing cows for a successful breeding season is critical for a cow/calf producer’s financial bottom-line. Two considerations often come to the top of the list when thinking about management strategies for setting the cowherd up for high pregnancy rates: 1) nutrition and 2) moving late calving cows up in the breeding season. Both of these topics have been recently covered in iGrow articles. Ken Olson’s article on nutrition from calving to breeding provides key nutritional management tips.

The two articles that address management of moving late calving cows up in the breeding season provides some potential management opportunities that have been effective in decreasing the length of the anestrus period. The first article is by Robin Salverson and the second article is by George Perry. There are several progestin protocols outlined in the 2016 Beef Cow Protocols developed by the Beef Reproductive Task Force that can be used for estrous synchronization with artificial insemination or to simply induce cyclicity in anestrous cows for natural service.

Vaccination considerations for reproductive pathogens

However, another management consideration is vaccinating cows for reproductive pathogens. Some producers elect to vaccinate cows (pregnant) during pregnancy testing due to management and labor constraints. For reasons that Russ Daly discussed about the impact of vaccinating pregnant cows with killed versus modified live viral vaccines, other producers utilize vaccinations in spring, prior to breeding time. It should be noted that the most common reproductive vaccine labels indicate administration to non-pregnant animals prior to the breeding season.

Vaccination Type

Does vaccination type (killed or modified live viral reproductive vaccines) have any impact on non-pregnant cows? Based on recent research the answer may be yes. Nine herds of well-vaccinated cows and heifers (n = 1,436) were given either inactivated (killed) reproductive vaccines at 30 and 60 days pre-breeding or modified-live viral (MLV) reproductive vaccines 30 days prior to breeding (Perry et al., 2016). Conception rates to AI tended to differ between killed and MLV groups (Table 1). Pregnancy success at 56 days after AI and overall breeding season was greater for animals that received the killed vaccine compared to animals that received the MLV vaccine (Table 1). When looking at calving distribution there tended to be a greater proportion of calves born between day 1 to 12 and 1 to 30 for the females that received the killed vaccine compared to the MLV vaccine.

Table 1. Influence of vaccine type on conception rates to AI and pregnancy success.


Inactivated vaccine

Modified Live Viral vaccine

Conception rates to AI



Pregnancy rates



At 56 days after AI



Breeding season




It should be noted that the magnitude of the differences from conception rates to AI to breeding season pregnancy rate decreased. This may be explained by the length of time from vaccination until conception. Results from a recent study in dairy cattle indicated no difference between inactivated and modified live vaccines when well-vaccinated cows were vaccinated at 45 days prebreeding (Walz et al., 2015). Results from another research study reported naïve heifers given the MLV vaccine had a greater percentage of abnormal cycles (38%) compared with the females given killed vaccine (one dose 14%, two doses 10%) (Perry et al., 2013). Of the heifers with an abnormal estrous cycle, 100% of heifers given the killed 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).

Impact on cow/calf operations

The type of vaccine could potentially impact a cow/calf operation’s number of calves born as well as the calving distribution. According to CHAPS 2000 benchmark data from 2010 - 2014, average pregnancy rates are 93.1%. So the questions becomes would producers notice the differences between these two vaccine types. However, getting three more calves per hundred cows would have a positive impact. Additionally, the benefit of older calves have been reported numerous time, so more calves from the first estrus cycle of the breeding season should improve the revenue from the calf crop. Assuming 2 pounds per day gain, older calves (one cycle = 21 days) would result in 42 more pounds to sell.

Questions about your specific vaccination questions – especially when changes to existing programs are considered – should be discussed with your veterinarian.