Out in the Field: Balancing Immunity and Fertility in Beef Herds

Balancing Immunity and Fertility in Beef Herds

By Lee Jones, DVM, MC, UGA College of Veterinary Medicine

For cow-calf producers, fertility is the highest priority for their herds. Of course, weaning a healthy calf is required for reproductive efficiency, too. It doesn’t matter when a cow loses her calf—if she doesn’t wean a live calf, reproduction didn’t happen and there’s nothing to sell. And while fertility is the top priority for cattle owners, for the cow, fertility is a luxury that only comes after all other metabolic needs are met.

That’s why when veterinarians make herd health recommendations, optimizing fertility is a high priority. However, these recommendations must also consider ranch and season prevention of infectious disease through biosecurity and disease prevention.

One part of a disease prevention program is the use of vaccines to stimulate immunity or disease resistance for individual cows, as well as the herd. Vaccines protect fertility of the cow herd, enhance colostrum quality by increasing immunoglobin G levels in the cow and colostrum and increase resistance in calves to calfhood diseases. Vaccine selection and timing in the cow herd can help determine what vaccines are used in calves. How producers market calves is an important consideration when designing a vaccination program. While vaccines are important, nutrition and biosecurity are the first two lines of defense in a prevention program.

Vaccines are used to increase resistance to common diseases in cow herds. They do this by stimulating the adaptive or acquired immune system which includes the cell mediated immune system (cells like T lymphocytes) and the humoral immune system that makes antibodies. Vaccines train the acquired immune system to quickly recognize the disease agents and eliminate them before severe disease occurs. The innate or natural immune system is required to activate the adaptive system. The innate immune system is dependent on adequate nutrition for it to function well. It’s important to understand that vaccines will not work in immunocompromised animals. That includes animals that are sick, parasitized or malnourished. Vaccines only work in healthy animals. In short, the animal makes the vaccine work.

Not all animals respond equally to vaccines. In fact, for some yet unknown reason even some healthy looking animals don’t respond to vaccines every time. Studies have shown that in some cases as few as half of the animals might respond. It is important to follow the label directions when giving vaccines. There are three important factors in vaccine efficacy—the vaccines, animals receiving and humans giving them.

Vaccines have to be stored, transported, handled and administered correctly. Not all vaccines work the same, so responses will likely be different in each animal and on each farm. Vaccines have antigens (things that stimulate the immune response ) and adjuvants (things that make the response better). Proper handling and administration are essential. Producers are reminded to use the right equipment and follow label recommendation and adhere to Beef Quality Assurance principles for the best response to their vaccination programs.

Nutrition plays a key role in vaccine response and immunity in cattle and all living things. Researchers at University of Georgia (UGA) saw a large vaccine response difference in calves supplemented with a trace mineral injection versus those that didn’t get it. This tells us that calves need to be on a good mineral program as well to make sure vaccines achieve their purpose. Even in humans, serum mineral levels have been shown to effect response to the fly vaccine.

Stress also affects vaccine response. In another study from UGA researchers, calves that went through a sale barn had a lower immune response to vaccines than calves that were weaned on a farm. Ranchers are advised to use low stress handling principles and update facilities to reduce stress in cattle when giving vaccines or completing other herd health procedures.

If animals don’t respond to vaccines, they remain naïve and susceptible to disease exposure and a source of transmission to other animals, so managing cattle to ensure optimum response is critical.

Viral vaccines come in two types: killed viral (KV) and modified live virus (MLV). MLV typically produce a faster, stronger, longer and more vigorous immune response in a single dose than a KV. KV vaccines require a first dose followed weeks later by a booster dose to “remind” the immune system. The second dose is called the anamnestic

response (“an” means no and “amnestic” means amnesia or forget). While we often used modified live in calves prior to the 2000s, we were restricted to using KV in cows and couldn’t use the MLV in calves while they were nursing pregnant cows for fear that the that the MLV vaccines virus might be shed from vaccinated calves to pregnant cows. In 2003, the first MLV vaccine was approved for use in pregnant cows.

The effectiveness of all vaccines depends on managing cattle to be in good health and condition at the time of vaccination. The immune system uses a lot of nutrients during response to vaccines. This creates competition with other functions like lactations and fertility. Vaccines, especially MLV, can cause short-term infertility, so it is highly recommended to give it at least 30 days prior to breeding…45-60 days is even better. This is problematic though for cows after calving. It means giving the vaccine three to four weeks after calving which means that cows are in a negative energy balance and may not achieve an optimum response to the vaccine. While some studies have shown a short term negative effect on fertility, others showed no direct impact whatsoever on fertility when using a MLV BoHV before breeding. The effect may be situation dependent. Several studies show benefits when used in heifers. In nearly all studies in this review, there was a slight advantage to overall season pregnancy rate to using the MLV. But again, it needs to be given at least 30-45 days prior to breeding, which makes it a problem in cows after calving.

So the question becomes: If KV may not provide the best protection and using MLV may have risks, what is the best approach to vaccinating cows?

A study from Auburn University challenged cows vaccinated with a combination KV or MLV. First, the heifers were vaccinated with two doses of MLV, then later with either KV or MLV then severely challenged with BVD and BoHV. The cows that had the combination had better protection than ones that only got MLV, so using KV vaccines after starting with MLV may be a viable option. In another study from 2002, research showed that both KV and MLV in a calf vaccination program provided higher antibody responses than only using one or the other. Using MLV followed by KV or visa versa resulted in a superior response at 42 and 63 days after vaccination for both BVD and BoHV, so using the combination may be the best approach in herd programs.

Since the label recommends not using in calves nursing pregnant cows that haven’t received the vaccine in the last 12 months, what is the best approach? Do MLV shed virus in nursing calves? The short answer is “yes,” but not likely and only under certain conditions, like severe stress. Research has shown stress can cause the virus to circulate longer in calves so low stress handling and adequate nutrition are essential. If cows have been adequately vaccinated and calves have little likelihood of shedding virus, then KV can be given to cows at pregnancy checking. This has the added advantage of being safe, effective and enhances colostrum for the next calf.

Another approach may also be to use two to three doses of MLV in heifers prior to breeding and then switching to KV vaccines thereafter. Producers are advised to work with their veterinarians to design an effective vaccination program and manage cows to optimize the vaccination response. FF

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