Control parasites now to protect performance later

Internal parasites have the potential to dramatically reduce performance in cattle. Parasite infestation increases disease, reduces milk production, negatively impacts reproduction and hurts feed conversion.

Some herds are more at risk of parasite infestations. While worms can be a problem in all operations, cattle on pasture are more likely to be exposed than animals on dry lots or in total confinement. It is vital to treat internal parasites. The health, efficiency and economic considerations make timely treatment the sensible choice.

However, if deworming is attempted without proper testing, or if it is done at the wrong time, it can lead to resistant parasites. A good practice is to use a fecal egg count prior to treatment. Not only will this test show you if treatment is warranted, it will also be an important source of information afterward to evaluate the efficacy of the treatment.

To gauge the parasite load of your herd, collect fecal samples from the same cohort group prior to treatment and send those to a lab. You need at least 20 fecal samples to get a good look at the level of parasitism. Collect and analyze another 20 samples 14 days after treatment to ensure it worked. MFA livestock sales managers can advise producers about collecting and submitting these samples for testing.

An effective anthelmintic dewormer should provide at least 90% reduction in fecal egg counts post-treatment. Unfortunately, certain classes of dewormers, such as piperzine, are less effective than they used to be, and some parasites aren’t as sensitive to treatment. That’s a key reason why the correct dose should be used appropriately. Dewormer products that are diluted or used at improper dosages are less effective. The most effective way to control internal parasites and prevent resistant worms is to treat animals with two or more classes of dewormers at the same time.

Fall-calving cows should be dewormed as should any cattle on heavily infected pastures. Deworming young stock, including weaned calves, replacement females and yearling bulls, is important in the fall as animals younger than 2 years old are much more susceptible to parasitism.

In the winter, pastured cattle are typically parasite free, though this depends on many factors, such as location and weather. If there are concerns about parasites, consult with your herd veterinarian.

Before turnout in the spring, treat animals to prevent parasite populations from rising during the summer. Deworming replacement heifers is critical. They can enter reproduction sooner and be more productive if they are free from parasites. Give replacements two successive treatments of dewormers 30 days apart. Treating them in this manner helps prevent the animals from shedding parasite eggs back into the pasture for the first 90 days of grazing. This improves the whole grazing season by reducing overall parasite contamination of the pasture. Effective, timely and consistent treatment year after year can have increasing economic benefits.

In addition to testing and deworming animals, it is a good idea to leave some animals untreated. While this sounds counterintuitive, there is strategy behind this advice. Not all parasites in treated animals die, so if you treat every animal with dewormers, the only parasites that remain can develop resistance, a selection pressure effect. As these resistant parasites reproduce, their offspring also have genetic resistance, which means you can rapidly lose the ability to treat animals with anthelmintics.

Leaving some of the animals untreated keeps some susceptible-to-anthelmintics parasites in the gene pool. This practice, known as refugia, helps keep dewormers effective much longer. It is the same approach that is used with certain stacked traits in corn hybrids, such as Bt corn.

Deworming strategically can help animals, herd production and profitability. Visit with your MFA solutions provider for recommendations on the best animal health program for your herd. 

View more articles from this October/November 2025 Today's Farmer magazine.