It’s not business-as-usual when it comes to managing worms in cow-calf herds – or it shouldn’t be.

Using the same dewormer every spring, sending the cows out to the same pasture, and then deworming again in the fall can lead to worm populations that are resistant to certain classes of dewormers.

Developing a parasite management plan with a veterinarian and making adjustments as needed is a good idea, said Dr. Joe Gillespie, beef cattle professional services veterinarian for Boehringer Ingelheim.

For northern U.S. cow-calf operations, the brown stomach worm (Ostertagi ostertagi) can cause more economic hardship than any other type of roundworm, he said.

While producers would suspect worms in poor-doing cattle, they may not realize that the brown stomach worm can also hurt the immune system of cows and their calves without giving much of an outward indication.

When young cows and their calves have worms in their gastrointestinal tract, there is irritation, and with irritation, the immune system may also be impacted. Cattle may experience subclinical effects, resulting in poor performance during late gestation and early lactation.

Bulls on pasture are susceptible to worms, too.

“You want to make sure that when cattle are coming off of that pasture – where there is a risk of exposure – that you purge the entire digestive system so that you don’t have those parasites staying within the cattle through the whole winter internally causing irritation and maybe some weight loss,” Gillespie said.

Some internal parasites have developed resistance to several classes of antiparasitics. Targeted selective treatments are becoming more common as part of a vet-directed parasite management plan to fight resistance. This involves leaving a percentage of the herd untreated to maintain a population of worms that have not been exposed to a dewormer.

Another part of the plan may include a fecal egg count reduction test. Fecal samples are taken from selected animals and sent to a lab for testing. Individual results are obtained on the number of eggs and types of parasites. Those animals are treated with the dewormer, and fecal samples for each animal are taken 14 days later and tested.

This allows the veterinarian and producer to understand how the number of eggs is reduced and if the product was successful.

“Your goal should be that you’ve eliminated greater than 90% of the eggs,” Gillespie said. “If you only eliminate 60%, that’s an indicator you are building some resistance and you need to change your management plan.”

A quick way to build resistance is to treat cattle at the wrong dose. On deworming day, running the cattle across the scale is important to ensure correct dosage.

“It’s commonly found that if you try to guess the weight of cattle, you can be 20% off. ” he said.

Treatment timing is also critical – whether treating for stomach worms or external parasites like lice. Understanding the life cycle of parasites can greatly help in reducing numbers and minimizing resistance.

Local veterinarians can help determine an appropriate parasite load and the risk from parasites based on the location, he said. The herd moving to a winter pasture may need a different deworming strategy than the herd moving to a dry lot for the winter, he added.

If cattle are entering an environment where they are not going to pick up new eggs, it might be appropriate to have deworming plans that differ from the deworming plans for cattle going directly back to grass.

“Our ultimate goal is to have a cow that milks and a calf that grows,” Gillespie said. “The brown stomach worm can have enough of an impact on that complex that it can reduce production, and dollars at the end of the day.”

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