Skip to main content
You are the owner of this article.
You have permission to edit this article.
Deworm calves, not cows in spring to keep dewormers effective
top story

Deworm calves, not cows in spring to keep dewormers effective

Slowing down resistance to dewormers is an important goal in 2021.

Already, several dewormers no longer work as well as they once did.

Dr. Joe Armstrong, DVM, University of Minnesota Cattle Productions Systems educator, recommends a strategic deworming protocol to slow down the development of resistance to anti-parasitic products. Armstrong recently proposed the following protocol at a summer management beef seminar put on by the University of Minnesota Beef Team.

1) Leave your mature cows untreated for parasites year-round. Mature cows can serve as a reservoir of non-resistant parasites that will out-compete resistant parasites.

2) Use an extended duration (120-150 days) injectable dewormer at spring turnout on calves and first and second calf heifers OR turn your herd out for 30-45 days and then bring the herd back in to treat your calves and first and second calf heifers with a “white” dewormer or pour-on dewormer product.

“We need to go to a strategic deworming model where we really are carefully picking who we treat, and we’re timing our treatments effectively to stop the development or slow down the development of resistance,” Armstrong said.

Calves need deworming because they have immature immune systems. But cows shouldn’t need deworming. Cows represent 30-40 percent of a cow/calf herd, and not worming them allows cows to serve as a non-resistant reservoir for worms/parasites.

“It is my opinion that cows and parasites have evolved together, and they should be able to co-exist,” he said. “Cows should be able to handle a low level of parasitism, and it shouldn’t affect their body condition, and it shouldn’t affect their performance.”

He suggests this deworming model also works for sheep and goats. If any of the mature ruminant farm animals can’t cope with a low level of parasites, then they should be culled.

When it comes to a white dewormer, Armstrong said there are many different brands. Take your location into consideration – are liver flukes a worry? Consult with your veterinarian on the best product for your situation.

“There’s research to show that the generic brands perform worse than name-brand products,” he warned. “I would consult with your veterinarian, and ask them which products provide the best value.”

“Internal parasites may cause reduced animal health, decreased rate of gain, and decreased responses to vaccines,” said Dana Adams, University of Minnesota Extension educator in Stearns, Benton, and Morrison counties.

A 2007 study on the economic impact of internal roundworm parasites in North America gauged the relative impact of various control methods.

“What they found was that deworming impacted cow/calf pair performance by increasing the weaning rate by 23 percent. In feedlots, deworming improved average daily gain by 5.6 percent and improved feed-to-gain ratio by 4 percent, which is second only to implants,” Adams said.

The parasites that cause the greatest damage are known as the HOT complex – Haemonchus, Ostertagia, and Trichostrongylus. These three groups of parasites cause economic loss in the billions of dollars annually to the cattle industry.

The parasites begin as eggs that pass through the feces and develop into larvae in about 6 days. The larvae then move up as high as they can on blades of grass. The ruminant eats the larvae, and the adult worms attach to the ruminant’s abomasum. The cycle repeats with the parasites living off of their ruminant host.

“Haoemonchus and Trichostrongylus can overwinter in the animals, but most often cannot overwinter in Minnesota’s pastures,” she said.

Ostertagia can overwinter in the cow, and it can also overwinter in the pasture.

“Ostertagia can be the focus of our control program, because it’s the most difficult of the HOT complex to control,” she said.

Unfortunately, the common medicines for controlling parasites may not be effective. Products that were 99.9 percent effective when they were introduced are now only 90-95 percent effective at best.

“This has nothing to do with the product. It is the development of resistance by those internal roundworm parasites,” Adams said.

With the rising of resistance, parasites will cause more problems and hurt ruminant production.

Parasite control in 2021 and beyond needs to focus on controlling overwintering Ostertagia in the months of April-early July. If these worms can be controlled in the spring in the calves, the number of eggs produced later in the season will be dramatically reduced.

“We really need to be concerned about the parasites that are reemerging from their overwintering time,” Armstrong said. “There’s a big difference between cows and calves. Mature cows are not the amplifiers of the situation. Calves are just pure parasite factories – so they kick out a ton of eggs to amplify parasite numbers.

“If we can target a specific time period and treat only the animals that need treatment, we can effectively control parasite load while maintaining the effectiveness of our anti-parasitic products for as long as possible.

Armstrong noted that this protocol can be so effective in some systems that producers may not have to treat for internal parasites in the fall.

“The exception to a strategic deworming model is when we have to deal with liver flukes, sometimes we can’t avoid treating everyone when cattle are at high risk of liver fluke infestation.”

Ideally, Armstrong said you should contact your veterinarian to set up a protocol, but he said he is available for anyone who has questions at

Minnesota Farm Guide Weekly Update

* I understand and agree that registration on or use of this site constitutes agreement to its user agreement and privacy policy.

Related to this story

Most Popular

Find the equipment you're looking for

Get up-to-the-minute news sent straight to your device.


Breaking News