Parasite control key to making cattle profitable

They’re tiny but mighty, and able to steal profits from cattle producers while remaining invisible.

They are gastrointestinal parasites, primarily roundworms, and they can reduce cattle appetites and corresponding weaning weights if left unchecked.

“You see very little sign of them quite often, but the impact is a loss of production,” said John Gilleard, a professor of parasitology at the University of Calgary.

“It’s an invisible kind of stealer of profit.”

Though there are various kinds of cattle parasites, the lowly roundworm presents the main problem in cattle, he told those on a recent webinar organized by the Beef Cattle Research Council.

A small parasitic load is expected but control is important to avoid production losses.

“It’s actually normal for livestock to have parasites. They are part of normal gut flora and when animals are grazing they will have some level of infection,” he said. “We’re just trying to keep them down to a level where you don’t see any … production impact.”

That impact can occur because a heavy load of roundworms affects cattle digestion and absorption of nutrients, said Gilleard.

Roundworms also suppress appetite so animals reduce their grazing and gain less weight even though they don’t look sick.

Controlling roundworms has become more complex with the worms’ resistance to ivermectin and related pour-on treatments, Gilleard said. Now such treatments are only partially effective and may become useless unless they are used judiciously in the future.

“It’s only going to get worse as we continue on in this vein,” he said.

A more targeted approach is needed and the strategies involve the measures on this list:

  • grazing management
  • monitoring worm burdens
  • responsible use of dewormers
  • biosecurity
  • selective treatment

Grazing management

Gilleard said about 90 percent of the larvae are in the bottom four inches of grass. About 75 percent are in the bottom two inches, so it’s important to move cattle before the grass gets too short.

This means stocking density has a major effect on roundworm parasite levels.

Larvae will aggregate where cattle spent a lot of time, and parasites can survive for months on pasture. That means a rotational system that puts cattle in the same place within a few months won’t necessarily reduce parasite ingestion.

Monitoring worm burdens

Fecal egg counts are a proxy for learning parasite load but don’t provide a precise measure. However, doing a count on a regular basis can help producers understand the status and show whether treatment has been effective.

Gilleard recommended collecting warm, steaming feces from about 20 animals. Keep the material cool but not freezing and send it to a lab for testing.

Sampling in the fall will indicate whether any control measures have been effective, and indicate risk of heavy parasite loads in the coming year.

Responsible use of dewormers

“We need more interactions between veterinarians and producers,” said Gilleard about controlling resistance to common treatments.

Veterinarians can ensure the correct product is used on the right animal at the right dose and given at the most effective time.

There are two major drug classes to control cattle parasites. Macrocyclic lactones include ivermectin and similar products that control internal and external parasites.

Benzimidazoles work against internal roundworms only and no resistance has been found so far, said Gilleard. Using a combination of treatments can maximize effectiveness and slow the development of resistance.

Cows, stocker cattle and feedlot cattle have different needs in terms of treatment, so ensure it is right for the animal, he cautioned. Dose should be based on weight, or at least on the heaviest animal in the group being treated.

Under dosing is a risk because it promotes parasite resistance to the treatments. As for timing, Gilleard said fall ivermectin treatments “cannot be assumed to provide good roundworm control.”


Cattle new to the herd should be treated to minimize the risk of introducing new parasites. Getting fecal egg counts from introduced animals might also be useful.

Selective treatment

About 80 percent of the parasites in a herd will be within 20 percent of the animals, said Gilleard. Leaving some cattle untreated can reduce selection pressure on the treatments and slow development of resistance.

He recommends leaving 10 to 20 percent of the herd untreated, but choose the ones in best condition.

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