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  • Megan Silveira, associate editor

The Why, What and When of Deworming Cattle

Three basic areas of concern about deworming cattle explained by a veterinarian.

“If you don’t look for it, you won’t find it,” said Megan Schnur, technical service veterinarian for Norbrook. “And I think that oftentimes, people forget to look for parasites.”

To fight parasites negatively affecting a cow herd, Schnur said producers need to integrate deworming. Schnur identified the basics of deworming — the why, the what and the when.


In addition to being a key part of proper livestock management, managing parasites can help your operation reach new levels of profitability, said Sonja Swiger, Texas A&M University associate professor and veterinary and medical entomologist extension specialist.

“Parasitic diseases will impair the health, reproduction, growth and productivity of cattle,” Swiger explained.

Producers expect their cattle perform to a high standard, and to meet that Schnur said solutions like dewormers are needed.


Schnur said the key is to use “the right drug for the right bug.” Once a parasite is identified, then the proper mode of action can be taken. She suggests working with a local veterinarian or extension agent to identify the pest at hand and a dewormer that will best target the parasite.

Dewormers come in two categories.

Schnur identified the first as benzimidazoles, which are commonly known as white dewormers. These tend to be oral and act in the animal’s gut. Schnur said these dewormers kill parasites immediately and have no long-term effects on the pests.

The second category, Schnur said, are macrocyclic lactones, or ivermectins. These dewormers offer extended effects on specific parasites.

Schnur recommends rotating dewormers, but the key is to switch between classes rather than just the brand name. There is also the option of concurrent or combination deworming, where more than one class of dewormer is administered at the same time.

“We’re increasing efficacy of that treatment because we’re broadening our spectrum with those two different modes of action,” Schnur said. “Doing this has a lot of benefits. The data would suggest it also helps to slow the resistance of internal parasites over time as well.”


The most fluid of these answers, Schnur said many factors can affect when a producer should deworm their herd. Personal safety, labor cost and other practical realities of herdsmanship contribute to the timeline.

More often than not, however, Schnur said producers find a similar rule of thumb for distribution.

“In general, we treat cattle when we work them,” she explained.

Those spring and fall seasons when livestock are going through chutes serves as the best time for treatment. While that’s convenient for most, Schnur added there’s more to the thought process than being in the right place at the right time.

During the spring, Schnur said a dewormer can help treat the animals before the start of an infection, and a follow-up again in the fall helps clean up whatever remaining issues with parasites still exist.

Editor’s note: Schnur presented during an online webinar, “Don’t Bug Me: Pest Control,” hosted by the National Cattlemen’s Beef Association (NCBA).

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