Misapplication Drives Resistance
Published: Friday, September 25, 2026
The biggest problem in deworming livestock right now is parasiticide resistance. That's according to Chris Grimm of Boehringer Ingleheim, the featured speaker at the Kosciusko County Cattlemen's Assn. Twilight Tour in Leesburg last Tuesday.
"If we're having resistance from one product within a class, we're going to build resistance across every product in that class," he said.
Resistance, he said, is a global issue. He gave examples dating back to 2004, including: the year generic Ivermectin was introduced. In 2006, resistance began to show in sheep and goats. In 2008, this resistance grew to include parts of the U.S. Resistance began to build and was confirmed in Australia in 2009, in herds of cattle in 2016, and in 11 out of 12 farms in 2017.
A part of the challenge is that there is not a one-size-fits-all solution.
"What works on this side of the county may not work on a farm on the other side of the county," he said.
The biggest driver of parasiticide resistance is misapplication.
"If you miss the withers and the shoulder and you don't get all the way back to the tail head, then some ends up on the ground or ends up on the side of the shoot or the alleyway, does it do any good on the chute? No, not really," he said.
Grimm said many people also have a hard time accurately gauging a bovine's weight, and this can lead to underdosing or overuse. He gave an example of a guessing contest at a fair: people were invited to enter their best guess about a heifer's weight for a chance to win a prize. The heifer weighed 1,250 pounds. Guesses ranged from 550 pounds to 2,300 pounds. The moral of the story, Grimm said, is to get an accurate reading on each individual bovine's weight before administering parasiticides.
Grimm said there are four pillars to sustainable deworming: management, diagnostics, treatment and refugia.
Management includes monitoring deworming efficiency with ongoing surveillance and routine diagnostic testing. Grimm said it is important to know which drugs to administer and when they are most effective. By using a spread across the three different classes of drugs, macrocyclic lactones, benzimidazoles and imidazothiazoles, farmers can maximize efficacy of dewormer treatment.
Other key aspects of management are limiting overgrazing, limiting soil compaction and facilitating a slow regrowth of pasture grass, among other things. This will cut down on the number of parasites that call that pasture home.
"Parasites only live in the bottom third of that plant," Grimm said.
By ensuring cattle are not feeding on grass that is too short, Grimm suggested rotating grazing once the grass reaches the height of a coffee cup.
Diagnostics and treatment encompass ensuring treatments have the highest efficacy possible. Grimm encouraged combination treatment to maximize profitability and productivity, reduce resistance and incorporate environmental and financial sustainability. The key to treatment is correct application of the drugs, as was mentioned above.
Refugia is leaving portion of the herd population unexposed to dewormers to build up immunity and reduce resistance in the whole herd.
"It maintains that biodiversity," Grimm said.
This strategy is commonly used in small ruminants, corn and cotton.
In a question-and-answer session following Grimm's presentation, he fielded questions about weatherproofing of dewormers, interspecies parasites and timing.
He said weatherproofing is different with each drug. For instance, something like Ivomac is not very rainproof and needs about a day to get fully absorbed. Eprinex, however, is considered rain resistant, and will not easily be washed away by weather.
Parasites tend to be species-specific. The only parasites that tend to cross over are between sheep and cattle.
Grimm said the timing of treatment is everything. For example, if a farmer treats cattle in the fall for lice and mange, the treatment will have likely worn off by the time those pests emerge in February. It is better to time it closer to when the parasite commonly emerges.
Other topics included vaccination timing, Boehringer Ingleheim's history and product recommendations.
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