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Sometimes, Less Is Really More When Deworming

Here’s a cattle example of the less-is-more principle: Deworming 90% of your cows could be better than deworming all of them.

The issue is resistance buildup by intestinal worms to common dewormers, says Doug Ensley, a cattle veterinarian with Boehringer Ingelheim. Producers have been using cattle dewormers for a long time, and almost all of the products attack the common cattle worms in the same way at the same site. 

“Over time, the worms adapt,” says Ensley. “No product is 100% effective. Maybe they were close to 100% at one time, but not anymore.” 

It’s why producers are due for a lesson in advanced deworming, he says.

When a few worms don’t die in the face of a dewormer, they are the ones that reproduce. The resistant worms gradually take over, and the dewormer becomes ineffective. 

Some parasitologists recommend that you only deworm 90% of your cows and leave the other 10% as an untreated refuge. 

This principle is called refugia, and it’s the same one used in corn to slow the development of pesticide-resistant rootworms. The refuge plants (or cows) allow a few susceptible worms to always survive. They cross with the resistant worms, dilute the resistant population, and maintain the population in a susceptible state. 

This can keep your favorite dewormers effective for the long haul.

How to test

Ensley says if you suspect your dewormer program isn’t as effective as it once was, you should work with your veterinarian to do a fecal egg count reduction test. When you are deworming animals, collect fecal samples from a few of them and have your veterinarian (or a diagnostic lab) do an egg count.

Keep track of the individual animals from which you collected because you will do a recheck 14 to 21 days later on those same animals. 

“You are looking for a 90% reduction in the egg count,” says Ensley. “If it’s not 90%, you may have resistance to your dewormer.”

At that point, you should evaluate every aspect of your program. Again working with your veterinarian, determine the exact parasites you have, choose a dewormer with the greatest efficacy, give the right dose, and evaluate again.

Also, adopt the refuge system. Ensley says you have the choice of which 10% of the herd you don’t deworm.

“Be strategic,” he says. “For instance, you may want to deworm 100% of your young cows because they are at most risk of rebreeding failure. The refuge can come from the healthiest cows.”

You don’t need to worry about the refuge animals, adds Ensley. They still get a herd-immunity effect. Reduce the worm load in a group by 90% or more, and every animal in the population benefits.

Are you good at guessing weights?

One of the keys to successful deworming is getting the dosage correct, says Doug Ensley, cattle veterinarian with Boehringer Ingelheim. It’s usually based on animal weight: 1 ml per 110 pounds of body weight, for example.

Ensley asked 33 cattle producers to guess the weight on a cow that they could all see. Guesses ranged from 873 pounds to 1,890 pounds. The cow actually weighed 1,400 pounds. 

“Lots of animals get underdosed,” he says. “Then people wonder why a product doesn’t work.”

Use scales for accuracy, Ensley suggests, and dose to each animal’s weight. If you don’t have scales, then dose the whole group to the weight of the largest animal. “You want as many dead parasites as possible.”

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