Dewormer Detective

Control Failures Becoming More Common for Some Dewormers

Sick calves let Adelaide Henry know it was time to call her veterinarian. He told her the generic pour-on dewormer she had used just wasn't doing the job. (DTN/Progressive Farmer photo by Boyd Kidwell)

Adelaide Henry, Liberty, North Carolina, knows all too well how confusing deworming decisions can be. The 19-year-old college student is breaking into the beef business by buying baby Holstein bulls from the dairy farm where she works.

Henry bottle-feeds calves for a short period then turns them out onto grass pastures. So far, she has successfully raised 40 head. Late last year, when she put two calves onto pasture, they showed signs of diarrhea and a lack of appetite.

She had dewormed the calves with a generic pour-on ivermectin, so she didn't think worms could be the problem. (Ivermectin is in the Macrocyclic lactone class.) Henry called in veterinarian Richard Kirkman, Siler City, North Carolina, to solve the mystery. After performing fecal egg counts and finding the numbers rising, he advised Henry to use a different class of dewormer. She tried Valbazen; its active ingredient, albendazole, comes from the Benzimidazole class. The diarrhea ended, and the calves began gaining weight. Mystery solved.


Whether you're a large cattleman or a small producer just getting in the business, the dewormer talk is a good one to have with your veterinarian. Some lower cost products that used to work well aren't as effective these days.

"It's cost-effective to have a surveillance program in place, so the producer and the veterinarian can devise a parasite-control program," Kirkman said. "In addition to reduced weight gains and lost income, parasite infections can have serious health effects for cattle."

An Iowa State University (ISU) study shows that in a herd with a heavy worm load, deworming is worth $201 per cow. To reach this conclusion, researchers counted additional weight gain (40 pounds) for calves at weaning, along with improved conception rates (10%).


If cows don't have worm problems, you won't get a cost-effective payback. This is why ISU Extension veterinarian Grant Dewell recommends parasite surveillance based on fecal egg counts. If the fecal egg count indicates a low worm burden, skip treatments. Fecal egg counts cost approximately $15 to $20 per sample. On a 100-cow herd, taking fecal egg counts from five to 10 cows will provide a good estimate of the worm load across the herd.

There are multiple protocols for defining worm burdens. With the McMaster Technique, for example, a mixed-parasite egg count over 800 per gram is considered heavy. However, there are several species of worms, and some are more dangerous than others. That's why professionals should do egg counts and veterinarians should make treatment recommendations.

By the time most producers can see a problem visually, they've probably lost more than enough money from reduced weight gain to pay for a deworming treatment.


A local veterinarian is the best resource when it comes to creating a plan for the optimum deworming time for an individual herd. Timing is critical to a successful parasite-control program, and the best times to deworm vary regionally.

"If I was only going to deworm cows once a year, mid-March would be my favorite time in our area," said University of Arkansas veterinarian Jeremy Powell. "The days are warming, the nights are balmy, and the grass is growing quickly."

The "spring rise" for parasite egg production typically occurs during this time in Arkansas. Treatment should be aimed at eliminating adult parasites in cattle before increased egg production gets under way.

"This strategy does a lot to minimize pasture contamination before young calves start grazing and worms potentially become a more serious problem for these growing animals," Powell explains.

His second-favorite time to deworm is around Labor Day, because there's a second rise of egg production for worms in the fall. This second deworming reduces pasture contamination during fall grazing, especially important for growing calves.

For Midwestern producers, Iowa's Dewell also recommends an early spring treatment, although the date will be later.

"Most of our cows in Iowa spend the winter in dry lots or stalk fields where they don't pick up worms. If your cows were dewormed in the fall, they should be clean when you turn them out," Dewell said.

"After the cows have picked up eggs while grazing for three to four weeks, you can deworm the animals and reduce pasture contamination before young calves start grazing."

If fecal egg counts show there are high levels of parasites at turnout, it's a good idea to deworm cows before turning them out to graze and deposit worm larvae in fresh pastures. After the cows graze for a month, Dewell suggests using a dewormer (fenbendazole) found in pellets, mineral mixes and pasture blocks to deworm again without bringing the herd into a working facility.


Veterinarians are growing concerned about the potential for worm resistance to generic ivermectin pour-ons. When these products were introduced 15 years ago, generic pour-ons were cheap and easy to apply. As a result, producers used them frequently, perhaps too frequently.

There's one school of thought that worms are growing resistant to generic ivermectin pour-ons because of overexposure to the active ingredient. There's another school of thought that the carriers used in generic products aren't as effective as those used in name-brand formulations.

At any rate, veterinarians now recommend using name-brand pour-on dewormers or injectable products. The injectables come with the added benefit of knowing each animal receives the correct dosage -- assuming accurate weights are being used on the animals being treated. It's important the dosage be tailored to each animal, not a herd average.

Dewell explains: "There's a tendency to leave the dial on the injectable applicator set for a 1,200-pound cow and not giving adequate dosage to your 1,500- to 1,800-pound cows."