As grass greens up and cattle head back to pasture, many producers are “throwing darts in an open field” when it comes to parasite control, says Tennessee Hereford breeder Ryan Proffitt.
The real-world frustration of deworming programs is knowing if they are working, Proffitt says.
Norbrook Technical Services Veterinarian Megan Bollin explains fecal egg count testing, targeted treatment, concurrent deworming, maintaining refugia and smarter pasture management can turn parasite control guesswork into a targeted plan that protects herd health, preserves dewormer efficacy and ultimately adds pounds to the calf crop.
Both Bollin and Proffitt agree a pragmatic roadmap for modern parasite control is anchored in diagnostics, targeted treatment and strong relationships with veterinarians.
Bollin shares five practical strategies to get the most out of today’s dewormers and preserve them for tomorrow:
1. Sit Down with Your Vet.
She encourages producers to map out a herd‑specific internal and external parasite plan with diagnostics built in.
“Your local vet should be your key partner in designing a program that fits your parasites, climate and management style,” Bollin says.
2. Use the Right Product at the Right Time.
Use the correct class, correct dose and consider concurrent deworming when resistance is a known issue. Your local veterinarian can guide you on proper treatment timing to avoid wasting money.
3. Manage Pastures with Parasites in Mind.
Bollin says pasture management is as important as treatment. Pasture type, quality, topography and drainage should all be considered in your plan, knowing we can’t always do much to change them.
“Only about 10% of the parasite life cycle is in the animal; 90% is on pasture,” she says.
Larvae generally stay below 4” on the grass blade. She says it is important to avoid overgrazing pastures below this height and manage stocking density accordingly.
“When you get a big rain after a drought, the larvae that had been waiting in the manure pats can quickly become infective and significantly increase the risk of infection, especially in young calves,” she says.
4. Prioritize High-Risk Animals.
Calves, stockers, bulls and replacements should be prioritized with the strictest parasite control and monitoring programs.
“Calves, replacement heifers and bulls are typically heavier shedders and more susceptible to the effects of parasites than mature cows,” Bollin explains.
Proffitt notes that many producers historically concentrate on keeping mature cows dewormed while underestimating calves’ role as carriers.
“We don’t need to, and shouldn’t be, treating every animal like we always have,” Bollin adds. “That has gotten us in a pickle with resistance.”
5. Measure and Adjust Treatment.
Bollin says it is important to know where you started. Get a baseline fecal egg count, understand your resistance patterns and monitor the efficacy of your treatment program. So many variables change from year to year: climate, weather conditions, new animals and other stressors. It’s critical to routinely evaluate your deworming program and avoid blindly doing the same thing year after year.
Diagnostics Are Essential, Not Optional
Fecal egg count reduction tests (FECRTs) are the most practical method we have to determine if dewormers are still working and at what level.
Bollin explains the process includes:
- Collecting rectal fecal samples and recording identification.
- Treating animals with product or products of choice.
- Resampling the same animals by taking rectal fecal samples, 10 to 17 days later, depending on the drug or drugs used.
The lab will count how many eggs per gram are in that fecal sample. There will be a pretreatment and a posttreatment sample. Bollin says the goal should be greater than 95% reduction.
Proffitt argues FECRTs are worth the hassle.
“If we don’t have a game plan on what we’re doing and we’re just rushing,” he says. “What did we win at the end of the day if we don’t know what we’re doing?”
Proffitt says testing tells him which cows he can skip treating, which saves him money.
Bollin explains in many cases mature cows 3 years and older, shedding low levels of eggs, on a good plane of nutrition, with no other stressors or health concerns (including liver flukes), should not need to be dewormed.
“This supports ‘refugia’ — intentionally leaving low-risk animals untreated to slow resistance,” she explains. “Because they’re mature, they’ve got a competent immune system that can actually fight off these parasites by themselves without a dewormer.”
To reduce costs further, Bollin says producers can pool fecal samples from multiple cows into a single submission.
Beating Parasite Resistance Starts at the Chute
Bollin says dewormer resistance, long documented in sheep and goats, is being seen more frequently in U.S. cattle herds.
She says overuse, underdosing and treating every animal regardless of need are major drivers in resistance.
“All of these deworming products, when they were originally approved, had very high levels of efficacy. We’re talking 99% and above,” Bollin explains. “As we’ve continually used these products, efficacy has been challenged because resistance has increased.”
If you’re using products that are no longer effective in your herd, you’re spending money on drugs that don’t work, and you’re not getting the production benefits. One way to restore efficacy when resistance is present is to use concurrent deworming.
Why Concurrent Deworming Matters
Bollin says concurrent deworming is using two dewormers from different classes at the same time. She stresses producers need to work with a veterinarian to avoid unknowingly pairing two products from the same class, which doesn’t provide the intended benefit.
She explains the benefits of concurrent deworming are:
· Higher overall efficacy when two partially effective products are combined. “Say you’ve got two products, for example, each with 70% efficacy. By using them together, you can raise your overall efficacy to levels exceeding 90%,” she says.
· Broader spectrum of parasite coverage.
· Models would suggest a slowing of resistance.
· Only sustainable long-term when used in conjunction with a refugia program. This means we don’t treat every animal. We want to keep a few “good” worms around that are still susceptible to the drug.
Bollin gives the example of pairing a benzimidazole, or a “white dewormer,” such as fenbendazole, albendazole or oxfendazole, with a macrocyclic lactone such as ivermectin, moxidectin or eprinomectin.
She emphasizes the industry unfortunately doesn’t have a lot of studies looking at this, but a study published in 2025 highlighted the benefits of concurrent treatment with fenbendazole in situations where resistance to macrocyclic lactones is likely.
Dung Beetles Are Valuable Allies
Dung beetles break up manure pats, exposing eggs and larvae to sunshine and dry conditions. Some dewormers are more compatible with dung beetle health.
“Two of the dewormers that are not harmful are moxidectin and fenbendazole,” Bollin says. “Those are two molecules that are generally safe for dung beetles, and those could be a good option to pair together.”
Application Technique and Dosing Accuracy Matter
Bollin says accurate body weights, not visual estimates, are critical, explaining underdosing is a key driver of resistance.
“One of the biggest contributors to resistance is that we are just not giving them enough active ingredient,” she says. “If you don’t have scales, it is best to treat to the heaviest body weight in the group, so that you make sure that they’re all getting enough.”
She also stresses the importance of storing deworming products correctly.
“Leaving them by the chute in temperature swings can reduce efficacy,” she says.
Proffitt also reminds producers to read labels and understand rain windows with pour-ons and to avoid mud or manure on hides. Bollin notes that injectables can provide more certainty that the animal is getting the full dose, whereas oral drenches can be spit out and pour-ons can run off or be groomed off by penmates.
Both Bollin and Proffitt frame parasite control not as one more chore on an overloaded to-do list but rather as a strategic, data-driven opportunity to protect animal health, slow resistance and convert good management into pounds sold.
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