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Don’t reach for your pour-on dewormer quite yet

Timing of application and proper dosing can make all the difference in how your pour-on takes on lice

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Ensuring the pour-on dewormer you’re using for external parasites like lice is effective includes applying it when it’s going to do the most good, not necessarily when it’s the most convenient. That can be challenging with everything else going on at the ranch.

“For Northern states, cattle are brought in for weaning and processing in September and October,” explained David Boxler, extension educator and livestock entomologist at the West Central Research and Extension Center for the University of Nebraska in North Platte. “Producers may think the dewormer they use at that time can take them through the winter and reduce lice populations. But our weather patterns are changing, and a warm fall means those lice aren’t even a threat yet.”

Lice hang out on an animal’s body all year long, but they’re sleepy in the summer. They tuck themselves inside folds of skin, between the legs and body — generally, anywhere that’s protected from direct sunlight — in order to survive. As the weather gets colder and haircoats grow longer, lice move up the animal’s body and feel protected enough to reproduce and lay eggs. But that sometimes doesn’t happen until late November or December.

“If you’re treating parasites in the fall with an injectable dewormer, that’s not going to knock down a lice population because it’s not even there yet,” said Joe Gillespie, DVM, Boehringer Ingelheim. “Pour-on dewormers are the most effective for lice, and they should be timed right before the levels of parasites are at their highest. It’s important to discuss correct timing with your veterinarian to get this right for your operation.”

Dr. Gillespie noted that some producers may believe they’re seeing a reinfection in early winter, but if they applied a pour-on before the lice migrated to the top of the animal, the product was applied too soon. “Lice populations, especially biting or chewing lice, can explode quickly once they start a new life cycle on the back and neck of the animal,” he pointed out.

“You can have a train wreck if you treat too early,” agreed Boxler. “Treat properly when it gets cold and is going to stay cold.”

Proper application ensures complete coverage

“Reading the product label is critical,” stressed Boxler. “Choose a product that is appropriate for the type of lice you have, and determine if you will need more than one application.”

Complete coverage is also important, and means taking the time to ensure a pour-on dewormer is distributed accurately. Carefully apply the product with an applicator gun along the midline (over the top of the back of each animal). Begin at the withers, and pour all the way down to the tailhead in a narrow strip to reduce runoff. Make sure the entire dose gets on the animal, not on the sides of the chute or on the handlers, to maximize efficacy.

“Sometimes, an animal doesn’t receive the full dose because things can get chaotic; animals are running down the alley as fast as they can go and you want to get through the process quickly,” Boxler said. “But product ends up flying everywhere. Take the time to apply correctly, and follow the label instructions because that will enhance product performance.”

The dosage, or amount of product applied, should also be adequate for the size and weight class of the animal being treated. Calculate the volume of pour-on product to be administered based on each animal’s weight, using a scale whenever possible. Weight tape or a cull weight slip are also options.

  • Under-dosing: If an animal is under-dosed, the amount of the product’s active ingredient is not present in the animal’s tissues at the necessary level and the product will not deliver the best lice control. Under-treatment will likely leave some of the parasites behind. That becomes problematic because the surviving parasites are potentially more likely to be resistant to subsequent and future treatments with products in the same class of dewormers.

  • Over-dosing: When you over-dose animals, you waste hard-earned dollars on product.

Lice infestations affect more than skin

“Lice irritate the animal, which, in a mild case, can cause damage to the skin and a small amount of hair loss,” Dr. Gillespie said. “But if the infestation is heavy enough, it can cause real production and economic problems.”

Cattle may spend all their time itching, scratching, licking and rubbing … instead of eating.

Boxler worked on a Nebraska study that showed a moderate-to-high lice population in feeder calves significantly decreased average daily gains (ADG) by 0.21 pound in the untreated group of animals versus those that were treated.1 That affects more than just ADGs. One study estimated that losses to the cattle industry from lice are more than $126 million annually.2

“Lice can also affect milk production,” added Boxler. “What’s worse, what cattle experience with lice infestations, including cold stress, can set up the animal for other issues including respiratory problems and a generally higher susceptibility to other illnesses.”

He’s passionate about finding ways to deliver effective parasite control, and has been working with everything from prototypes of traps for stable flies to essential oils for cattle horn flies.

“We’re battling parasites without the promise of new parasiticide modes of action,” Boxler said. “While we’re always searching for other unique options, it’s important to follow label directions and use the products we do have available correctly to enhance product efficacy.”

If producers can get lice populations under control, cattle will overwinter better. They’ll grow a good haircoat for protection, steers will gain weight more efficiently, and cows will be in better condition for spring calving. Properly applying a pour-on dewormer at the right time can help animals do all these things they’re supposed to do, without lice getting in the way.


1 Gibney VJ, Campbell JB, Boxler DJ, et al. Effects of Various Infestation Levels of Cattle Lice (Mallophaga: Trichodectidae and Anoplura: Haematopinidae) on Feed Efficiency and Weight Gains of Beef Heifers. J Econ Entomol 1985;78(6):1304–1307.

2 Drummond RO, Lambert G, Smalley, Jr. HE, Terrill CE. Estimated losses of livestock to pests. CRC Handbook of Pest Management in Agriculture 1981;1:111–128.

©2021 Boehringer Ingelheim Animal Health USA Inc., Duluth, GA. All Rights Reserved. US-BOV-0370-2021-A


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