The headgate rattles as another calf rushes out. The day is beautiful in a crisp way that only autumn can provide.

Those beautiful fall days make for more-pleasant fall processing, a busy time for any operation. But, with fall days staying crisp-but-not-cold longer in the season, your strategy to combat external parasites may need to change.

Lice are a common winter parasite — even southern states have to deal with the tail louse — and fall processing used to be the time to combat them, said David Boxler, livestock entomologist, extension educator and researcher with the University of Nebraska’s West Central Research, Extension and Education Center.

“Because of changing weather patterns, if an endectocide is given at fall processing, there is a chance part of the lice population isn’t active yet. Delousing won’t be fully effective if done too early,” explained Boxler.

Summer sleepers

Lice over-summer instead of over-winter like many parasites, noted Boxler. They hide on cattle in folds between the legs and body and the dewlap. One type of louse can even over-summer in the ear. Once they get cold, they wake up and start reproducing.

“The problem is that producers will apply products for external parasite control early in the fall, and expect those products to continue to work through the entire winter,” said Joe Gillespie, DVM, Boehringer Ingelheim. “The lice don’t become an issue until several months into the product’s duration of use.”

Sometimes, that means producers think a second infestation occurred later in February, or blame the product for not working. But, it’s likely the rest of the population woke up and reproduced after the original dose.

“If your risk of external parasites is February or March, then you need to be applying products in December or January to protect those animals through that critical period. If you apply those products in October, the likelihood of protecting against external parasites is very low,” said Dr. Gillespie. “You may have done a good job of protecting against the internal parasites as those cattle move into winter grazing, but that has very little impact on the external parasites.”

Strategically balance lice control and feed intake

In December or January, once the weather really starts getting cold, when you observe scratching, hair loss or an unthrifty animal, Boxler recommends putting that animal in a squeeze chute and performing a two-handed hair parting to look for lice. Focus on the top line, the withers and the face for this hair parting.

If you find 1–5 lice per square inch, that’s a low population and should be monitored. If you find 6–10 lice per square inch, mitigation is recommended. If there are more than 10 per square inch, you should treat as soon as possible.

In addition to being a comfort concern, a heavy infestation of lice can decrease average daily gain by 0.21 pound per day.¹ Winter is notorious for high input feed costs, so maintaining economic balance requires strategy. You don’t want to waste money on a product at the wrong time, and you don’t want parasites stealing the feed resources cattle need in the winter. Taking care of internal parasites is still a good idea at fall processing time.

“If you have both the internal and external parasite loads taken care of — by removing those irritations — cattle will be more likely to utilize what you feed them during those winter feeding periods,” Dr. Gillespie said. “Hopefully, that gives you an animal that’s more prepared when spring comes.”

There are four species of louse, so picking the correct product is necessary. Boxler explains that one type of louse feeds on hair and skin, and three others are blood feeders.

“An injectable actually is directed more for the sucking-lice species than the chewing louse,” stressed Boxler. “The pour-on is more effective in controlling the chewing louse, and also controls the other three blood-feeding species.”

Appropriate dosing is essential in controlling external parasites. Underdosing can occur if inaccurate weights are assumed, or if the product is not fully applied to the animal because things went haywire in the chute. Different products need one or two applications, so follow label instructions.

Underdosing, even if done at the correct time of year, may still leave you with a bigger lice problem. Part of the current population won’t be killed, and their offspring can be resistant to the product. Since new ingredients aren’t available, we all have to be good stewards of the tools we have, Boxler and Dr. Gillespie agree.

Because each region of the U.S. has different times and types of parasites that cause the greatest burden, Dr. Gillespie suggests building a strategic delousing plan with your veterinarian, who knows the parasite load in your region of the country.

Fall processing days already involve plenty of multitasking, so save the external parasite control for when it will be most effective.

To learn more about internal and external parasites, visit https://www.veterinaryentomology.org/

_Reference:

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.}

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