Not treating mother cows during horn fly outbreaks can decrease the average daily growth rate of calves by 12%.

And when it comes to growth rates of yearling stockers and lactation rates of dairy cows, both can decrease by about 16% when horn flies aren’t kept in check, according to Texas A&M AgriLife Extension.

But are control efforts worth it when you take into account both time and money?

Studies by the University of Georgia have documented average net returns of about $11 per animal when subtracting control costs from the value of calf weight gains, said veterinary entomology Professor Nancy Hinkle, who has studied horn flies for more than 30 years.

That equates to an additional potential profit of $8,800 for an 800-cow ranch. Based on numbers like that, Hinkle said, most producers find that control efforts are well worth their time. An added benefit: the health of both cows and calves improves.

“Even though one horn fly only takes a small amount of blood from a cow with each feeding, when you have hundreds and sometimes even thousands of these flies on each cow, imagine the aggravation and pain,” she said.

And when cows are being tormented constantly, their calves aren’t getting the milk they need.

But to effectively control horn flies, Hinkle stressed, a well-thought-out integrated pest management plan focusing on adult flies as well as larvae must be executed and followed.

“Since horn flies develop in wet cow manure, some producers have found it effective to drag their pastures and break up the manure pats,” she said. “If that is done on a weekly basis, it can have a significant impact on horn fly production.”
Since this may not be practical for many operations, Hinkle noted, another effective method is to rotate pastures on a regular basis, which reduces the amount of manure accumulating in one area.

Hinkle agreed with Bayer Animal Health technical services Veterinarian Larry Hawkins that a combination of ear tags used to control adult horn flies and feed additives to help prevent larvae from developing in manure is an effective one-two punch.

“Some feed-throughs provide almost 100% elimination of horn fly larvae, and ear tags can offer very good horn fly control through much of the season,” Hinkle said.

Hawkins added, “I think those two complement each other well, especially as we get toward the end of the fly season.”

Hinkle and Hawkins concurred on another point, and that agreement came with emphasis: insecticide resistance has become a major issue with horn flies, and, because of that, cow-calf, yearling, stocker and dairy producers are encouraged to rotate between active ingredients in ear tags and modes of action on a yearly basis.

As ear tags lose potency later in the season, Hinkle and Hawkins recommend that they be removed and other modes of action be implemented, such as pour-ons, dust bags and back rubbers.

If removing ear tags isn’t practical in late summer or fall, they should definitely come out before winter.

“If you leave them in, all that does is select for resistant flies that will overwinter and come out next spring as an even bigger problem,” Hinkle said.


Hawkins added, “You need to do things that fit with your management system, but it’s important that all producers work to keep the resistance problem down. Because of that, ear tags should be removed by season’s end regardless of your management program.”

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