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Tips to help keep horn flies off your cattle

Even a couple hundred of the tiny horn flies can knock 20 pounds off a calf

flies on livestock

A few years ago, a friend in Saskatchewan asked me to help him treat his beef cows before they went on pasture with pour-on insecticide for horn flies. His job was to push up the cattle in the chute and my job was to use the applicator-gun and squirt pour-on liquid along each of the cow’s back. We ran through about 200 cows and he was amazed that we used about half of the insecticide he had calculated.

I never told him, until there were about 10 cows left, that I discovered the pour-on jug had a breather-hole that I fail to punch open to allow air to replace the horn fly insecticide we were using. Two weeks later, he had to run his cattle through the chute once again because of an “unusual heavy” infestation of horn flies out on pasture.

It doesn’t take many

Oddly enough, it doesn’t take that many horn flies to have a significant pest problem on pasture. A nominal count of 200 horn flies on the back of each cow or calf can:

  • Increase animal stress; make animals skittish and reduce grazing time.
  • Decrease forage intake linked to a 20 per cent decrease in milk production.
  • Encourage bunching and increased heat-stress severity.
  • Spring calves are 15 to 20 lbs. lighter at weaning time.
  • Reduce cattle immune function to fight disease.
  • Spread of disease among cattle such as anaplasmosis.
  • Replacement heifers have higher rates of infertility.

As this picture at the beginning of this article illustrates, one horn fly is very small (about four to five mm) and seems to get lost when hundreds of them are present on the back of an individual animal.

Horn flies are much smaller than a common stable, house or common face-fly; gray in colour with two dark stripes running along its body. Its head has two reddish compound-type eyes and has biting-mouth parts which it uses to pierce and suck blood from its cattle host; taking up to 20 to 25 blood meals daily.

Just imagine the explosive implications of 1,000 horn flies on each cow that seem even more attracted to the herd’s breeding bulls! Case-in-point: the University of Arkansas research estimates that a heavy infestation of 1,000 horn flies/cow can draw about 10 cc of blood per cow per day.

Effective treatments

Luckily, when a population explosion of horn flies does occurs, there are several proven methods that effectively can reduce horn flies counts on cattle by as much as 90 to 95 per cent. Most of these programs involve commercial insecticides, such as two timeless chemicals, namely: organophosphates and pyrethroids, both of which kill horn flies by disrupting vital nerve functions. Another class of insecticides called endectocides (ivomectin) can also be used in a similar manner.

Here is a breakdown of three of the most popular applications that dispense these insecticides in order to control horn fly numbers:

  • Ear tags — Plastic ear-tags are impregnated with either organophosphates or pyrethroids. One tag per season is recommended per cow. Most people should be aware that the efficacy of the insecticide in ear-tags is good up to about five-month control. Most people are instructed to wait until June to maximize the ear-tag’s full benefits for tackling peak populations of horn-flies that occur in late-July and August in Western Canada. It’s also common practice for producers to alternate organo- and pyrethrin-based tags every other year to reduce the chance of horn fly resistance. It is also recommended to remove ear tags at the end of the grazing season for the same reason. Note: a new ear tag has come out in recent years that contains both of these insecticides.
  • Back rubbers/oilers — A back rubber is usually a big, thick chain and mesh wrapped in a durable material and is treated with insecticide diluted with diesel fuel. Producers should check them on a weekly basis to see if they need to be recharged. If the pump-type back rubbers are used, it is a good idea to check them to assure that they are in good working order. It has been my experience to see the odd one leak insecticide all over the ground. Furthermore, the standard recommendation is to set them up to force cattle to pass by them. I asked a few cattle people about this advice and most of them said that cows are more than willing to regularly use them in order to rid themselves of horn flies.
  • Pour-on insecticide — Efficacy of pour-on insecticides often depends on the type of active insecticide used (re: endectocides) as well as its carrier liquid dispensed on cattle. For example, in the past; it has been my experience that generic types of pour-ons had some difficulties in insecticide absorption rates upon cattle due to carriers that companies employed.

About the author


Peter Vitti is an independent livestock nutritionist and consultant based in Winnipeg. To reach him call 204-254-7497 or by email at [email protected]



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