The impact of Horn Flies

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By Dwayne Summach, P.Ag.  M.Sc.

Regional Livestock Specialist

Saskatchewan Ministry of Agriculture, Kindersley

Fly season is well underway and while most people consider flies a nuisance, cattle producers should deal with that nuisance as an economic threat. Horn flies are the species that hang out on the back and sides of the cows all the time. About the only time the female flies aren’t on an animal is when they leave to lay their eggs in a fresh cow pie.

Research conducted at the Lethbridge Research Station concluded that as few as 40 horn flies per cow warranted control measures to improve animal performance and productivity. A study conducted at the University of Nebraska determined horn flies typically reduced calf weaning weights between four and 15 per cent when no control measures were taken – that’s 20 to 75 pounds on a 500 pound calf. The average reduction in average daily gain in long yearling steers on grass was measured at four tenths of a pound a day.

Fortunately horn flies are susceptible to various control efforts. Insecticide carrying ear tags, when used as directed, can be very effective at making the animal’s back far less attractive for horn flies to hang out for an entire grazing season. One drawback of using ear tags is a cow only has two ears, and usually they are already occupied. Also, horn flies will develop resistance to the insecticide and therefore producers should alternate between using synthetic pyrethroids and organophosphates annually.

Some pour on products, normally used for parasite control, provide weeks of horn fly control. Timing the application to provide protection during fly season is generally viewed as inconvenient.

Self-application devices such as grooming brushes and mop applicators can deliver insecticides to the back and sides of cattle that use them, providing an effective method of horn fly control. These applicators should be situated so that all animals are required to use the device in their routine travels, typically on their way to water or to access salt and mineral.

For more information regarding fly control or other related topics, please consult your veterinarian; contact a Saskatchewan Agriculture Regional Office; or the Agriculture Knowledge Centre at 1-866-457-2377.

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