Control Flies Before They Control Your Cattle

By Arden Wohlers DVM
Extension Veterinarian
University of Nebraska Panhandle Research and Education Center

In June, July, and August when the number of flies per animal reaches 200, it becomes economically important to have a fly control program in place. That number of flies can reduce weight gain on a grazing animal up to ½ pound per day. This reduced weight may translate into 30 to 40 dollars lost revenue per animal for the summer. The losses are a result of the flies annoying the animals so they move around instead of eating. The flies can also spread viral, bacterial and parasitic diseases that impair efficiency.

There are thousands of species of flies, but only a half dozen may become a significant economic consequence to grazing cattle. Horse flies and deer flies are of limited problem. Stable flies and house flies are more often associated near buildings with feed yards and corrals or decomposing feed and manure. Heel flies that once were a critical issue has became a great deal less bothersome after years of control of their larva, the cattle grub. The primary concern is the face fly and the horn fly. If we control face flies and horn flies, the overall fly problem will be in check.

The first step in fly control around the farmstead is to establish good drainage for mud holes and eliminate build up of manure, bedding and other debris that create fly producing areas. However, such measures are not practical in the pasture environment, so we must rely on insecticides. It is necessary to rotate through different classes of insecticides and the methods for applying them as flies are able to develop resistance and circumvent control from continuous application of a single method.

Insecticides can be applied through ear tags, back pours, sprays, back rubbers, dust bags and orally in the mineral. The various chemicals that can be used are pyrethroids, organophosphates, ivermectin and spinosyn. Back pours and sprays are excellent because they give nearly 100% control but they need to be reapplied every 3 to 5 weeks. Rubs and dust bags are good but there is continuous upkeep and some animals don’t use them unless they are situated in the pasture properly. Insecticide ear tags are good but must be used correctly. This involves placing them in the ears near the start of active fly season, using two tags for every animal in the pasture, and alternating the chemical on an annual basis. The oral larvicides can be effective when added to the mineral mix if the program is started before fly season and the mix is continuously available. The most effective programs will use a combination of two methods. A common program in this area is one insect tag per animal plus a complete fly knockdown once or twice during the season with a back pour or whole herd spray.

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