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Deworn cattle, control flies to get grazing season off to a good start

Early in the grazing season, worm control and fly control go hand-in-hand for cattle producers. Spring pastures can expose the herd to both internal and external par­asites that threaten health, perfor­mance and, ultimately, profitability.

Economically damaging pests such as lungworms and round­worms may have overwintered either in the pasture or in the cattle themselves. Horn flies, the primary external parasite affecting cattle, can start to show up in heavy numbers in April, depending on the weather.

Thus, controlling parasites with a multi-faceted approach is an im­portant management practice before your herd hits the pasture this year.


If you dewormed your cattle in the fall, they should have remained fairly clean all winter. However, only 10% of the worm population will be found in the animals. The other 90% is in the pasture and will emerge when conditions are right.

That’s why it’s important to treat cows and calves with a dewormer prior to pasture turnout. Consider a long-acting injectable to control worms for an extended period— enough to cover the entire grazing season. Other options include pour-on, oral and feed-through products.

Deworming cattle provides many benefits. Reducing internal para­sites improves immunity, which leads to better performance, feed consumption and conversion. With improved immunity, vaccination response should also be better.

Effective parasite control requires proper pasture rotation. Deworming causes the cows to shed parasite eggs. Producers should plan to move cows to fresh pasture after de­worming to avoid recontamination.

Always follow label directions. While you may see early-season control of flies when using a pour-on dewormer, this is not an appro­priate use of the product. In some cases, worms have become resistant to a class of dewormer because producers were using a pour-on dewormer as a fly-control method.

Fly control

Kicking off your fly-control program before grazing begins is also a smart move. Spring cattle-working tasks, such as pregnancy checks, weaning or branding, can provide a conve­nient opportunity to apply insec­ticide ear tags. They will provide several months of fly control.

Your biggest concern is horn flies. These season-long pests will show up in the spring and stay around until October. They are the most economically damaging external pests in cattle, costing producers some $1 billion in lost produc­tion annually. When horn flies are controlled, growing cattle gain an extra 1.5 pounds per week. Wean­ing weights can increase 12 to 15 pounds for calves nursing cows that have had fly control.

Horn fly pressure will be greatest in July and August. If fly tags lose potency in late summer, there are other options. Oilers or dusters can be used during peak season. They need to be located at mineral sites or water tanks where every animal will walk underneath.

Spraying cattle periodically while out on pasture is a possibility, and pour-on fly treatments can be used on cattle that can be run through a chute. Pour-ons provide up to three months of control. The economic threshold for a second treatment is 200 to 300 flies per animal.

Feed-through insect growth regulators (IGR) can be added to mineral during the grazing season. They’re particularly effective for flies that grow in manure, namely horn flies. IGRs control the insects’ development so they don’t mature and reproduce.

Stable flies and horse flies also ir­ritate cattle. Stable flies are generally seen on the animal’s legs and appear earlier in the season than horn flies. The best way to reduce stable fly pressure is to clean up around hay rings and bunk feeders.

Horse flies, on the other hand, are difficult to control. Their larvae usually develop in semiaquatic areas, making it difficult to prevent their growth. Only the females feed, and they only do it for short peri­ods of time. It’s important to control horse flies, however, because they can transmit anaplasmosis. If you want to try treating for horse flies, a pyrethroid can help. Central Life Sciences has a 6% pyrethrin prod­uct, Pyronyl Crop Spray, that can be used as a surface spray or in a misting system in barns, stables and on cattle in feedlots and pasture.

No matter what method you choose, control of internal and external parasites is one of the best investments a cattle producer can make. Talk with the livestock experts at your local MFA for advice on the right products and timing for your operation.

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