Especially if you live in a humid area, where flies might be more abundant, flies are more than an annoyance to cattle. They also carry disease. Some types of flies feed on cattle, causing anemia and affecting the production of milk. You might not be able to completely eliminate flies, but you can reduce their numbers among your cattle, choosing from a handful of means.
Try back rubbers. These are posts wrapped in burlap or rubber and sprayed with insecticide. Hung at the proper height in a high-traffic area, your cattle will slip under them frequently. As the post rubs against their backs, it distributes the insecticide. Many animals enjoy rubbing against objects, so once they figure out how to use the back rubbers, they'll do it on their own, without any need for an incentive. You can make your own back rubbers or buy them ready-to-use at farm supply stores.
Spray your animals with insecticide, available from farm supply and online stores. This works best if you have a small number of animals. After all, hand-spraying a thousand cows is nobody's idea of fun. Keep in mind that sprays are temporary, and you will have to reapply the product every few weeks -- more often if it rains, as the water can wash it off the animals' backs.
Buy fly control ear tags. These tags -- which look very similar to identification tags -- sprayed with insecticide will deliver a constant supply of chemicals to the animals, killing flies on the skin. With this method, you'll have to remove tags before winter to prevent flies from becoming accustomed and immune to the chemicals. After winter, you'll start again with fresh tags.
Add liquid insecticides to the feed. Known as feed-through chemicals, these are absorbed slowly by the body, making the skin toxic to the larvae but not the adult flies. Some companies sell insecticides in the form of salt blocks, which cattle lick to ingest to absorb the chemicals, or as special mineral supplements with insecticide added to them.
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