How Does Feed Through Fly Control Work for Horses? | Breaking the Fly Life Cycle in Manure

Tired of slapping flies off your horse while you tack up? Feed through fly control doesn’t kill adult flies buzzing around your barn. Instead, it breaks the fly life cycle at its source, targeting larvae that hatch in your horse’s manure before they ever grow wings. Here’s how it works, what it kills, and the one mistake that makes it fail.

The Mechanism: From Feed To Manure To Dead Larvae

Feed through fly control products contain an insect growth regulator (IGR) or larvicide mixed into a daily ration. The active ingredient passes through the horse’s digestive tract with little or no absorption and ends up in the manure. Flies lay eggs there, and the eggs hatch normally — but the larvae can’t finish developing because the product disrupts chitin formation, the hard outer shell an insect needs to grow. Farnam confirms the process: the product is given daily, passes through unabsorbed, and breaks the life cycle of house and stable flies. The horse’s body never really processes it — the manure is the battleground.

Daily feeding matters more than anything else. The product only works while moving through the horse, so manure must be treated continuously. Miss days and you leave gaps where larvae survive.

Which Flies Does It Actually Stop?

Feed through fly control targets exactly two species: house flies and stable flies, both of which breed in manure and decaying organic matter. But it does nothing for insects that don’t develop in manure. Bots, deer flies, mosquitoes, and horse flies bypass the manure route entirely, so this isn’t the tool for those problems.

The time frame is forgiving but not instant. Suppression is usually noticeable within two weeks, with peak effects around six weeks of consistent feeding. Results begin once the first treated manure hits the ground, but only if the horse keeps eating the product daily.

Active Ingredients and Product Types

Different products use different chemistry, and the choice matters for your horse’s health and your comfort level.

Diflubenzuron

This chitin synthesis inhibitor is the active ingredient in SimpliFly Feed-Thru Fly Control, and UConn Extension guidance also identifies it as a larvicide option. Farnam markets SimpliFly as made without organophosphates.

Cyromazine

Veterinary sources describe cyromazine as an IGR that similarly inhibits chitin and exoskeleton formation in fly larvae. It’s another non-organophosphate route to the same result.

Tetrachlorvinphos (TCVP)

This older organophosphate larvicide used in Equitrol inhibits cholinesterase in flies, making it effective — but with a different risk profile than IGRs. That distinction matters if your horse has health concerns.

Novaluron

How To Use It Right (And Why Partial Programs Fail)

The most common mistake is treating one horse in a barn full of them. Flies don’t respect stall assignments — if they breed in untreated manure in the next paddock and bite your treated horse, you’ve wasted the scoop. Farnam advises dosing by weight and feeding daily in the ration. MultiFly’s retailer explicitly recommends it for every horse on the property.

Timing is the other half. Start before fly season and keep feeding until cold weather or first frost. UConn Extension stresses treating manure before larvae mature — once they’ve pupated and emerged, you’re starting over.

Feed through fly control is one layer of a complete program, not a replacement. Manure removal and barn sanitation do most of the heavy lifting; the feed through product catches larvae that removal misses. If you’re comparing options, our roundup of the top feed through fly control products breaks down the best picks for different barn setups.

The horse’s body isn’t the target — the manure is. That’s the whole point. The product acts in the pile, not the pony, which is why it’s considered safe for daily use when dosed correctly. Just remember: it’s a manure-breeding fly solution, not a universal insecticide.

FAQs

How long does feed through fly control take to work?

Suppression usually shows within two weeks of daily feeding, with peak effects at about six weeks. Consistency is key — the product only works while the horse is actively excreting treated manure, so skipped days set you back.

Can feed through fly control be used with other fly products?

Yes. It works best alongside topical repellents and fly sprays, which handle adult flies while the feed through product targets larvae. Manure removal and barn sanitation are essential partners, since the product only affects flies breeding in treated manure.

Is feed through fly control safe for all horses?

Most products are designed for daily use and pass through with minimal absorption, but ingredient matters. Organophosphate-based products like those using TCVP carry different considerations than chitin inhibitor IGRs. Consult your veterinarian if your horse has health issues, is pregnant, or is on other medications.