Do Fly Supplements for Horses Work? Feed-Through Control and Safer Choices

By Dr Duncan Houston

A horse stamping, swishing its tail and rubbing against fences needs effective fly control. Adding something to the feed sounds convenient, but products sold as fly supplements do not all work in the same way.

The useful question is which insects are causing the problem and where they are breeding. A product that acts in manure cannot be expected to stop every insect arriving from neighbouring land.

Quick Answer

Labelled feed-through insect growth regulators can reduce development of certain flies in treated manure. They are part of a wider control programme, not an immediate repellent for adult flies. Evidence for garlic, vinegar and herbal feed supplements as reliable repellents is limited, and natural ingredients are not automatically safe. Start with manure management, physical protection and identification of the fly problem.

How do feed-through products work?

Some products contain insect growth regulators, such as diflubenzuron or cyromazine. After the horse consumes the correct amount, active material passes into manure and interferes with immature fly development. It does not make the horse’s skin an invisible insect shield.

University of Connecticut Extension explains this approach and the importance of consistent intake and manure management. Only consider products labelled for horses and legally available in your location; instructions and restrictions vary.

What can each approach realistically do?

Is garlic a safe alternative?

Garlic should not be treated as a harmless default fly remedy. An equine feeding study documented red blood cell changes and Heinz body anaemia at high garlic intake. That study does not establish that every small exposure is toxic, but it clearly undermines the idea that more garlic must be better.

There is also insufficient justification for promising protection from apple cider vinegar, brewer’s yeast or diatomaceous earth in feed. Avoid dusty powders around airways and do not add multiple supplements without reviewing the complete ration.

What else could be causing rubbing?

Lice, mites, skin infection, contact irritation and insect-bite hypersensitivity can look like a simple fly nuisance. Sweet itch may require a targeted plan for biting midges and damaged skin. A horse losing hair or rubbing itself raw needs a veterinary assessment rather than progressively stronger supplements.

How to build a practical fly-control plan

1. Identify where and when insects bother the horse, including legs, eyes and mane or tail.
2. Remove manure and wet bedding regularly; address spilled feed, wet hay and leaking water points.
3. Review manure storage or composting so it does not remain a breeding source beside the stable.
4. Use appropriately fitted physical protection and inspect underneath it daily.
5. If considering feed-through control, check the target species, label, individual intake and whole-yard participation with your vet or an equine adviser.
6. Track the response using comparable observations rather than changing several products at once.

Penn State’s horse-farm fly guidance supports combining sanitation with selected control methods. A neighbouring breeding source can limit results even when your own programme is well managed.

When should a vet be involved?

Contact your vet for bleeding sores, persistent swelling, marked distress or a painful, squinting eye. Breathing difficulty, collapse or severe weakness warrants emergency help. These problems need assessment of the horse, not just a change in fly-control product.

Common mistakes

• Expecting a feed-through product to repel horse flies immediately.
• Leaving manure management unchanged.
• Assuming a shared supplement bucket gives each horse the correct amount.
• Increasing garlic because flies remain present.
• Using a rug that causes overheating or hides skin damage.

Prevention through the season

Plan before local fly numbers rise, following product labels rather than a universal calendar month. Keep storage secure, review skin daily and adjust turnout or shelter access to the insects involved. Maintain normal social contact, movement and forage access while reducing exposure.

Frequently asked questions

Will a supplement stop all flies?

No. Different flies and biting midges have different breeding sites and behaviour.

Can I use feed-through control for just one horse?

It may have limited effect if untreated manure remains a major breeding source nearby.

Does natural mean safer?

No. Safety depends on the ingredient, dose, horse and evidence.

Choose control methods by what they actually target. A consistent yard plan usually matters more than an impressive ingredient list.

For help understanding your animal’s care and preparing questions for your veterinarian, visit ASK A VET™. Seek in-person emergency care immediately for urgent signs.