Choosing the Best Fly Supplements for Horses

As spring brings warmer weather, it also brings something far less welcome for horses and their owners: the return of flies.

Flies tend to be one of the most irritable insects for horses and their owners. If you feel like you are losing the battle against flies in your barn, you may want to consider feeding your horse a fly control supplement.

What Is a Fly Supplement for Horses?

Fly supplements are products that can be fed to horses in an attempt to control flies.

Oral fly supplements are traditionally used in conjunction with other products that can be used topically on the horse or in the environment.

Like most other oral horse supplements, fly supplements for horses come in a form that can be ingested by the horse by top-dressing the feed.

Fly supplements work by two main methods:

1. Reducing the number of flies by interrupting the fly life cycle

2. Making the horse less appetizing as a bloodmeal for the fly

There are two main types of fly supplements: insect growth regulators (IGRs) and fly deterrents.

Insect Growth Regulators

IGRs are substances in the fly supplement that pass through the horse’s body unchanged, into the manure. It is in the manure where they do their “magic” by inhibiting fly larvae from reaching adult stage.

Diflubenzuron and cyromazine are the two most common IGRs. They work by inhibiting chitin from forming the exoskeleton so that fly larvae cannot transition to adult flies. Horses do not have exoskeletons or chitin, so the IGRs do not affect the horse.

Two products that contain diflubenzuron and cyromazine are Tribute Equine Nutrition Essential K and Solitude IGR, respectively.

Fly Deterrents

Fly deterrents are substances in the fly supplement that make the horse less appetizing for the flying insects to bite. This is similar to how topical bug spray works: It masks the smell of the horse, making the horse less noticeable to the fly.

The two most common feed-through deterrents are garlic and apple cider vinegar. Hilton Herbs Bye Bye Fly, for example, is pure, cold-pressed garlic, while SmartPak SmartSupplements SmartBug-Off contains apple cider vinegar.

Ingredients in Horse Fly Supplements

Garlic

Garlic is known for its strong odor and taste. Feeding garlic to a horse may work like bug spray—masking the horse’s scent so flies are less likely to notice or bite.

Contrary to popular belief, garlic supplements do not work by making the horse smell like garlic all the time.

Some care should be taken in the amount of garlic fed to a horse, as it can cause Heinz body anemia in horses (similar to when dogs eat onions and garlic). Commercial supplements have a concentration of garlic that is below the threshold for causing Heinz body anemia.

Apple Cider Vinegar (ACV)

ACV is an acid produced by fermenting apples. Its topical applications are highly touted for being antibacterial and antifungal, but it is questionable as a feed-through.

Note that ACV is an acid, so attention must be made to the fact that an acid is being added to the gut, when generally we are trying to reduce acid to prevent ulcers in horses.

Do not feed more than one cup of ACV per day, and do not use in horses with a history of ulcers.

Diatomaceous Earth

Diatomaceous earth is a particle derived from fossils. On the microscopic level, the granules are very pointy. The sharp granules cause damage to the fly’s exoskeleton and lead to their demise.

When added to horse feeds, diatomaceous earth passes unchanged through the horse’s body and into the manure, where it helps prevent flies from landing on the manure to lay eggs.

Diatomaceous earth is more commonly used topically by sprinkling it around livestock areas and over manure. It is only effective in its dry state, so if it is a wet or rainy spring, it may not be very helpful in fly control.

Diflubenzuron and Cyromazine

Diflubenzuron and cyromazine are both insect growth regulators. These chemicals pass through the horse’s body unchanged, are excreted in the manure, and inhibit fly larvae from developing into adult flies.

IGRs do not kill or repel the adult flies but instead decrease the amount of new flies hatching from the manure. So it may take several weeks to see any difference in fly numbers.

If your horse lives in an area where there are many other horses, or if they are stabled at a boarding barn, feeding an IGR will not produce good results if all the horses are not fed the supplement. The flies hatching from neighboring horses’ manure will be equally attracted to your horse.

Does My Horse Need Fly Supplements?

Depending on the horse’s environment and routine, some may benefit more from fly supplements than others, including:

• Horses who live outside in warm climates

• Horses in a herd that have a higher concentration of horses per acre of land

• Horses stabled where manure is not picked up daily or where the manure pile is close to the pasture and barn

• Horses who cannot be brought inside during periods of high insect activity

• Horses who have an insect bite hypersensitivity with symptoms of hives, hair loss, or rubbing/itching

• Horses who have a pre-existing skin allergy

• Horses performing at a high level of activity where fly bites and itchy areas would cause discomfort under the saddle area

• Older horses with poor immunity or Cushing’s syndrome

• Horses who are especially bothered by flies, exhibited by running the fence line or pacing, or other behavior issues

• Horses who have an eye condition or disease and will not keep their fly mask on, or the mask is causing discomfort to their face

How To Choose a Fly Supplement for Horses

Decide whether you want a highly effective fly supplement or a gentler, more natural option—especially if your horse has a sensitive stomach or is a picky eater.

Keep in mind that these supplements will only help reduce fly numbers and not eliminate all flies. More is not better in the case of these supplements. Feeding more than the label recommends could result in toxicity. “Natural” alternatives can also be toxic in high amounts.

Always ask your vet for their recommendations for fly supplements for your horse, as some ingredients can react to other medications.

When used in conjunction with other fly control methods, fly supplements can be a useful aid in the horse owner’s summer arsenal of insect-fighting supplies.