Do Fly Repellent Supplements for Horses Really Work?

I board my horse at a facility with approximately 30 other horses. There is adequate pasture space for all. And fairly good manure management. My question is, do fly repellent supplements work? I know that for larval killer, all horses must be on it. But will the repellent kind like garlic mixes work at all? And if they do work I would like to start my horse on them soon so that they are protected for the coming fly season. – SM

Dear SM,
This is a tough one, as the evidence to support the use of garlic and other products (apple cider vinegar, diatomaceous earth) to repel insects is mostly anecdotal. That is, numerous owners report excellent results from giving their horses garlic to deter biting bugs. In fact, many people swear by these natural products and continue to use them year after year.

Due to the widespread popularity of garlic in horses, several research studies have evaluated the safety of this herb since very high amounts are known to cause a specific kind of red blood cell disorder. A paper published in 2005 showed that horses that would voluntarily eat more than 0.25 grams of freeze-dried garlic per kilogram of their body weight twice daily did develop Heinz body anemia. To put this into perspective, an 1100 pound horse would have to eat 4.4 ounces of pure garlic in two separate meals. That’s 125 grams or 125,000 milligrams. Now, this is much more garlic than any equine supplement contains or recommends be given. For example, there is 1,500 milligrams of garlic in one serving of SmartBug-Off Pellets, or about 83 times lower than the upper safe limit as determined by this particular study.

Try giving garlic to your horse and see if you notice a difference in the number of flies, mosquitos and even ticks that think your horse is a tasty morsel. If you do, then I wouldn’t hesitate to keep him on a daily garlic-containing supplement for the duration of the season. If you do not notice a difference, then unfortunately garlic doesn’t react with your horse’s individual chemistry to discourage these pests. And because supplements that contain garlic work on an individual, horse-by-horse basis, if your horse is the only one in the barn that receives garlic, lucky for you!