We're getting to the time of year when horse owners begin feeding their horses garlic as a means of repelling insects. Many horse owners do this without much thought. Some feed garlic very thoughtfully, because they associate the human benefits of garlic in the diet to their horses.
While there is some evidence in humans that applying a dilution of garlic oil topically (on the skin) will repel insects, there is no scientific paper published that provides evidence that feeding garlic to horses repels insects. In fact, research indicates that garlic is dangerous for equines. The 1972 study published in The Journal of the American Veterinary Association stated that the toxic dose for horse is less than 5mg per kilo of bodyweight.
Here are some pertinent quotes from the research:
Horses will voluntarily consume sufficient quantities of garlic to cause Heinz body anemia. The potential for garlic toxicosis exists when horses are chronically fed garlic.
Horses showed improved hematological parameters 4-5 weeks following removal of garlic supplementation.
Nutrient Requirements of Horses 2007
I've no doubt those of you who are feeding garlic to your horses are doing so because you want only the best for them--the best health and the highest degree of comfort. That's why I feel it's important for you to understand it isn't enough to say garlic is safe just because you haven't seen any ill effects in your garlic-supplemented horse. Depending on the dose, and the frequency and duration of dosing, there could be low-grade deleterious effects, due to red-blood-cell damage that's not enough to cause a 9-1-1 situation, but just enough to cause a mild anemia that might not be outwardly evident. It might affect your horse's stamina, energy level, or resistance to disease.
If you're sceptical, please take a look at the resources listed below. Are you sure that feeding garlic is actually repelling insects? Even if you strongly feel that it is, are there other ways to achieve that outcome that does not have scientifically proven associated harm?
If your goal is to repel insects, there are many herbal and chemical topical sprays that will achieve that outcome, particularly when used in conjunction with a cotton or mesh fly sheet and a well fitting fly mask.
Nutrient Requirements of Horses - National Reserach Council 2007
American Journal of Veterinary Research, Association of maximum voluntary dietary intake of freeze-dried garlic with Heinz body anemia in horses, Wendy Pearson 2005
Acute hemolytic anemia caused by wild onion poisoning in horses, "Journal of the American Veterinary Medical Association," 1972; pages 160/323 Pierce, K.R., et al.,o 327.
The Great Debate: Feeding Garlic To Horses
Dr. Karen Hayes, DVM, MS, 2002
Lewis L.D. "Equine Clinical Nutrition," Williams and Wilkins, 1995, page 480.
Kobluk, Ames, and George, "The Horse; Diseases and Clinical Management," Saunders, 1995, page 1,083.
Knight A.P. and Walter R.G. "A Guide To Plant Poisoning Of Animals In North America," Teton NewMedia, 2001, page 186.
Murphy M., "A Field Guide To Common Animal Poisons"; Iowa State University Press, 1996, page 160.