Jane van Lennep answers:
Garlic seems to work for some horses and not others in keeping flies away – it’s the sulphurous smell that’s supposed to be effective. But as with all things, you can have too much of a good thing and too much garlic (or onions, which are closely related) can cause a problem called Heinz Body Anaemia, which can result in damage to the red blood corpuscles and lead to the horse becoming anaemic. In experiments, it took several pounds of fresh onions to induce the condition, but it is possible that smaller amounts fed over a long period of time might also cause it.
How much to feed is not a fact, more a guess, but as a guide, multiply your horse’s weight by two and divide by 100, then feed up to that much dried garlic in grams. So if your horse is 500kg, that’s 10g maximum of garlic per day. Bear in mind, though, that fresh garlic is thought by many to be too ‘strong’ for horses, so dried might be better. Fresh garlic is around four times heavier than dried so on that basis, your 500kg horse would end up having up to 40g per day of fresh garlic – a lot more than two or three cloves!
Garlic is a useful herb with antibacterial properties. It’s also a good source of organic sulphur, and as well as thinning the blood and improving the circulation, it also thins mucous so that coughing is more productive and, therefore, recovery from respiratory infections is quicker. Before antibiotics, garlic poultices were used on open wounds and it was the discovery of the high level of sulphur in garlic that led to the development of the sulphonamide drugs.