Helli.c
Member
I have heard adding small amounts of garlic granules/powder is good for keeping flies away in summer but are there any other benefits to it? 
I've found it's also good for the immune system as it acts as a natural antibiotic and also has anti-inflammatory properties. It was a godsend for my last horse and also my new boy in helping to clear up excess mucus post surgery/infection and also seems to help reduce coughing episodes too.
From Dr Gettys website:
GARLIC MAY NOT BE WORTH THE RISK
Do you feed garlic to help keep the bugs away? The active ingredient in raw garlic is called allicin (also known as N-propyl disulfide) and can lead to Heinz Body anemia. Commercial products use a heat processing which supposedly destroys all of the damaging allicin, but also reduces the insect-repelling properties. It's true that the strong odor will help keep bugs away, but most folks love the real smell of horses, and would not enjoy one that reeks of garlic. Bottom line... the jury is still out on the long term safety of garlic-containing products. Until we know more, feeding garlic may not be worth the risk.