I used to use garlic too a long time ago...then I read an article about the possible links to some serious diseases, particularly anemia. What about trying a feed through fly control supplement? I think you can get Equitrol for under 20 bucks.
Which serious diseases, other than anemia?? I would rather use garlic than a feed-thru control if that feed-thru control is chemical based.
It is the allicin in raw, unprocessed garlic, that MIGHT cause anemia if fed in massive doses.
I have been feeding processed equine garlic to three of my four horses seasonally for seven seasons and they pass their physicals every fall with flying colors.
I use it as a tick repellant and it is about 90% effective. Given where I live and that the deer ticks and lone star ticks are lined up at the gate every morning, 90% is fantastic.
I start out at 1/2 teaspoon twice daily in mid-March and work them up to a slightly heaping teaspoon twice daily by late May. I discontinue in the late fall, whenever it's cold enough to stop the ticks from crawling all over everywhere.
The fourth horse does not get garlic because he has ulcer issues, so I have to spray him.
For my part, I would not feed garlic to a horse that has colic or ulcer issues or that has been sick in a way that might cause anemia.
Hope this helps