I have started three of my four horses on garlic in mid-March every year for the last four years.
Depending how bad the ticks are, they come off garlic no later than mid-November. For some reason this year, I have not seen the Fall surge of ticks that I usually do, so they are off garlic already.
From my four-year experience, garlic doesn't seem to do much at all to control flies, but it is at least 90% effective in controlling the ticks.
I see it every year because my fourth horse doesn't get garlic. He was starving when I rescued him 15 years ago and has always had a finicky stomach. He also had an ulcer Summer, 2004, so I don't "test the waters" by giving him garlic.
He is the one I have to keep sprayed with Repel-X during tick season; the other three don't need sprayed because the garlic runs a good tick defense.
So from my personal experiences:
1. I recommend seasonal use of garlic for tick control, but it seems to be worthless for flies.
2. I would not feed it to horses that are prone to colic and/or ulcers.
3. I feed a little less than the recommended amount. The recommendation seems like a lot to shove down a horse's throat. That is probably why my tick control is only ~90% effective, but I will take 90% any day :)
3.1 I have also learned to NOT add the garlic until I am ready to feed because it permeates the feed, making everything taste different. I don't have any feeding issues doing it this way.
I buy equine garlic by the 20# tub from http://www.herbs4horses.com
I always have enough left over from the current season to start the next season because it seems tick season creeps up on me before I know it and I want something already in the cupboard - lollol
Garlic is water soluble, meaning it won't build up in the liver or kidneys. It passes thru their system quickly.
The ACV does work to some degree for fly control, but I have reached a point where it doesn't work at all. Those HUGE black flies with stingers bigger than a vet's needle plow right thru the smell of the ACV. I have just started using chemical fly spray for the first time this year, except for the Arab who can't eat garlic, but I quit spraying him with Repel-X as soon as the ticks disappeared.
I was using Zonk until I ran out and had to open up the gallon of Bronco. They both work the same, IMO, but each person has to figure out what works best in their area.
Hope this helps.