I'd say 1/10th of a coffee can if not less. You don't need much...I think the scoop that my BO uses is maybe 1 oz or so...so she won't need too much...you probably could just give her a handful of it in the grian
A suggestion if I may: I have tried both garlic powder and garlic granules and have found them to be incredibly ineffective. I swtiched to putting cloves of fresh garlic in their feeds and then it started working. The powder and granules seem to lack the factor that repels flies. Just a thought ;)
Feed 1 pound of the onion tops per day to a healthy horse for 3 days, along with his regular feed, then 4 pounds on Day 4. From Day 4 through Day 8, his packed cell volume (or PCV, meaning his red-blood-cell percentage) dropped about 23 percent, from 30 to 23.
I don't crush them but I cut them into quarters. It depends on the size of the cloves as to how many I put in. If they are small or thin I put in about 6. Medium sized 4-5, large 3-4. That is divided out into two feeds. I tried in less than that but it didnt seem enough at first. This is the smallest quantity that I've found effective.
I tried the garlic granules for about 6 mo. And didn't have any success with it. I had them up to about 4 tablespoons a day. Never tried the fresh cloves though. They just eat them? Interesting!! Took a while to get my guys eating the granules, then they just starting chowing down like it wasn't there. I gave up on the granules, too expensive if it's not working.