I actually read about this in Horse & Rider magazine recently. They tested several "recipes" and found it didn't work any better than what you buy, though it was cheaper. However, you have to spray it every 30 minutes, so it was MORE expensive by that rule (the spray you buy is good for several hours). I'll give you the link if interested and you can't find it. I have not heard of giving it in their feed, so I am at a loss there.
"If you talk to the animals they will talk with you and you will know each other. If you do not talk to them you will not know them and what you do not know, you will fear. What one fears, one destroys." - Chief Dan George