I visited a stud today and noticed that not a single horse had flies on it. There were flies biting me, but no flies on the horses. The owner showed me the feed they use, and said the fly inhibitor was in the feed. So I went to the feed source to ask about the ingredients. He told me they were no longer putting the ingredient in the feed as it "there was something wrong with the tag". So I looked up "insect growth inhibitor" and saw this product, cyromazine. It is widely available and I could not find anything negative. It works by the horse ingesting the chemical and it goes through the horse. When the fly breeds on the manure, it cannot mature and reproduce. So, I saw a lot of flies around the barn, but not a single fly on the horses. I had dropped my little hrose off for training and the flies were on him. And they were biting me. Does the horse's body emit something detectable to the fly to keep it from biting the horse? I have a call in to our local county agent about the product.