Problem with horse supplement?

I have tried Max E Glo as a supplement for my horse and he started losing hair within a week. Has anybody ever experienced this problem? I asked the vet if it could be an allergy to the rice bran or something and he said he doubted it, he'd never heard of such a thing. Let me add that this was not my normal vet, but an intern who was the only one available for a farm call. He saw the missing hair and was puzzled because it didn't look like rain rot (which I knew it wasn't) or any other common skin condition and told me to stop using it and see what happened. I stopped and had no more hair loss problems. Several people have suggested I try this product and they just love it. Any other suggestions?