Corn oil is high in Omega 6 fatty acids, with almost no Omega 3's. Omega 3's are what help skin, coat, and hoof growth. Horses already get plenty of Omega 6's in their hay.
Flax seed and meal is very high in Omega 3 fatty acids, so you should see improvement in the horse's coat as he/she sheds and grows in a new coat. Agway BROWN Flax Seed 25lb BULK BAG on eBay! Buy Flax Bulk Brown Flax Seed Golden Flax Seed - Granary Bulk Foods - Appleton, WI - Granary Bulk Foods - Appleton, WI (920) 830-3311 Brown Flax Seed 25lb - Nuts & Seeds - G - N - Bulk
Your local feed store should be able to order you bulk flax or flax meal. I prefer to feed flax meal. My feed store gets 50 lb bags for about $28, which is much cheaper than online prices. Flax Seed Meal 50 LB View Cart
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I don't have any experience with garlic, but I have had decent luck using Apple Cider Vinegar to help with flies. I feed 1 cup of ACV during fly season in my horses' feed.