If you are feeding a vitamin fortified feed then be very careful about feeding any additional supplements. As an oil based vitamin Vitamin A can be overdosed and the overdosing is more of a worry than underdosing. Horses on green pasture rarely suffer from Vitamin A deficiency.
Skin health/strength can be compromised by the lack of minerals Zinc and Copper and safe supplementing of these is far easier than giving additional vitamins. A soil sample can be easily collected and tested for these minerals. If you think your horse may be deficcient in Vitamin A you need to talk to your vet.
Please read this information sheet before dosing your horse with any additional oil based vitamins. These are vitamins A, D, E & K are all oil based which means they are stored by the body and if given too much can be toxic.
Vitamin A in Horses
Note to cherie - My horses live totally on grass, live out year round, uncovered - they get no hard feed, no supplements but get a multi mineral block that they have free access to. They don't get mudfever or any other problems - vet has only seen them for routine vaccinations. While you may think your treatment with Vitamin A has been the reason for the health of your horses it is more likely that your general care is good and even if you didn't give the Vitamin A they would still become healthy in your care.