Vitamin A is very important so if you have a 1000 pound horse that is healthy our vet recemends 15,000 IU daily. Lactating mares need about 36,000 IU or more daily. You would have to give a horse 10 X the recemended level of Vitamin A to overdose it. The fact is, the less healthy the horse, the more vitamin A is an absolute must. Vitamin A will get rid of rain rot, weepy eyes and poor overal skin conditions. It works, it makes sense and it is not expensive.
If you are not a believer try it for 2 weeks and see if it makes a difference. Its alot less work, and you dont need to worry about picking off scabs and causing your horse a great deal of discomfort. We have had zero cases of rain rot, we feed vitamin A daily in a supplement during the winter months and as everyone knows Canada has a very cold, wet, nasty climate during the winter. It works!