In some cases during the sheeding season some horses will loose all the hair in patches before growing new hair. We have a couple horses at our track that go thorough that every year and the hair does grow back. I would also recomend a great product out there that will incourage hair growth and you can buy it as a shampoo. Its called Shapley's MTG. I use some of it on a bald shop of my gelding that he has had for over 2 yrs and now have hair growth!!! You can view their site here and also order from them: http://www.shapleys.com/index.html