Although there are many excellent whitening shampoos on the market, one of the best ways to deal with urine stains in your horse's tail is to help prevent them. Every once in a while soak your horse's tail in a white vinegar/hot water mix. The properties in the vinegar eliminate mineral deposits and will actually smooth the cuticles of the tail making it much more difficult for urine to be absorbed. And the vinegar aroma is also a natural fly deterrent. Bonus!