People often hose off their horses with clear water to remove heavy sweat or caked-in mud several times a week. The clear water is not harsh to the horse's hair coat and doesn't strip it of the natural oils. Washing a horse with harsh shampoo can strip the natural oils from the horse's coat, so always use a mild shampoo if it is necessary to wash the horse. The most important thing is to rinse thoroughly to make sure all the shampoo is removed. If you are showing, wash the horse two days prior to the show and that will allow enough time for the natural oils to get back into the hair coat. After that, if you need to rinse or spot wash, you won't lose all the natural oil in the hair coat. You should also take the weather into consideration. The best time to wash a horse would be on a warm (70o F or higher), sunny day. Horses can become chilled just like people, if bathed on cool days.