This is one of the better articles on colic and impaction I have seen. Some time horses that won't pass can be loaded into a trailer, this response will encourage a bowel movement and may help get things going. If I notice a horse is not drinking or has sweated more than normal (foaming or salt marks on hair) I will dip my dry hay in water before feeding. This removes dust and wets the hay, which helps in getting a little more water and not so dry food into the horse.

Rick