Summarize:

Your horse's nutritional needs will vary according to the amount of fresh grass she has consumed while out to pasture and the level of activity she has had. Evaluate your horse's needs every day to determine whether to subtract or add from her regular feeding amounts. If your horse has been out to pasture all day and has been eating lots of grass, she will not require as much hay. If your horse has had a hard workday with lots of riding, then you will need to provide more food to help replenish the extra calories she burned. Don't feed your horse right before or right after she has finished doing something strenuous because blood flow will be diverted away from her organs and that can interfere with digestion. Plan feedings around your horse's scheduled activity. If your horse will be doing something extra strenuous, schedule her feeding for three hours before the activity. If you need to change your horse's feed, do not just switch to the new feed. Start by replacing 25% of the old feed with the new feed. In two days, replace 50% of the old feed with the new feed. Two days after that, replace 75% of the old feed with the new feed. Then two days after that you will be able to give your horse 100% new feed. In addition to making food changes gradually, you should also feed your horse at about the same time every day. Horses perform better when they have a regular feeding schedule. Making drastic changes to your horse's feed or feeding schedule can lead to equine colic and founder. Equine colic is a condition that causes severe abdominal pain and may even require surgery. Founder is a condition that causes poor circulation and can even lead to the separation of the hoof from the foot. Founder is often fatal.
Adjust your horse's food intake as needed. Schedule feedings for an hour before or after you have ridden your horse. Make gradual changes to your horse's diet.