For horses, consistency in their diet is key to good health. Horses develop a microbial population in their intestines that is specific to what they are being fed. Changing it can cause digestive upset that can lead to colic and, in extreme cases, laminitis. According to a recent SmartPak Webinar, 10 Guidelines for Smarter Feeding, changing hay can increase the risk of colic by a factor of 10; changing other feeds increases the risk of colic by a factor of 5.
It takes time for the gut to develop a new bacterial population so most people recommend changing no more than 25% of your horse’s ration at a time, giving its intestinal bacteria time to develop. Of course, if you feed just a handful of grain or a small amount of ration balancer you can change over more quickly than a horse eating 8 to 12 pounds of a complete feed, so use your judgment.