If you are starting to add more feed to your horses diet you should use a step wise manner to introduce it to your horses diet, especially if adding grain.
The reason behind this is to allow the enzyme activity and the microbes a chance to adjust and increase their ability to utilise and digest the feeds correctly. Adding grain quickly to the diet can disrupt and damage the microbes in the digestive system leading to complications such as hind gut ulcers and also colic. These such things want to be avoided at all costs! Happy Riding Everyone!