Fibrous feeds produce the greatest amount of body heat during the digestive process. Hays, which are commonly fed to horses in winter since pasture isn't as readily available, produce considerable body heat. Oats, which has a fibrous hull, also produces body heat. Corn, a low-fiber feed, won't produce much body heat. This is in contrast to what some horse owners think.
Some owners think that corn is a winter feed, and oats is a summer choice.
The opposite is true. If you know you're in for a cold night, feed about 10-20 percent more hay. You may even want to feed a hay of slightly lower-quality than normal. The horse has a thicker haircoat in winter. Body heat is trapped in the haircoat.