Making sure the hay is a high quality forage will keep the horses from developing that “hay belly”. Large amounts of indigestible, tough, stemmy, hay lays in the large intestine of the hindgut, causing the intestine to stretch, giving the appearance of the hay belly. (But make sure the hay belly isn’t from worms, as commonly seen in young horses that have poor nutrition).
(Photo from last winter)
Provide water that isn’t froze over! Horses won't drink freezing water, or as much water as they should. Keep the water thawed. Some people even cover their tanks and only leave a small opening for the horses to drink. My horses would probably destroy the cover, nibbing with it all the time! We have put a large piece of white foam insulation under the tank, cutting it the same size as the water tank. That lasted until the horses kept pawing at the edge of the tank, breaking up the styrofoam.
Provide salt, so they lick the block when they need to. I also feed a small amount of a granular mineral salt mix with their grain. Either way will make the horses thirsty, and drink more.
Grain is optional. I just starting graining everyone again, as I have young growing horses, mares in foal, and some working horses, even though they aren't working too hard right now. Grain does not provide the digestible heat that hay does though.
I give a high fat grain, as the high fat helps the horse to utilize the nutrients. I also grain with a product high in vitamins and minerals.
I don't use winter blankets, but if you do, make sure they are waterproof!
Check their hooves, and now may be the time to trim before winter hits harder. But I don't like a hoof too short, as they may bruise their sole on the frozen ground. Keep the hooves cleaned out from ice balls. Hooves won't grow as fast during the winter, as nutrition is going to keep the horse warm and not to coat shine and hoof growth.