I feed 2 flakes of alfalfa mix hay twice per day to each horse. I feed a small amount of complete feed once a day to deliver Hoffmann's minerals, hoof supplement and glucosamine (for the old guy). I also some extra grain (16% sweet mix) if it gets really cold. My guys have free choice water and salt lick. We have no grazing at all in the winter.
That said, my horses don't work much in the winter, but I do add additional hay according to the temperatures and the amount of work they do.
I have one of those thoroughbreds who needs quite a bit of food in order to keep weight on. Not as much as other thoroughbreds I know, but way more than some of the quarter horses he is boarded with. In the winter, he usually does just fine on his normal grain amount with a quarter cup of corn oil added to it. It helps him keep a nice little layer of fat without making him a roly poly pony. Other than that, he gets 2 flakes of hay in his stall morning and night, and all day access to a round bale with the rest of the heard.