OK... so I've heard that horses are fine out in the cold as long as they have a run in shed & lots of hay. My 2 horses have a pretty good winter coat & have stayed out thus far this winter. I have been told not to blaanket because it makes their coat lie flat & the air can't circulate. We have a barn, but, they aren't used to being in it & I also think they will stay watmer if they can be close together...
I've never put one of my horses in the barn, they are outside all year long. I've haven't had one get sick on me yet. The only time we stall our horses is when are mares are ready to foal, we take them out of the pasture 30 days before they foal so as to get them off of the fescue and feed them hay only (and a little "mare & foal" grain). Then we leave them in the stall with the new foal for 2 - 3 weeks.
If you don't have a heater (and you may have needed one before you read this!) you will have a trough full of ice!
My horses stay out too, unblanketed-however I do feed extra hay if it's below freezing. I feed by weight, not flakes, at 20percent of horse's body weight routinely. The energy required to digest hay helps them stay warm. I also try to keep hay in front of them 24/7, just have to put more out on the colder days.
Drinking plenty of water will help keep horses from colicking, and they are more likely to drink water that is not almost frozen. I put a tank heater in my troughs as soon as we start getting freezes at night. My horses are drinking almost as much water now as they did during the summer. TSC has several different tank heaters, as does our Co-Op.
I recommend a trough heater. It will help save you time, and it will keep the water a constant temp. so the horses are more likely to drink it. A must this time of year! You don't want an impaction colic on your hands.
They should be fine outside in the cold. They can withstand up to -40 with a full winter coat. I always check the wind chill factor, and if you have a hard-keeper..hard to keep weight on, or older horse that is hard to keep weight on, you might consider blanketing on a really cold night to help w/the wind chill. I blanket mine to keep them dry and warm, even though they have full coats of hair. Their hair fluffs back out when they roll if you take the blanket off.
Mine don't even have a run in shed now. They have plenty of cedars and a good wind break at the side of the barn. I am using a friend's pasture until I can buy a place. I am recently divorced (well since May) and they had a barn there, and never used in in winter. I feed about as much hay as they will eat. It is a grass hay, not bermuda or alfalfa. I really believe in them grazing all day. Keep them brushed though, if their hair matts up they don't have the insulating protection they need.