How heavy of a blanket...?

For outside on those super cold days, I'd say a heavyweight (300 gram fill).

http://i940.photobucket.com/albums/ad245/WyattsMom...

(The second part isn't correct. Horse shedding has very little to do with heat, it has more to do with length of sunlight.) But that's a very good estimate.

But, obviously, every horse is different. If you have trouble with keeping weight on a horse, a heavier blanket may help. On the contrary, if you have a very fatty horse with a really thick coat, you may not need as much blanket. But, that chart works as a good general guide.

I would get a medium one and a heavy one to account for the difference in temperature. I do not go by weight, but what the blanket feels like and what it is designed to do.

If you are going to exercise the horses during the winter it would be good to give them a belly and neck clip to help them - you should not put a blanket on a damp horse as it will cause the lining of the blanket to become damp and the horse could get a chill. With a clip like this they can be exercised and still live out quite happily.

I would recommend a heavyweight blanket paired with a hood if they are kept outdoors or partially outdoors. These can be expensive, but you can save money by getting a high quality one that will last you a few winters. If they are stall bound, a medium weight blanket and hood may suffice depending on how truly bitter the weather is.

If you keep them blanketed, their coats will often be in better condition and you will not have to feed them as much food to stay warm. Not to mention they will be very happy that someone cares about them enough to keep them rugged up and cozy warm!

Just make sure to groom them daily and check for any health issues as you normally would if they were not blanketed.