My horses have never been blanketed in the cold. In North Dakota, we can also get to -20 below with a -50 below windchill. Very cold. My horses have always had windbreak and plenty of free choice hay. Outside. And I can count on one hand the number of times I've seen one of them shiver over the last 20 years, which they were put inside in those cases. Most healthy horses are quite capable with dealing with cold. But every horse is different.
I actually just experienced a horse seminar last week which discussed a horse's calorie needs during cold winter months. When the temperature drops below 30 degrees F is when a horse needs to start tapping into some extra calorie energy to stay warm. Therefore the horse needs to 1) eat more calories 2) be blanketed. Horses naturally have some 16 different "heating levels" just based upon how "puffed up" their hair coat is, which allows air to be trapped around the hair, warming the horse. You'll notice on colder days the horses look fuzzier. On warmer days, they look less so. That's their hair naturally standing on end, or laying flat to adjust to the right amount of warmth.
ALL horses need windbreak. Wind is the one thing there hair coats can't defend against. Wetness is also a factor that they have a hard time staying warm with, because it smashes the hairs down. If their is wind or rain, horses need more calories to stay warm. You may heed to supplement grain to reach to calorie needs.
If you are going to blanket, DO IT RIGHT. If you're horse is only a "little bit cold", you cannot just throw a little lightweight liner sheet on the horse to at "a little more warmth". But adding any blanket, you just squashed down all their hair -- their only warmth mechanism. So if you choose to blanket your horse, you've got to committ to it, and be ready to change blankets when the temperature changes around the clock, and to have a heavy enough blanket, and possibly a neck hood, when the horse needs it.
Have a blanket ready, but why don't you just watch him when the temperature gets cold? If he is shivering or seems to be dropping weight (make sure you FEEL your horses every day as their puffy hair coat can hide lost weight), then you'll need to start blanketing him.
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It is not enough to know how to ride; one must know how to fall.