It's depends. If your horse lives in a frigid climate and grows a thick woolly coat that makes him look like a Sasquatch, you're probably fine. However, if you see shivering and/or weight loss that's a different story.
Location, Location, Location
If you live in the south and it was in the sixties yesterday and this morning the wind chill is 10 DEGREES (which is exactly what's happening right now!), you bet the horses at Pure Joy Horse Haven are wearing blankets! Their coats are not ready for this kind of weather, nor are they physiologically acclimated for such an extreme temperature drop. Not only is it cold but the wind is wild and that huge of a transition can put a lot of stress on your horses' systems, which blanketing mitigates.
Shelter Helps...to a Point
Don't assume your stalled horse is comfy unclothed. Horses generate heat through movement. When movement is limited, a chill can set in. Of course shelter makes a difference, but when it's really bitter out, it may not be enough.
Just Feed Lots of Hay
It's a common belief that if a horse has hay in front of him, he'll stay warm. Horses have an internal furnace that runs on fiber, which hay provides. It's very efficient, but it isn't always enough. Even though the furnace is getting enough fuel, cold air could draw heat away faster than your horse's ability to maintain a steady core temperature.
Let's do the Math
A horse can only eat so much hay in a day. Let's say as an example your horse eats enough hay to provide 10,000 calories. In order to avoid losing body weight when the chill is on, he needs 15,000 calories. What's the result? The horse's inside will be less than his output. He'll shiver to mechanically increase heat. That's a normal response but takes a lot of energy to sustain. Eventually, the horse will start to lose weight. Of course, adding blanket insulates the horse and reduces his caloric output so he can better maintain physiologic equilibrium.
But Wait. There's More
Other factors to consider when you're deciding whether to blanket or not are:
The horse's health
The horse's current weight
Coat length and density
If you have an older horse with no shelter who's a little skinny and has a thin coat and the wind is howling during a driving rain, yeah... put a blanket on!
Every horse is an individual. You may have one that laughs at gale force wind and temps in the teens, while you can hear his field-mate's teeth chattering from a mile away. Do whatever works best for that particular horse at that particular time.
Cold Weather Feeding
When the temps plummet, hay alone may not be a rich enough source of calories for your horse. In fact, a hay-only diet doesn't provide adequate nutrition for any horse, but we'll save that discussing for another blog.
To increase your horse's ability to keep his furnace fueled, you can add high energy food like beet pulp, alfalfa, and ground flax to your horse's meal. Fiber is the BEST source of energy. Adding a healthy fat source can help on those really cold nights too.
It's also VERY important to make sure drinking water is unfrozen and easily accessible. Add a generous amount of loose white salt in your horse's bucket. 1.5 tbls min per 1000lbs 2x day and provide a white salt block Salt will trigger the thirst response and help your horse stay well hydrated.
Which Blanket is Best?
Your options are a non-insulated rain sheet, a light-to-medium weight fill, a blanket with heavy insulation, or combining two of the above.
Putting a waterproof sheet on in wet weather might be all your horse needs to be comfortable. Usually horses handle rain without batting an eye. If their skin gets soaked through and the air is brisk, they will lose body heat very quickly.
Heavier weight insulation will create a more effectivebarrier between your horse and the elements. Experiment with different options to see which offers the right amount of protection depending on the circumstances. As a general rule, choose a blanket that provides just enough to keep your horse comfortable vs. more than he needs.
Horses radiate tons of heat. Trapping it against their body creates a very unhealthy situation that can cause physiologic harm and emotional distress.
The Bottom Line
Yes, horses are well-adapted to cold weather, but they will appreciate a warm pair of jammies when the mercury dips lower than they can comfortably handle.