Fall is when neighbors to the south breathe easier, but for northern folks, fall means winter readiness. And for horses this could mean packing on some extra weight to prepare when higher energy requirements are needed. Go in a little on the chubby side by increasing the hay. It doesn’t hurt to add some corn or vegetable oil to the diet but that can be an expensive way to go. If you do have a hard keeper, grain with fat added can be a good option. Sugar beet pulp is a nice fiber source high in soluble carbohydrates. It is easily fermentable fiber so you don’t have to worry so much about digestive problems. Incidentally it’s a high-energy source that is low in phosphorus [unlike some grains], which is an extra benefit because the horse won’t have a high-phosphorus excrement that’s released into the environment.
Horsepeople who live in the Great White North know that when it’s a super cold day they can warm their hands by pushing them deep inside a horse’s coat. That heat is produced inside the horse’s digestive tract, which is stoked by fermentation. In other words, horses have their own little central heating system. You can help keep that warmth flowing by providing the right materials. For the horse, this means fiber.
When a horse digests long-stem fiber (hay), microbial fermentation occurs and heat is created. This has a benefit to the horse in times of cold weather because fiber is digested slowly and heat is sustained for quite a long time.
Although it’s difficult to pinpoint a precise amount to feed, an extra flake of hay on a cold day would be a good idea.
But what is really cold to a horse? The thermoneutral zone (TNZ) is the range of environmental temperature at which the animal uses minimal energy to maintain body temperature--- the "ideal" temperature for comfort. The TNZ for the horse is lower than for a human. If the outside temperature is in the single digits and lower the horse will need more fiber to stay warm. If it’s in the ‘30s then requirements aren’t as great. However, factor in wind and cold rain too, which will increase energy requirements.
Other calories gained from fat and grains, such as corn, can still be used for warmth although it won’t produce that long, sustained heat. A horse will use that feed for whatever purpose it needs, whether it is running a race or keeping warm, but the fiber automatically generates heat.