Understanding when your horse needs additional assistance from you in an effort to stay warm will help keep your horse healthy and happy this winter. Each horse will have a different set of requirements based on his age, hair coat, body weight, and it will vary depending on the outside conditions (moisture and wind).
"Critical Temperature" is a term used to describe when your horse needs to produce extra body heat to maintain his core body temperature. A mature horse with good body weight will have a critical temperature around 30 degrees F (-1 degree C) at the beginning of winter. This critical temperature will drop to 15 degrees F (-9 degrees C) once the mature horse (in good body weight) has developed a good winter coat and has gained 100 pounds. Critical temperature will swing up or down depending on a horse's age, body weight, and hair coat. Young, old, and thin horses with no winter coat (or short-hair) will have a critical temperature around 40 degrees F (4 degrees C).
How can you help your horse produce extra body heat? Simple. Increase your horse's caloric intake by 15-20 percent for each ten degrees F the outside temperature falls below your horse's critical temperature. For example, if you have a 1000 pound horse and feed him 20 pounds of grass hay per day, then you would increase his daily feed by three to four pounds for each ten degrees F the outside temperature falls below your horse's critical temperature.
You can also help your horse stay warm by providing shelter from moisture and wind. Blankets and rugs can also be used for young, old, thin, and short-haired horses to help them stay warm, but keep in mind you will most likely need to increase their caloric intake as well.