To blanket or not to blanket...that is the question many horse owners ask themselves as the days grow colder.
Horses in the wild have many natural abilities to help them keep warm during harsh winter weather. Wild horses grow thick coats for warmth, and instinctively seek shelter from wind, snow, ice and cold rain. If given plenty of hay, fresh water, and allowed to grow their own natural winter coat, many horses can spend the winter outdoors without a fancy turnout rug or blanket.
Some situations, however, do call for blanketing a horse during the cold winter months.
Showing in Winter
Keep a horse's coat show-worthy during muddy, wet winter months can be a difficult task. Unless your barn offers heated wash stalls, it's almost impossible to bathe a horse in the winter. Turn-out rugs and stable blankets can keep the worst mud and manure from your horse's coat so that a bit of elbow-grease the night before the show can bring out the bloom in his coat and make him blue ribbon-worthy once again.
Riding Exclusively Inside During the Winter: Indoor Arenas
Indoor arenas are great for schooling your horse during winter months, but rigorous exercise makes horses sweat more. If they have heavy winter coats, it may make it difficult for perspiration to evaporate during winter schooling sessions. Riders schooling indoors during the winter often clip their horses to keep them comfortable during school sessions. Clipped horses, however, should be blanketed, because clipping removes their natural protection against the cold.
Horses That Don't Grow Thick Winter Coats
Some horses just don't grow a thick winter coat. Many Arabians, Thoroughbreds and other so-called "hot" blooded horses do not grow a thick enough coat to offer adequate cold protection in very frigid temperatures.
Moving Horses from Warm to Cold Climates
If you've just purchased a horse from Florida or another warm climate and you're shipping him to Minnesota in January, it makes sense to blanket him. Horses may need some time for their bodies to adjust to the colder winter months.
Old or Sick Horses
Some older horses have difficulty remaining warm during winter months. Illness can also make some horses more susceptible to the cold and damp. Follow your veterinarian's advice if blanketing is recommended for your horses.
Sources:
http://www.drsfostersmith.com/pic/article.cfm?aid=1599
http://www.petplace.com/horses/to-blanket-or-not-to-blanket/page1.aspx
http://www.equisearch.com/horses_care/health/winter/to-blanket-your-horse-or-not/