Associate Professor, Animal Science
Iowa State University
A horse’s adaptation to cold weather is either acute (immediate) or chronic (long-term).The immediate response of a horse to a sudden change in temperature is to change its behavior. Horses seek shelter from the cold and wind, or huddle together, to decrease heat loss. Horses will stand with their heads away from the wind, their tails set low and into the wind. Horses stop foraging and stop moving to conserve energy. On the other hand it is not unusual to see horses running in cold weather, which increases muscle contraction causing heat production. Shivering and other voluntary muscular activity can also generate substantial body heat. For chronic exposure, horses require 10-21 days to acclimatize to the cold. Horses develop a heavy winter hair coat. The coat acts as a tremendous insulator. Cold weather will cause the hair to stand up (piloerection), trapping and retaining body heat.
The thermo neutral zone is when metabolic heat production does not need to be increased to maintain thermo stability. The lower critical temperature (LCT) is the temperature below which metabolic heat production is increased to maintain core body temperature. Once the LCT is reached, physiological changes and human intervention, such as shelter and/or extra feed, are needed to help the horse cope with the cold. The LCT for young horses can range from 12.2 degrees F to 32 degrees F and in adults the LCT is as low as 5 degrees F.
When cold weather persists at temperatures below the LCT, then an increase in dietary energy is required. Dietary energy is the only nutrient that must be increased for horses kept at temperatures below their LCT. The main source of dietary energy (calories which convert to heat) is obtained from feed. Other sources of heat include the sun, muscular activity and mechanical heat in barns.
For each decrease in coldness of one degree F below the critical temperature there is an increase in digestible energy requirements of one percent for body temperature maintenance. Forage (hay) is the most desirable method to meet a horse's elevated energy requirements. Forages contain higher fiber then grains. Fiber is utilized through bacterial fermentation within the cecum and large intestine. Much more heat is produced in bacterial fiber fermentation than in digestion and absorption of nutrients within the small intestine (grains). The result is that a greater amount of heat is being produced through the utilization of forages.
Feeding good quality grass hay is the simplest way to ensure the horse will meet its energy requirement in the cold. If a horse is eating a round bale or large square they should be fine in terms of eating enough to maintain energy balance. If you limit feed, feed 2X per day, the horse will need 1 to 3 flakes of extra hay per day. Energy intake is the most critical factor in determining how readily a horse develops a tolerance for cold. If a horse does not eat enough energy to offset the heat loss due to the cold, the horse loses weight. The extra cost of feed to rehabilitate a thin horse back to normal will equal or exceed the cost of the feed that should have been fed to maintain the horse’s body weight during the cold.
Horses should have access to some type of shelter – a timberline, natural bluff, or a shelter. The shelter is typically a 3-sided shed. Recommendations for the size of shed are 100 sq. ft./foal; 120 sq. ft./yearling and 150 sq. ft./horse. Horses do conserve up to 20 percent more body heat in a shed compared to an open exposed area.