Lower Critical Temperature for HorsesBy Kentucky Equine Research Staff · February 25, 2011
The thermoneutral zone (TNZ) for horses is defined as the range of temperatures in which the horse maintains its body temperature with little or no energy expenditure. Essentially, the TNZ is the temperature range wherein the horse does not have to work to raise or lower its body temperature.
The lower critical temperature (LCT) is defined as lowest temperature in the TNZ and is the temperature below which the horse must increase metabolic heat to maintain normal body temperature. Above the upper critical temperature (UCT), horses must work to lower body temperature, usually by dilating blood vessels in the skin, sweating, and/or panting.
Body temperature is produced and maintained by several normal metabolic processes, including the heat of nutrient digestion. Heat is normally produced in most chemical reactions and by muscle contraction. In horses (nonruminants) and ruminants (such as cattle), the fermentation of fiber by gastrointestinal microbes produces large amounts of heat. Normal rumen temperature in cattle is usually around 102° F (39° C). Body temperature is also affected by feed intake and types of feed consumed. Increasing digestible energy intake increases production of body heat.
Body temperature of horses is influenced by the ambient temperature, wind, sunlight, precipitation, and relative humidity. The most common causes of cold stress that horse owners battle against are wind and precipitation (rain or melted snow). This emphasizes the importance of blankets or windbreaks so horses can remain dry, which in turn helps them stay warm. Young, thin, or older horses are less tolerant to cold than other horses.
The TNZ for a given horse is also influenced by the temperatures to which the horse has been accustomed. Full acclimatization takes about 21 days, but horses usually adapt to temperatures in 10-14 days. The wide range of the LCT in adult horses extends from 41° F (5° C) for horses in mild climates to 5° F (-15° C) in natural-coated/unclipped horses adapted to very cold temperatures. Young horses and foals have higher LCT than adult horses. Defining the UCT is more difficult and researchers have suggested that it can range from 68-86° F (20-30° C).
Several studies have confirmed that cold weather increases the need for dietary energy and that feeding free-choice, good-quality hay is the simplest way to provide additional digestible energy in mature, idle horses. The most important management steps to decrease cold stress in horses are to provide an area where horses can escape from wind chill, keep horses dry, and increase dietary energy to provide more calories that the horse can use to stay warm.