What exactly is Critical Temperature and how is it managed?
Horses, like all mammals, have a thermoneutral zone (TNZ) which is defined as the temperature range in which the horse does not have to change metabolic heat production to maintain a constant core body temperature, which is normally between 37.2 – 38.2°C (Reece, 1991). The bottom end of this range is called the Lower Critical Temperature, or LCT, in which the horse must increase metabolic heat to maintain normal body temperature and the upper end of this zone is called the Upper Critical Temperature or UCT, where the horse must work to lower body temperature.
Wind speed, precipitation, and humidity are just some factors that effect TNZ. Thankfully, changes in temperatures do not typically happen immediately which allows horse acclimation to decreasing temperatures in about 10-14 days. Full acclimatization takes around 21 days.
Lower critical temperatures can vary drastically from one region to another and by the individual horse as well. According to McConagy, 1994, horses have coped well in a wide range of ambient climates from 135°F in northern Australia to -40°F in western Canada and have been able to maintain their internal body temperature. To maintain an almost constant body core temperature there has to be a thermal balance. The simplest form is expressed as “heat production = heat loss +/- heat storage" (Bianca, 1968). As mentioned above, the rate of heat loss is governed by factors such as wind speed, difference in temperature and vapour pressure (Morgan et al, 1997).
The horse is a large animal, relatively small body surface in relation to body mass and it has a wide thermoneutral zone (Morgan, 2007). As an example, McBride et al (1985) measured a thermoneutral zone for mature Quarter horse geldings of between 5°F and 50°F. In stating this, there are many parameters that affect LCT and estimating LCT is a multi-factorial challenge where the horses physiological state, metabolic rate, feed intensity, feed quality, age, size, ration surface area to body mass, housing, activity, acclimatization, season, and various climatic factors come into play. However, as a VERY GENERAL GUIDELINE, in the humid temperate areas of North America, horses reach their LCT around 20°F and their UCT between 60-70°F.
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.