Thermodynamics of Horses in Cold and Heat Research Paper

The Current Problem

Horses are homeotherms in that they are able to adjust their body temperatures in response to temperature changes in their external and internal environments. However, the major problem is that horses are unable to regulate temperature under extreme conditions of temperatures. As homeotherms, horses dissipate immense heat during exercise, which can be difficult to regulate. Usually, metabolic heat, which emanates from strenuous exercises, causes hyperthermia, a thermal condition that threatens the lives of horses. Hyperthermia is common among horses because they have the capacity to produce excess metabolic heat, unlike humans, who can only produce limited metabolic heat. Hodgson et al. state that horses have the capacity for very high metabolic rates because they can take oxygen at an excess rate of 100ml/kg/min (1161). The high rate of oxygen uptake increases metabolic rates and generates excess heat, which causes hyperthermia. Fundamentally, adult horses have a low surface area to body mass ratio, which makes it difficult for them to dissipate heat easily when they experience hyperthermia.

Horses also have the ability to adapt to low temperatures in the environment, particularly that of subarctic regions. Acclimatization is an important adaptive response that horses exhibit when they are under different climatic conditions. Climatic factors such as the radiation of the sun, relative humidity, velocity of the wind, rain, and ambient temperature influence the microclimate of the equine environment. In this case, ambient temperature affects the productivity of horses and is subject to other climatic variables. The collective effect of temperature and other climatic variables is effective ambient temperature, which has a marked influence on the productivity of horses. Since horses are homeotherms, they can regulate their temperature in response to thermal changes that are higher or lower than the thermo-neutral zone. In winter weather, horses experience lower critical temperature, which compels them to increase their metabolic heat to maintain their body temperatures within the thermo-neutral zone. According to Cymbaluk, lower critical temperatures for mature horses, yearlings, and newborns are -15⁰C, -11⁰C, and 20⁰C, respectively (65). In this view, lower critical temperatures among horses vary according to age, breed, and climate.

Possible Solutions

The first solution to hyperthermia among horses is to give them cold drinking water. During exercise, horses dissipate their metabolic heat through sweats and respiration, which cause the loss of water. Hence, drinking water replaces the water lost through sweats and respiration. According to Hodgson et al., without consideration of the weight of feces and urine, 90% of the weight that horses lose during exercise is water (1162). Therefore, it means that water plays a central role in the dissipation of metabolic heat among horses. The second solution to hyperthermia is bathing horses with cold water. As horses dissipate metabolic heat through the skin, cold bathing is an appropriate solution because water absorbs heat from the skin through evaporation. Essentially, during hyperthermia, peripheral blood vessels dilate to dissipate the heat through the skin. Cold bathing increases the rate of heat loss through the skin, for horses have a low surface area to body mass ratio. The solution is to protect horses from solar radiation and increase airflow so that horses lose heat through the convention.

The solution to hypothermia in winter conditions is to feed horses with additional food. Since effective ambient temperature below the lower critical temperature increases the production of metabolic heat, horses require additional energy to maintain their temperature at thermo-neutral zones. Cymbaluk states for every decrease in degree Celsius in ambient temperature below the lower critical temperature, a mature horse weighing 500kg requires additional 1535kJ (70). In this view, horses need digestible energy intake to maintain their temperature at a thermo-neutral zone and increase their productivity. Another solution to hypothermia is sheltering horses to protect them from the low ambient temperature. Cymbaluk states that sheltering horses in a well-protected mechanical shelter conserve 20% of energy among horses. Hence, shelter reduces heat loss significantly and thus, reduces the demand for energy as a source of metabolic heat. Heating barns using electricity or coal is also a solution to hypothermia. However, it is expensive to maintain the temperature of the barns at a thermo-neutral zone.

Questions

1. To what extent exercise causes hyperthermia?
2. Does the weight of horses determine their predisposition to hyperthermia?
3. How do changes in blood flow alleviate hyperthermia and hypothermia?
4. What is the appropriate composition of the diet for horses in winter conditions?
5. Does total insulation of horses determine energy demands among horses in a winter environment?
6. What is an appropriate model of a shelter that conserves optimum heat?

Interesting Quotes

• “Respiratory heat loss, estimated from the temperature difference between blood in the pulmonary and carotid arteries and the cardiac output, was estimated to be 30,19, and 23% of the heat produced during exercise at the three intensities” (Hodgson et al. 1161).
• “Despite having a sweating capacity that is almost twice that reported for human, the low surface area to mass ratio of this species imposes great demands on the thermoregulatory system during moderately intense exercise” (Hodgson et al. 1169).
• “In sweating animals, such as humans and horses, a major proportion of the heat loss during exercise occurs via the skin” (Hodgson et al. 1167).
• “Maintenance digestible energy (DE) intakes should be increased 2.5% per Celsius degree decrease in effective ambient temperature for adult horses and by 1.3% per degree decrease for growing horses fed for moderate gain at temperatures below LCT” (Cymbaluk 65).
• “An unprotected, equine mechanical model was reported to have a 26%, 18% and 9% greater climatic energy demand, under British conditions, than a sheltered-rugged, exposed-rugged, and sheltered-unrugged” (Cymbaluk 70)
• “To heat an uninsulated barn to 10⁰C when outdoor temperatures are between -15°C to -30⁰C may require heat input of 55 to 1230 watts for each horse model, respectively” (Cymbaluk 70).

Ideas that Come to the Mind

In the analysis of the two articles, the ideas that come to mind are how metabolic activities contribute to hyperthermia and how winter weather causes hypothermia. The metabolic activities of horses can occur at a very high rate, which overwhelms the thermoregulation mechanism. Low surface area to mass ratio limits the rate at which horses dissipate metabolic heat leading to hyperthermia. Moreover, effective ambient temperatures below the lower critical temperature elicit metabolic heat and increase energy demands among horses. When energy is not sufficient, hypothermia occurs since horses are unable to generate metabolic energy to maintain their temperature within the thermo-neutral zone.

Works Cited

Cymbaluk, Nadia. “Thermoregulation of horses in cold, winter weather: A review.” Livestock Production Science 40.1 (1994): 65-71. Print.

Hodgson, David, Laura McCutcheon, Sherell Byrd, William Brown, Warwick Bayly, George Brengelmann, and Philip Gollnick. “Dissipation of metabolic heat in the horse during exercise.” Journal of Applied Physiology 74.3 (1993): 1161-1170. Print.