Horse thermoregulation
Horse thermoregulation is a heat regulating device. It helps maintain body temperature within ± 2 ° C. We take rectal temperature as a benchmark.
=> the “standard” may vary:
- stalion : 37.6°C
- foal: 39.3 ° C
- lower in the morning than in the afternoon (we can reach 1 ° C difference)
Where does the thermoregulation of the horse take place?
- LSkin is the place of heat dissipation to the outside. Dissipation depends on the difference between skin temperature and ambient temperature.
=> The skin, the cutaneous tissues and the fat of the subcutaneous tissues are bodily insulating systems.
- The fat is particularly important.
- But fur also has a major insulating role. It retain a more or less important layer of air (with a quick thickness adjustment by the erection control).
Thermogenesis and thermolysis
Thermal equilibrium results from the balance between heat production (thermogenesis) and heat loss (thermolysis).
- On the one hand, the basal thermogenesis results from metabolic activity. When the horse moves, feeds ... it causes additional thermogenesis. Muscles produce more than 80% of body heat during work!
- On the other, the thermolysis is accompanied by a feeling of cold. It is due to:
- radiation (radiation emitted by a wall, a surface).
- conduction: by contact with a material that is not at the same temperature (solid or fluid).
- convection: by renewing a fluid (air, water) around the horse. The loss is accentuated in a draft.
- evaporation of water.
Thermal neutrality
There are 2 lethal room temperature extremes that exceed temperature control capabilities:
- too cold (hypothermia)
- and too hot (heat stroke).
=> thermoregulation will take place between the two.
- for an adult horse accustomed to moderate climatic conditions: its thermal neutrality zone is entre + 5 °C et + 25 °C.
- for an adult horse used to cold climatic conditions: its thermal neutrality zone is entre -15 °C et + 10 °C
=> il faut 3 semaines à un cheval pour s’habituer à de nouvelles conditions climatiques.
Estimate the heat experienced by the horse
We may think it is particularly "hot", but this is misleading. The accuracy of our interpretation depends on many factors such as:
- if we are acclimatized and how long we have been exposed to heat;
- how hydrated we are;
- if we rest or if we work and how hard we work;
- our individual tolerance to heat;
- whether we are in the sun or in the shade and whether it's windy or not ...
We measure the 5 components of thermal stress:
- temperature in the shade (hence the value of planting trees);
- the amount of humidity in the air (often measured in% relative humidity): to check live humidity level, at your place, have a look there ;
- the strength of the sun (the one that determines the index of our sunscreens): in cloudy weather the strength of the sun is less strong;
- the reflection of the radiation on the ground (hence the interest to reflect on the coatings chosen for equestrian surfaces - and also for the tracks)
- wind speed (remember to spot air currents in your home)
=> to get an idea of the "temperature / hygrometry" component allowing you to consider or not an activity with your horse, see this article here
1 - Fight against heat by facilitating thermolysis:
- To begin with, with redistribution at the level of the skin : the subcutaneous capillary networks (deep and superficial) make it possible to modify the insulating qualities of the skin. Those are “radiator” zones (ears, tail) favorable to caloric losses (short and not very dense hairs). They allow “long-term” regulation.
- And also by redistribution of blood to the limbs : particularly significant heat losses at the extremities. The oxygen supply to the muscles must be assured, therefore the blood flow is maintained. But, there is a redistribution of blood in the limbs.On a hot horse, you can see the veins under the skin
=> venous return borrows the saphenous vein (it's a peripheral vein). The arterial blood arriving at the level of the skin (not cooled by exchanges of heat against the current with the venous blood), is hotter: this facilitates the thermal losses. The ascending venous blood warms up on contact with the arterial blood which cools down as it descends. The anatomy provides for heat exchangers: veins surrounding a ventral artery. This allows the arterial blood to warm the venous blood coming from the skin and cool it down before it reaches the skin.
Showering the inside of the hind limbs helps thermoregulation: but this technique is very wasteful in water. . It is important to take this into account:
- to organize rainwater harvesting, for example, to supply horses' showers,
- or to favor other means of thermoregulation.
Brain case
Brain temperature should not increase by 1 ° C. A special irrigation allows its control with a dissociation between brain and body temperature.
- In the horse, the internal carotid artery passes through the cranial cavernous sinus, allowing counter-current heat exchange and thermoregulation of the central nervous system.
- In the case of the mounted horse, the pair of throaty air-filled pouches that envelop the internal carotid arteries are another thermoregulatory mechanism.
=> Showering the horse on the top of the head will optimize the radiating effect of the ears and promote thermoregulation of the brain.
Case of the testicles
In the horse, spermatogenesis requires a testicular temperature lower than body temperature.
There are anatomical devices that reduce testicular temperature:
- the muscles that relax when the ambient temperature rises and move the testicles away from the body,
- the warm blood of the testicular artery is cooled by countercurrent exchanges in the testicular veins,
- there is also no adipose tissue,
- but abundant sweat glands.
=> Showering the male at the level of the testicles will increase the thermoregulation effect.
2 - Fight against heat by increasing losses:
First :
L’evaporation of water. is an efficient cooling process. Under normal temperature and humidity conditions, 2 phenomena allow the loss of about 25% of the heat produced in the animal at rest.
- La sweating is predominant in large species.
The skin can be cooled to a temperature below room temperature, resulting in an exogenous supply of energy. It is carried out by the elimination of water by the sweat glands.
Horse sweat contains glycoproteins. They will promote the dispersion of sweat on the coat and therefore increase evaporation. This glycoprotein is responsible for the scum found in areas of friction (foaming properties).
=> When we shower a horse we will trigger a mechanism comparable to sweating:
- a first loss by convection (body heat will pass through the water which is heated by contact),
- and when you then pass the heat knife to leave only a thin film of water,this is the evaporation mechanism which is added (hence the interest in doing so).
=> if there are drafts where the horse is left to dry, this increases the loss (hence the precautions to be taken not to leave the horse anywhere, because it can really catch a cold ... ).
=> it is important to learn to pass the heat knife not to dry the horse but to leave just enough water to promote perspiration.
Secondly
Thermoregulation is one of the functions of the respiratory system. Thermal polypnea results in a sudden acceleration of respiratory movements.
The movements of air on a moist surface (nasal mucosa with the nostrils opening as much as possible, and tracheal) ensure a significant loss of water and therefore energy. It is usually accompanied by an increase in saliva production, which can therefore lead to a considerable increase in thermolysis.
WARNING
A horse can lose up to 15 liters of perspiration per hour of intense activity in high heat AND its perspiration contains 4 times more salts than that of humans.
1) we always leave water available to the horse at all times (without letting him jump on it).
2) it is necessary to plan supplement with electrolytes :
- of course, we leave a salt stone available at all times, but we know that the horse will not always consume it.
- We will therefore also be able to put salt on the hay or in the supplements distributed.
3) it is useful to learn to check the horse's hydration level with the skin fold test (Pinch the skin at the point of the shoulder and note the time it takes for the skin to return to its place):
- immediate replacement: the horse is not dehydrated
- in 2 to 3 seconds: dehydration is slight
- in 6 to 10 seconds: dehydration is really important and you have to act
- beyond that, the horse is in danger of death !!!
3 - Two ways to fight against heat by reducing thermogenesis:
1) Reduction of endogenous thermogenesis:
- reduced food intake: we will leave hay at will but do not supplement with foods rich in energy (of course we adapt on a case-by-case basis: this is not to put the horse in difficulty: in the case of the horse usually mounted, his “sporting activity being reduced, so too are his needs),
- food consumption at night: we can observe the behavior of horses and notice that their consumption will decrease during the day to focus on the cooler hours
2) And reduction of exogenous thermogenesis:
The surrounding environment can provide energy.
- adoption of a position vis-à-vis the sun minimizing the exposed surface;
- shaded areas are favored (natural, with many hedges, judiciously arranged according to the course of the sun - or artificial, with shelters, shade sails and others);
- the choice of floor coverings matters:
- we know that dark colors and mineral materials absorb heat and then release it.
- it is also known that light colors have dazzling reverberation properties. The best compromise has to be found.
4 - Other thermoregulatory mechanisms
There are mechanisms for regulating body temperature via:
- des nervous feedback mechanisms,
- and through temperature control centers located in the preoptic area of the hypothalamus.
=> there are temperature sensors that determine whether the body temperature is too high or too low.
- heat sensitive neurons
- others sensitive to cold
- as well as skin thermoreceptors.
=> the hypothalamus acts as a thermostat.
Thermoregulation in equine permaculture
The design of the site will allow:
- to locate the microclimates over the seasons;
- to choose the components to take advantage of favorable microclimates and reduce unfavorable microclimates.
One Comment