I had a phone call tonight from a lady wondering if Miniature Horses are as hardy as the big horses. In many ways they are, but it is important to know that babies can over heat, dehydrate and die. The babies can fall asleep in the sun, not realizing they are hot and overheat. After about 8 weeks, they wake up more easily and realize to move into the shade, but we watch them very closely when they are young.
Also, here in Arizona, it is not unusual for a June day to get over 105 degrees. Obviously, shade is a must. One mistake many mini owners make it to build a short barn. The heat is held in down closer to the minis. Even though, they are small, it is still a good idea to build your shades up high.
Many of us install fans in our barn. These not only help with temperatures, but are a huge help during fly and mosquito season.
Then, there are misters. We have not found them necessary, but they are a good way to drop the temperature 10-20 degrees!
After a heat related death of a horse at a show, this equation was given for measuring if it is too hot to work your horse. I have never used this as a monitor, but it is interesting to play with in your head... Add air temperature and relative humidity and subtract wind speed. If your total is 180 or higher, don't ride. If it is 130-170, use caution. If it is 130 or below, they say you can ride.
Here was their rating and rationale for caution when the total is above 130:
Less than 130: All go—horses can function to cool themselves assuming
adequate hydration.
130 – 170: Caution—a horse's cooling mechanisms can only partially
function as intended. Some cooling management procedures will need to be
performed.
180 or above: Stop—a horse's cooling systems cannot and will not
function adequately. All cooling procedures will need to be utilized to
keep the horse out of serious trouble.
Why is it an issue for the horse when heat and humidity combine to
equal 180+? What doesn't work and why? What are some of the
physiological ramifications? What are some of the symptoms?
Heat is produced by muscles in the metabolic conversion of chemical
energy to the mechanical energy required for muscle contraction and
limb movement. Seventy-five to eighty percent of the chemical energy
is converted to heat, which moves from the contracting skeletal
muscles to the surrounding tissues by the flow of lymph and blood.
Assuming a comparable rate of exercise intensity, the rate of
cooling, or heat loss is affected by air temperature, wind velocity
and humidity. (Werner, 1993). Heat can also be lost in a fourth way,
conduction, which is a direct transfer of heat from the skin or feet
to surfaces in direct contact (such as an ice bag on the skin).
So, thinking about the chart and the equation:
Temperature (F) + relative humidity (%) – wind speed (MPH), we see
how the ability of the horse to cool itself in these four different
ways will be affected:
• In cool temps with low humidity, heat loss through convection and
conduction can be as much as 50%. Heat can also be lost through
radiation, with as much as 60% of a body's heat lost in this way when
air temperatures are cool. The numbers in our equation would add up
to much less than 180, and the horse would have no difficulty cooling
itself.
• As temperatures rise, the thermal gradient for heat dissipation is
reduced, resulting is less convective, conductive and radiative heat
loss and more evaporative cooling. The evaporation of water from the
skin surface is the most important means of heat dissipation in high-heat/low-humidity conditions. So, when we get a high temperature
reading with low humidity, a horse may still not have difficulty
cooling, but if temperatures are extremely high with no wind, we
might get a result above 140, which would means our horse needs our
help cooling off.
• With high humidity, sweat cannot evaporate as easily and so the
ability of the horse to cool itself in this important way is reduced.
When high humidity is combined with high temperatures, (which we just
saw reduce the effectiveness of radiant, conductive and convective
cooling), the horse has now lost all four means to cool itself and is
in a dangerous situation, subject to a greater rate of heat
accumulation within his body.
This was on one of the yahoo groups. I can't vouch for the validity of the formula, but it's an interesting thought process to consider.
