Heat stress and sunburn
It's 98 degrees in the shade on an August afternoon, and you need to tackle the cross-country phase of your event. Will that be too hard on your horse?
It all depends, according to Dr. Christian Rammerstorfer, on the physical fitness of the animal and its state of acclimatization to hot and humid conditions. "If your horse is acclimatized to the hot and humid weather, its body is better at moving blood to the skin, making it easier to thermoregulate," the equine exercise physiologist notes. "Blood flow to the skin facilitates heat loss as heat is transferred from the body core to the surface."
Blood moving to the skin is a good example of conduction, one of four physical processes to transfer heat away from the body available to the horse, along with convection, radiation and evaporation. It is the process of heat transfer between two surfaces in direct
contact with each other - in this case, the horse and air. "The parts of the body with a high surface-area-to-mass ratio, such as the limbs and head, allow for maximal conductive heat loss," Rammerstorfer says.
Convection occurs in fluids and gases as the particles are mixing within that fluid or gas. Temperature differences result in some particles being more or less dense than others. The rising of less-dense particles and falling of denser ones transfers heat either to or away from the horse, Rammerstorfer explains. Air circulation helps convection, so having your horse out in the breeze (in the shade, of course) or in front of a fan can prevent heat stress on a hot day.
Sunshine is the most significant radiation factor. "If your horse is standing in the bright sunlight, the amount of solar radiation absorbed may be significantly greater than its own metabolic heat production," Rammerstorfer notes. "Your horse is going to get hot."
Evaporation - accomplished by sweating or panting - is the most important heat loss mechanism during and immediately following exercise. But in high humidity, little or no heat transfer occurs, and evaporation can become ineffective.
For example, if a horse is exercised on a hot day (105 degrees) with low humidity (25 percent), he won't lose much heat via conduction, radiation or convection, but evaporation will remove a tremendous amount. His body core temperature may rise to 104 degrees at the highest.
But work him at the same temperature in high humidity (75 percent), and the sweat will run off the horse instead of evaporating; far less heat will be removed. This can lead to a core temperature of 108 - which cannot be sustained for long without resulting in heat stroke or heat exhaustion.
"If it's real humid out, you need to hose or sponge off your horse with cool water after every workout," Rammerstorfer strongly advises.
When hosing off the horse, let the water flow over the large muscle groups until the respiration rate has returned to close to normal. When finished, do not use a sweat scraper, but leave excess water in the hair coat as it will remove more heat as it evaporates.
One other tip: Do not feed large amounts of hay in the morning on a hot day with high humidity.
"Fermentive heat production in the hindgut occurs for several hours after consumption, adding to the total metabolic heat production," says Rammerstorfer. "Hay particles draw water into the gut, which can compromise its availability for the sweating response. In addition, hay and water in the gut can create excess bulk, decreasing the horse's performance capabilities."
Horses that come to the South and Southeast from northern or western (arid) areas need five to seven days to acclimatize, even more if they are not fit.
"During the acclimatization period, treat your horse as if he was unfit," notes Dr. Christian Rammerstorfer. "Start with light exercise, then become progressively more intense."
Rammerstorfer suggests monitoring the respiration rate during the walk and trot - of both the acclimatized and unacclimatized horse. "Whatever you do, if your horse starts panting (80 breaths per minute or more), move him into the shade. Do not continue to exercise as long as he is panting."
Take the saddle off immediately, provide fans and hose the horse off using a cool, low-pressure water flow until the respiration rate has returned to normal - eight to 10 breaths per minute.
Taken from Horse and Horse