Any discussion of heat management would necessarily address muscle mass, skin color, level of fitness, etc. etc. I`ll leave those topics to others and discuss a thermo-dynamic concept called "latent heat of evaporation." You don`t hear much about it in endurance but it`s surprisingly important.
The heat generated by muscular action can be dissapated in a number of ways, but a major process is for blood to be shunted to surface capillaries for cooling. For example, the capillary modeling we see on fit horses is nature`s way of increasing surface area to enhance air-heat exchange (same principle as a car`s radiator).
Water (sweat or sponging) on the horses skin absorbs a certain number a calories or BTU`s (measures of thermal energy) when under going the transition from a liquid to a gas. This is where the real cooling action occurs.
In a high humitity situation, the liquid-vapor transition point is elevated because it is pressure-dependent. In other words, in high humitity water vaporizes at a higher temperature due to increased atmospheric pressure (this is why a car has a pressurized raditor cap). Thus the ability of sweat or sponge water to vaporize and absorb heat is dramatically reduced in high humitity conditions.
The folks in the East have learned this lesson well and they sponge or spray their horses constantly, almost at every road crossing. They have learned that in high humity, "conductive cooling" is much more effective than "evaporative cooling."
I have also seen crews in the East apply ice water the horses under stress, then let it sit for a few seconds and then scrape it off-- then they do it again and again. I`m not sure how they keep from causing muscle cramps, but maybe someone from the East will jump in and explain the process.
One last thought. The XL Performance equine temperature monitors are really interesting. They cost about $170 but provide insight that is not normally available. For example, yesterday was a moderate heat/high humity day here at the CEC. Three of our horses did interval work on the same course, at the same pace, carrying the same weight. Although heart rates were comparable, working surface temps varied: 99.5 - 102.1 - 103.2. Why the variations exist is beyond the scope of this post, but I do know that the data will cause us to manage these horse differently under similar high humidity conditions.
Happy trails and cool, dry air...
Ramey and Cynthia Peticolas-Stroud
OK, I`ll pick up on Ramey`s excellent discussion about heat and humidity, having done all of my endurance competitions on the east coast. We have all learned to sponge "on the fly" with a wet sponge dipped in every puddle and stream that we trot or canter past. Sponge it on, wait a few seconds, sponge it off. Doing this, I`ve had my horse stay cool and sometimes even dry out between water. Getting ready to compete in heat and humidity means training in the heat of the day (don`t over do it), not clipping that winter coat until just before ride season, and perfecting that sponging technique.
In vet checks, if there`s shade, get in it. Keep sponging the horse with cool or even ice water. We always kept the ice water for the lower legs and large veins of the fore and hind legs. Some people stand their horses in ice wraps or boots, using old support boots placed loosely around the leg and pouring ice/water into them. It`s important to put the water on, then remove it to keep the cooling going.
Two interesting thing I learned at the Hilary Clayton Sport Horse seminar this spring were: From research done for the horses at the Olympics in Atlanta, putting ice cold water on the horse did not affect them, even putting it on the large muscle masses. I, for one, am not going to be the one to try this for endurance!! The theory is that the ice water only cools to just below the skin and will not cause cramping. Someone else try it, not me!! The second thing, which I find makes sense, is the build up of heat under support boots (brush boots, sports medicine boots, etc.). They`ve measured the temperature of the tendons under these things and found it to be quite high. If you use them, get them off as soon as you get into the vet check and cool your horse`s legs. The other thing they have found is that after the first few minutes of wear, none of the boots, including the sports medicine boots, provide much support. It seems that once the material is stretched as the horse strides out, it does not have enough "rebound" to continue to provide a lot of support and your horse is probably better off learning to use himself without outside support. If you are using them for protection against the elements or because your horse overstrides, that`s fine.
Barb, Wind Dancer, Whisper, and Katie
In the hot and humid Northeast, many endurance riders keep the cold water principally on the large veins on the insides of the forelegs and the jugulars running along the bottom side of the neck. This cools venous blood returning the to the heart. Dumping cold water over the large muscles of the back and hindquarters of most horses is asking for big trouble. In fact, if there is a coolish breeze, I avoid sponging even the shoulders, as they can also cramp up. It`s not unusual to keep a rump rug over the hindquarters while simultaneously sponging the forelegs.
Of course, there are exceptions: when I crewed for Jeanne Waldron and Rambo in 95-degree heat at Gladstone last summer, she had me put water all over him. But his surface temperature was so hot that it warmed the water on impact.
Bobbie and Fine Print in Maryland (cool so far this spring)
Have never seen a problem with cold water on hot horses, PROVIDED the weather is hot! Am amazed how people will do it sometimes almost as a reflex, not stopping to think what the conditions are. My rule of thumb? If I wish someone would do it to me, then my horse probably would benefit from it. On the other hand, if I am still standing there with my jacket on, best to limit sponging to jugular grooves and insides of thighs and forearms.