There is no general consensus on this as it depends on MANY factors such as:
Health/age of the horse
Weight of the horse
Physcial fitness of the horse
Whether there is a breeze or not
Availibility of shade or not
Color of horse
The good news is that since there is no general consensus, there is no right or wrong answer. Of course, some may try to say it is this or that temperature, but they do a disservice to the horse owner because what is too hot for one horse may not be too hot for another.
Know your horse's comfort level, and consider the area where you live and the current humidity....and go from there.
Here's my personal factors to consider:
Our horses live in a very high altitude, on top of a mountain. That is good news and bad news. Good news is that there is almost always a breeze blowing. Bad news is that because of the high altitude, the suns rays are closer, which equals hotter and an increased risk of skin cancer, especially for our gray OTTB. Hence, we use a fly/grooming spray with sunblock in it. They have no shade in the dry lot, but their grazing pastures do have shade from trees....
Our OTTB has a high baseline body temp, he is always hot/warm to the touch.....high metabolism, I suppose. He is also a dark dark gray, some parts of him are black, so he absorbs the sun's rays more than a light colored horse, which makes him feel like he is baking when you touch his fur on a sunny day. Overheating causes him to gulp his water in large amounts while in pasture, and he has had an episode of colic/severe stomach cramping from this. From observations last year(10 minutes grazing in the sun with no shade on an 86 degree day with high humidity and he was sweating and foamy all over his body)..... he does not do well in the sun/heat.
Our Belgian is fat and hairy...but she is a light chestnut. Her fur does not absorb heat as much as our OTTB....but her weight and her hair density puts her at risk of overheating and electrolyte issues as she sweats copiously in the sun on hot and high humidity days...... she does not have high metabolism, and her baseline body temp is lower than the OTTB.
Between the two, the lighter colored, fat hairy Belgian does ONLY slightly better in the heat than the dark colored, normal weight OTTB. Both only do light "work".
Neither do as well in the heat/sun, theoretically, as a normal weight, light colored, physically fit quarterhorse would.
Both our horses have stalls with two dutch doors each, and a well ventilated barn with both ends of the barn walls and one side wall equipped with sliding doors the size of the wall to improve air flow. They also have stall fans and a barn fan. Still, we would like them out in the grazing pasture with the shade trees and mountain breeze whenever possible.....and that will be a day to day decision, based on the wind, temp and humidity.
To generalize: knowing our horses' body conditions(dark, hairy, fat, etc) and the environmental conditions they live in: they would need to come in to their stalls during the hottest part of the day more often than alot of other horses.
HSI(heat sensitivity index) should NOT take the place of knowing your horse's ability to withstand the heat. HSI is a generalization and should not be something you base your decision on. JMO!!
Last edited by Beauseant; 05-14-2011 at 10:00 AM.