We have 3 horses. All are big (18.1-18.3) Belgians, and I would say their workload varies from light to moderate. It is rare they have a long, hard day. We do keep them in good, fit, shape though. The vets and our Amish friends frequently comment on how fit and healthy they appear.

We have noticed that in the summer, one of the horses seems to heat up more than the others. Even standing in the field, it seems his respiration is faster on a warm day. We don't work them during the hottest part of a summer day, but we do what we can. Yesterday, we did a simple wagon ride, almost entirely walking, for 3 miles (half shaded), in the morning, so temps were in the mid-high 80's. By the time we returned, the heated horse was breathing at 120/min, while the teammate was closer to 70/min. The teammate cooled down twice as fast too. We spent extra time hosing the hot horse to cool him. He always seems to be fine, going strong, energetic, and alert...just hot, sweaty, and breathing fast. I have noticed that he and his half-brother sweat, but not as much as the third horse who will be soaked when they are only half-wet. They sweat fine, just not as much. They don't get frothy unless they have been worked pretty hard under a good load. In the past, we gave taken temps and the hot boy always seems to be at least a degree hotter than the other horses. I am curious if anyone has dealt with this before. It only seems to be an issue on hot days. Are some horses just more tolerant than others?

My team is pretty closely matched and related. When worked one always has a higher respiration rate than the other. I can't find any reason other than individual difference.
Bob

Just like people are different so are animals. I sweat something awful, can work fine doesn't bother me except I am soaked and when I stop and start to cool down I get chilly. Pain when working firewood in the winter. Need to take things off and then cover up quick when I quit. Been that way all my life. Talked to my dr. said make sure I stay hydrated, and don't worry unless I quit sweating all together.

In Germany I wasn't confronted with this, but when I came to the US I learned very fast to watch the heat. In this context one thing the late Elmar Lapp taught me when I visited and spent a couple of days with him: On hot days BEFORE work he would hose his horses down so that they started the day cool.
I mentioned in a post some time ago, never hose the body first, that would just lock in the heat, start with the legs.
It will be good to continue watching that horse closely and also to see how he is working with his team-mate. Some horses seem to think they have to do all the work, while the other one is just keeping the traces taut enough on a rolling load that he appears to be working.

Either he has a minor underlying cardiac issue, that hasn't presented in any obvious form or he just isn't as heat tolerant as the others.

It could just be an individual difference, I'm not as heat tolerant as some other people that I know.

I had a Percheron cross mare that was the same way. Definitely more noticeable on humid days. I never had her checked and have since sold her but always wondered if her breathing was compromised.