The direct answer for a powerfully built horse
A Quarter Horse in a heatwave should work at dawn or not at all, live in moving air with constant shade, and be cooled as an athlete rather than simply turned out and forgotten. The breed's trademark depth of muscle through the hindquarters, forearm, and neck is an engine that produces serious heat during even a short lope. A Quarter Horse who feels strong for ten minutes in humid heat may already be storing more warmth than his skin can release, so the decision to shorten the ride belongs at the trailer, before he is saddled.
Sweating: the sign that matters most in this breed
A Quarter Horse who stops sweating during work, or who sweats only in patches while his coat stays dry along the neck, is showing the warning owners most often ride past. Reduced sweating in a horse working in heat removes his main cooling route at exactly the moment muscle heat peaks, and it can appear in a fit gelding who coped well the previous summer. Pair that with a breathing rate that stays high during a long walking break, skin that feels unusually hot over the hindquarters, or a horse who becomes dull or irritable under saddle. Any of those signs in this Quarter Horse ends the session: walk him into shade, strip the tack, and begin cooling while you decide whether your veterinarian needs to hear about him today.
Cooling a big-muscled horse properly
Run cool water continuously over this Quarter Horse's neck, back, and quarters, scrape it away once it has warmed against his skin, and repeat without long pauses, because a single hosing leaves a warm layer doing nothing. Walk him slowly in shade between cycles if he is steady on his feet, offer small drinks often rather than one huge bucket at once, and stand him where air moves — a fan at a stable door helps a stalled horse far more than a closed barn aisle. Avoid hauling in a closed trailer on a peak-heat afternoon even for a short trip; a stationary trailer in traffic is the hottest box a Quarter Horse will ever stand in.
Turnout, feed, and the rest of his hot week
Turn a Quarter Horse out overnight on safe footing, bring him in before the ground bakes, and make sure the field shelter fits his companions too, because the horse ranked lowest stands in the sun at the shelter edge. Keep hay in front of him but shift larger grain meals to the cool evening, and rinse the salt and dried sweat from under his saddle pad and girth area daily so hot-week skin does not break down. If he lives out with a pond, check that the bank has not become deep mud that traps a tired, hot horse at the water's edge.
One heat plan that every handler can see
The risk with a family Quarter Horse is mixed messages: one rider told he is on dawn work only, another who saddles him at noon because nobody passed it on. KinsLe Family Sharing keeps this Quarter Horse's heat limits, cooling routine, and vet-call signs on his shared profile, so whoever brings him in works from the same page as the person who rode him at sunrise.