I think it must be a combination of both! Poet and I did an unintentional experiment over our Christmas break that seems to show that the sunbleaching that occurs in some horses over summer is a combination of sun and sweat.
Where we live flies can get bad, so my horses have flymasks on during daylight hours. It has been hot (like seriously hot… 40 degrees celcius/100 F plus) so they sweat a fair bit behind their ears where the strap for the masks sit.
Check out the bleaching pattern though on Poet who is liver chestnut and bleaches out in patches every summer… he has bleached severely where he has sweated around the mask strap, BUT under the strap, where it wasn’t exposed to any sunlight, he has maintained his coat color. Funky huh!!
Looking at him this morning he is bleached badly around his flanks and on his shoulder where he sweats the most too. So sweat + sun + a certain color and coated horse = bleaching, even when the diet is well and truly adequate for copper and zinc (thanks to FeedXL and pasture analysis I know this).
It’s interesting to note that Popcorn, who has an entirely different coat both in colour, length, thickness and even feel doesn’t bleach anywhere, ever. So specific coats seem to bleach a lot more than others. And our climate obviously contributes significantly too!