I just want to make th egeneral remark that horses and donkeys are designed to process bulks of low quality (read low nutrition) food. In nature, they survive in area's where cows don't, because cows need better food.
As a consequence, I think there are more problems caused by too heavy food than by too low nutritional food.
I give my horses hay all year round, but this is hay that comes from natural land, and its not as heavy as the high nutricional hay that is produced for cattle. It also has a high diversity in grass and plants; it's not a monoculture.
A horse is in fact not a grass eater like a cow, in nature it eats anything from grass to trees. It's a dustbin!
Beside this hay, I give them a handful of grain to keep them friendly, and to create some diversity again.
Another remark is that this rule applies more for natural breeds than for the more domesticated breeds: a Fjord will suffer more from high nutritional food than a Thouroughbred.
So general rule: HIGH amounts of LOW nutritional value.