I'm sure the question of feeding horses is largely dependant on terrain, length of stay in any particular place & how many horsies you have to feed.
Lots of horses in a long drawn out siege in rocky, barren territory = lots of hungry horses.
Relatively few horses just passing through on the steppe are going to have little problem.

Personally I've never been a big fan of cav heavy armies & I rarely have more than 4 cav units.
Much more often only 2.
OK, so the exception is Naginata Cav... mmmm Naginata Cav, though even then I mostly only had 6 units in a frontline army