I recently saw a study, UC Davis, I think, which said horses can be maintained on free choice alfalfa only. Adult horses. As for kidney problems, only if the horse has an existing condition.
I used to feed 50/50 alfalfa and oat hay, to balance the high calcium and protein but low sugars with the low protein, low calcium but high sugars in oat hay. Now im doing 25% each alfalfa, oat, 3-way and orchard/Timothy. Mainly for cutting calories
ETA: I can't find any decent grass hay, and the orchard/Timothy is by far the most expensive(oat 10$, 3-way 10$, alfalfa 17$, orchard/Timothy 21$)