On lush pastures horses need less hay than in barns. Empty pastures can be used to grow hay so you don't have to buy it, or converted into a memorial garden. In barns you can keep stallions and mares together while in pastures you have to be careful as they can randomly breed if of age and unbred for the month
Pastures you can grow hay in them if they are empty and if you have an unbred mare in with a Stallion that can breed they can random breed at some point