Go to your local feed store and ask for a grass pasture mix that would be good for horses. Look for blends with timothy, brome, and/or orchard. Avoid alfalfa. Oat grass is another good option.
what you plant also depends on where you live. Western climates and far northern are vastly different from us here in the south and what we can use as pasture.
I bought a pasture mix the one our feed stores had for horses had more alfalfa in them. This mix has some just not a ton. It has timothy and clover and orchard.
I feed my geese horse quality grass hay and whole grains in the winter. My hay guy has about 50 geese and he says the geese devour his oat hay in the winter. He is bringing me a couple of bales to sample.