Pick grasses native to your area, so your climate & soil type doesn't kill it off.
No matter what you plant, horses always have a favorite spot they will eat the grass down
Planting grass seed is an instance where, the less elaborate one tries to be, the healthier the horse
My horses are eating off 22 acres of what I call "virgin" pasture. That means it was cow pasture 40 years before we bought it.
It's never had fertilizer put on it and we put weed killer down once in ten years; that was the year after we came out of a "Exceptional Drought" status and the weeds wanted to Rule.
We had plenty of rain that year and the native grasses came back. My neighbor had planted something different where they dug up to run city water and only a very small percentage of it came back