Horses destroying pastures?

Pasture Maintenance.

You don't have enough pasture to keep the grass growing well, so you need a dry lot to put the horses on when the other two pastures are getting overgrazed and when it's wet. This can double as an outdoor arena, as long as it's big enough to hold all your horses comfortably.

Horses will always eat all the good grass and leave the weeds. They'll eat the grass down to the crowns, which tends to kill it, and makes room for the weeds. To prevent the weeds from taking over, you have to keep the grass mowed - not as short as a lawn, ideally about 4-6". I do it every couple weeks.

Their hooves are very hard on a pasture. If it's dry, or frozen, they damage the grass crowns so it can't grow back well. If it's soft, when they run they smash it in and dig it up. When it's muddy... omg the damage.

Once a pasture has been overgrazed, it takes a long time to bring it back. Since you have two pastures I assume you know about rotating them? Use one for a while till it looks a little lean, then shut them out of that and use the other while the first one regrows. Don't let either get actually overgrazed; you'll have more grass over the long term if you move the horses as soon as it's starting to be stressed.

Rotate the horses into the dry lot (with hay) any time both pastures are stressed; return them to pasture slowly to prevent colic/laminitis.

If you pasture 24/7, you might want to extend your pasture life by putting them into the dry lot either during the day or all night. Not only do they eat less, they do less damage that way.

Other things to do:

Clean up the manure, especially in a small field. While composted manure is great fertilizer, fresh manure actually will kill the grass. So clean it up frequently. If there are dead spots from pee or manure, sprinkle them with lime.

Fertilize. If you're in the US, most states Extension Services will test soil for you to recommend what you need. You take samples from all over the field, send it in, and they reply with answers. Fertilizer helps a lot.

Keep the horses off the pastures in the winter. If you have enough acreage, winter pasture is great, but as yours are allready overgrazed, they don't help the horses in winter and the horses just further destroy it.

Good Luck.

Farming 101

Scarf the pasture with a harrow or disc and over seed it and rake the seed in

I'd use a local " pasture" mix and over seed that with a local annual grass for cover

If you are in North America. . Your late I'd plant in the fall.