Since you're in CA you may have restrictions I don't. If you can burn, once the cows eat it down a bit, I'd burn off the pasture and then turn everything under and re-seed with grass. If you can't burn, then I'd turn it all under and re-seed. Once it's starting to come up, watch for the weeds and spray a weed killer. Ideally, I'd spray a pre-emergant weed killer before anything came up in spring and then after the recommended time, I'd re-seed the grass. You'd probably need to establish a 'sacrifice' lot to keep the horses off the newly seeded pastures for a while but in a year you could have a lush green pasture established.
Because the drought has killed off everything here, I'm going to have to re-seed but I have to wait until I know the drought is over. Right now, I can't burn and it would be a waste of time and money to spray or re-seed. I'm looking forward to the end of the drought so I can get my pastures deep in grass again, I remember vaguely, when things were green!