rehabbing a field?

Have about 22 acres of what has been used as horse pasture for 20 some years.

It's not getting grazed enough to keep it down, so the better half has been having it brush hogged a few time a year.

I'm thinking, now that we have a decent tractor, that we partitiion is off and turn 10-12 acres into hay fields - the smaller fenced areas will be more intensively used (6 horses at the moment, usually has 2 borders also, so 8) and instead of mowing for no reason can turn the un-pastured acres into hay and get something out of it. We're spending 4k or so a year on hay.

I've brush hogged it all and some of it is hay, a few acres pretty good (used to be well grazed and mowed paddocks..maybe 4 acres) but most of it is very weedy - last year before we got the tractor it was only brush hogged in mid august - so everything went to seed.

I'm mowing about 5" high and doing it just about the time the weeds flower but don't seed - read that this will increase hay and reduce weeds.

Is that enough?

Can we spray and with what?

And seeding new hay..how and when? All I've found is putting in new pasture...nothing on reseeding an existing pasture.

rehabbing a field?