I "renovate" most of my pastures every couple years by high density/rotationally grazing it down low first time through in the spring, drag it with a Wingfield harrow to spread the manure, then no till drill (rented GP Solid Stand 10 ft drill works well) in a good innoculated persistent red clover in the small seed box with a good local grass blend in the big seed box. The "high density" grazing also spreads manure.

If you don't have cattle to graze it down then soil sample, mow it down low, spread whatever dry fertilizer is needed then drill a variety of seeds. One nice thing about a coulter drill like the GP is that the coulter will incorporate some of the broadcast fertilizer while planting. 

I would never destroy what is already growing there.

Forget the spray! Spraying will kill a lot of useful plants. The only thing that is a weed is something the cows won't eat.