I would cut the dead material down after seeding and fertilizing. Throw and mow. You could probably do it with out cutting but each plot is going to react different. Throw and mow worked great for me.last season but I have some plots in sandy soil. Dont know how great it would work in a heavy clay. Cutting dead grass is a lot easier than cutting green grass. You could skip cutting and go right to tilling after spraying, saving time that way.

Technically speaking gly works best on young actively growing weeds and grasses. It kills from the roots, and it does not kill seed. If your goal to remove all weeds and grasses for this year and next, then bushhog, wait 2-3 weeks and then spray, but if you disk, well then you've just brought up more dormant seed that will resprout.

We disk before planting just don't bush hog before we spray. We spray wait 4-6 weeks disk and plant.