crig and ron are right.

what is the point in spraying roundup, and then plowing/disking/tilling the soil? when you work the soil, you are going to kill the existing vegetation and bring new seeds to the germination zone. there is no sense in spraying it if you're just going to plow it under. and the when you plow/disk you're just going to have new seeds to deal with. seeds can live for many years.

till or do whatever you want to with the soil, let the weeds come up, then spray, then plant.