Hello Rooster.

Roundup only kills green. The seed has no green. Therefore you could plant the seeds even days before the roundup app and still be ok.

I often spray roundup then overseed the same day.

Depending on what the field has for growth determines what prep work needs to be done. Care to elaborate on what the field has in it now?

Froggy

I agree that you can seed right after round-up, but I usually wait 2 weeks. That way if anything was not killed by the round-up, you can spray again.

If you have the time and it is an old field or weedy area, you may want to spray once in the spring and once in the fall. That way you will get the fall weeds as well as the spring/summer ones.

Good site preparation is absolutely key.