I don't spray Roundup before plowing. It's a waste of time, fuel and herbicide. When you plow something under that is green, it's going to die. Why bother spraying it if you are going to kill it with a plowing operation?
We never use Roundup on the plots at our places. We disk it up in the spring before the big bluestem grass starts to grow and plant it immediately. Inmid to late summer the big bluestem grassturns brown.We brush hog it close, remove most of thematerial, disk it and plant.