I am starting from scratch with a green field site. Should I spray the area i want to plant out before rotavating?
You can rotavate a couple of days after spraying and the grass will still die although it will look green for a while.
You could have used black plastic or old carpet to kill the grass but that should have been done months ago.
More on vegetables at: https://gardenie.wpengine.com/gerrycategory/types-vegetable-growing/?id=5628