It is not supposed to kill the regular grass per the manufacturer's label.
Some of the reviews on the product indicate great results with no other grass kill but other indicated it killed their entire lawn.
When we had nutgrass in our lawn we treated it with the killer but only where the nutgrass was which fortunately was not throughout the entire lawn. It did make the spot treated brown and die.
The other recommended treatment is to pull it out but that's not practical if you have a lot of it.
Sorry I can't tell you more specifically. Good luck!