There are two herbicides that I know of that that just kill grass. One is sethoxidim which is generic poast herbicide. The other is clethodim which is sold under the name grass out max. They can be used to kill grass in any broadleaf crop but will kill anything in the grass family such as corn. I feel either would work for your needs but don’t know for sure. I’m sure you can Google them and read the product label to find out. I know in N.C. they can be bought over the counter as they are not restricted use herbicides. All my research led me to decide they were save to use even in my vegetable garden ( I use clethodim mostly as that’s the one my local farm supply carries and my cost is about 70 bucks a gallon) and it works great. It also works great keeping the grass under control in my clover food plots