roundup will kill them. You can spray Poast on the plot which will kill your grasses but not the broadleaf weeds.

What to spray on brassicas to kill weeds?

In food plot plantings, sethoxydim is commonly used to control grasses in legume and brassica plantings. Clethodim is very similar to sethoxydim in that it is a grass-specific, selective herbicide. It is often purchased under the trade names Arrest Max, Arrow or Select.

Will glyphosate kill vegetable plants?

Roundup, a widely available weed and grass killer, uses the active ingredient glyphosate, a nonselective herbicide that will kill nearly any plant it touches. Although it can quickly kill sprayed plants, it's generally safe to use around vegetable gardens when applied in accordance with the instructions.