accent (or generic version) is fairly decent on johnsongrass, and it is approved to use in sweet corn.  However, johnsongrass needs to be small - around 15-18" tall.  I suspect your johnsongrass is way taller than that.  Also, Accent is pretty weak on crabgrass.

Impact (Armezon) does a decent job on crabgrass, and is approved to use on sweet corn; however, it will not do much to the johnsongrass.

We've used Option on sweet corn.  Does a decent job on crabgrass, and works on johnsongrass.  Sweet corn needs to be small, crabgrass needs to be small, and johnsongrass needs to be in that 15" size or so.

Check labels on all these products for appropriate info....... weed size, crop size, rates, adjuvants, etc.