The call of the week is how to control ryegrass and, in a few other cases, fine fescue in emerged corn (Video of ryegrass and fescue in corn). Several have reported these grasses recovering from glyphosate + dicamba or in a few instances, glyphosate + rimsulfuron burndown. The questions that follow are why did they recover and what is the best way to control them now that the corn is up?
With respect to the escapes from glyphosate + dicamba the reason is likely a combination of the grasses being glyphosate-resistant and/or dicamba hindering the grass control provided by glyphosate. In the case where the grass has recovered from glyphosate + rimsulfuron, the ryegrass is likely resistant to both herbicides.
The question about what to do now really needs to be tailored to the grass species in question. Ryegrass is a winter annual and is moving toward the end of its life cycle. As such about the best option is to go ahead an apply the typical POST corn herbicide premix which is often Halex GT + atrazine or Capreno + glyphosate + atrazine. Both of these tankmix applications should move the ryegrass along toward the end of its life cycle.
With respect the fescue, it has typically been moved into the field with tillage or some other kind of soil movement. It is a perennial and as such glyphosate and rimsulfuorn are not that effective this time of year as translocation of those herbicides down to the growing point is very limited. Fortunately, atrazine is very effective on fescue and should do a good job cleaning up this grass.