By Larry Steckel, Extension Weed Specialist

The corn across Tennessee looks to be in pretty good shape. The maturity of our corn crop has some variability due to some starting planting in early March while other fields were planted the last of April. 

From rating corn weed control trials and walking a few corn fields this week, it is very apparent that the pre-emergence herbicides that were applied in March to mid-April started breaking last week. These applications however, were not applied in vain as they have kept the weeds down until now. The corn in these fields can be cleaned up much easier as the weeds are very small.  

Weeds in corn fields where no pre-emergence was applied have grown very large now and will be more difficult and expensive to control. There have been some corn fields already treated most notably with Halex GT, Capreno, Armezon/Impact or Realm Q. These herbicides, like all corn herbicides, work better when tankmixed with atrazine. Unfortunately, for corn that is over 12 inches tall, that is no longer an option.  

Due to the limited amount of days fit to spray, there are a number of corn fields where the corn is well over the 12 inches tall that will need to be sprayed. One good substitute for atrazine on large corn to be tankmixed with some of the premixes is Status. These and other herbicides offer effective post-emergence options even on large weeds, but there are cutoffs associated with all these.

Crop stage and/or crop height is used to determine the cutoffs for a given herbicide. Often the label states the limits at whichever (crop stage or crop height) comes first. Crop stage can easily be done by counting the number of collars that are fully open. For example, in the photo above, there are four open collars so the corn is at the V4 or four-leaf stage.

Below is a list of post-emergence corn herbicides and their respective crop height and stage cutoffs.

*Note that if atrazine is added (to herbicides such as Callisto or Capreno), the cutoff would be reduced to 12 inches or 8 leaf.