Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
I have seen a lot of corn that is pale after a postemergence herbicide. A lot of fields were sprayed last week after days of cloudy overcast weather but were sprayed during those days with full sunshine. The corn plants have not had a chance to build up cuticle layers and they were very susceptible to herbicide injury.
The amount of postemergence herbicide(s) that the plant absorbs is impacted by the waxy cuticle on leaf surfaces. Cool, overcast weather results in thinner cuticle layers that can allow more herbicide to enter the plant. Herbicide formulations, spray adjuvants, and fertilizers play a role in herbicide absorption and being cautious with what goes into the spray tank can reduce the risk of injury. If spraying during cool, overcast periods, switch to “softer” additives if the label allows it; for instance, methylated seed oils (MSO) increases the risk of injury over crop oil concentrates (COC); and non-ionic surfactants (NIS or 80-20’s) reduces the risk further. Consider using the lower allowed rate of surfactant or nitrogen. Be sure to read the label and see what is allowed by the manufacturer.
We do not want to see herbicide injury, and some is more apparent than others, but crops often outgrow early-season injury with little to no impact on yield. Under these cool, cloudy conditions be cautious about the adjuvants and tankmixes you use to avoid injury. Remember, if the crop is more sensitive to the herbicides during these conditions, chances are, so are the weeds.