The early-planted corn fields should have been sprayed postemergence for later emerging weeds. In later planted fields, morningglories can be a challenge. Glyphosate is quite effective on small morningglories, but when they start to “run” or develop the vining growth habit, glyphosate is not as effective. If morningglories are an issue for your field, you should consider an additional 1.25 to 1.5 lbs of atrazine with your postemergence sprays in order to provide good residual control. However, this requires that applications be made before the corn is 12 inches tall.
The weather patterns lately have resulted in situations where the risk of crop injury from postemergence herbicides is higher. Specifically, prolonged periods of overcast skies, high humidity, and warm temperatures followed by bright skies. If the postemergence herbicides were made as the days turned hot and sunny, the risk of injury is greater. We assume this is because the wax layer on the leaves has not developed and the leave surface is “tender” and prone to absorbing a lot of the herbicide spray. In addition, the leaves are more susceptible to “burn” from spray additives such as methylated seed oils (MSO), crop oil concentrate (COC), and/or nitrogen. If spraying during these sensitive periods, switch to “softer” additives if the label allows it; for instance MSO increases the risk of injury over COC and non-ionic surfactants (NIS or 80-20s) reduces the risk further. Consider using the lower allowed rate of the surfactant or nitrogen. Be sure to read the label and see what is allowed by the manufacturer. Certain herbicides can cause leaf burn and the risk is greater under the conditions described above; this includes atrazine, Aim, Cadet and Basagran. If labels allow a range of rates for additives or nitrogen, use the lower rates to reduce the risk of injury.