Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
We have had conditions that have resulted in a lot of tender plants. Specifically, prolonged periods of overcast skies, cooler weather, and plenty of rain. If postemergence herbicides are made as the days turn hot and sunny, the risk of injury is greater. This is because the wax layer on the leaves may be thin and the leave surface is “tender”. This, coupled with many fields needing postemergence herbicides, means we need to pay attention to adjuvant selections. Adjuvants are needed to increase herbicide coverage, increase adsorption across leaf surfaces, and improved performance. However, this can also increase the risk of crop injury. So consider:
- What is required on the pesticide labels? Herbicide manufacturers will not stand behind their products if the label is not followed. Given a choice of adjuvants, non-ionic surfactants (80:20) are safer than crop oils or methylated seed oils. Also, given the choice of nitrogen sources, dry ammonium sulfate has less risk than liquid nitrogen fertilizer.
- If the crops are tender and more susceptible, then weeds are likely to be more susceptible (meaning weeds of the same size are more susceptible under these types of conditions). It may not be necessary to use the same level of adjuvants as you would for the same size weed under drought conditions.
- What is included in the tankmixture? Tankmixing can increase risk of injury. Other pesticides can increase risk of injury, for instance herbicide formulations may have similar properties to an adjuvant (i.e. emulsifiable concentrates can have adjuvant properties and add to the adjuvant load).
- If weeds are larger than the labeled heights for control, then consider using adjuvants and rates that allow for more “activity” (i.e. switching to crop oils or using nitrogen, using higher rates)
Be sure to match the adjuvants you use to the growing conditions, and the weeds and crop susceptibility at time of application.