Hot and dry considerations should have farmers reconsidering herbicide application

Prashant Jha
Iowa State University Extension
Ag News

Most herbicide labels caution against applications made under environmental stress such as the prevailing hot and dry conditions in Iowa this year.

This is because of increased risk of crop injury and/or decreased weed control. Weeds growing under hot, dry conditions are often difficult to control because of the development of a thicker waxy cuticle, which serves as a barrier for herbicide absorption.

Such conditions can also reduce herbicide movement inside the plant due to slowed rates of translocation and metabolism. Reduced weed control under hot and dry conditions is a concern for systemic herbicides including glyphosate and postemergence grass products. Addition of full labeled rates of adjuvants is recommended to improve weed control with systemic herbicides when plants (weeds) are under stress.

We are experiencing quite a bit of corn response after being sprayed with postemergence herbicides this summer in Iowa. Although I don’t have specific details on the products being used, the injury is most likely caused by hot and dry weather conditions at the time of herbicide application.

Most contact herbicides, like carfentrazone, atrazine, fomesafen, lactofen, fluthiacet and acifluorfen, become more active at high temperatures (90 degrees or above).

Increased activity of these herbicides under hot weather conditions may improve weed control, but can also increase risk of crop injury. Crops may also show injury symptoms from spray adjuvants added to the contact herbicides under hot and dry conditions; however, the injury is temporary and the crop eventually recovers.

Precautions can be taken to minimize the risk for crop injury with contact herbicides under hot weather conditions. Use lower labeled rates rather than higher rates since the weeds will be more susceptible to the contact herbicide at high temperatures. Crop injury can be minimized by applying the contact herbicide in the evening as the temperature starts to decline. Using the lower labeled rate of a recommended adjuvant will also reduce injury potential from a contact herbicide applied at high temperatures.

As we start making postemergence applications for weed control in soybeans, it is important to revisit what we have learned from past years regarding off-target movement due to inversion and herbicide volatilization (under hot and dry conditions).

Spraying during temperature inversions can cause issues with off-target movement of herbicides. Dicamba injury due to inversion often shows up in low lying areas of the field. Most inversion occur between two hours before sunset until two hours after sunrise and can occur multiple times in a month.

Avoid making herbicide applications when wind speeds are below three miles per hour. Dicamba product labels recommend to stop spraying at least two hours before dusk, and start spraying at least two hours after dawn to avoid off-target movement through inversion.

Dicamba labels also prohibit applications if the current or forecasted air temperatures exceeds 85 degrees to reduce potential for off-target movement through volatilization. Consult the Enlist (2,4-D choline) and Xtendimax, Tavium, and Engenia (dicamba products for over-the-top applications in dicamba-tolerant soybean) labels for additional application restrictions and stewardship guidelines.