With severe dry weather hitting some areas of the Corn Belt this year, a number of no-tillers have voiced concerns about the effectiveness of post-emergence herbicide applications in both no-tilled corn and soybeans.

Quick Concerns. Problems with glyphosate performance usually become evident within 10 to 14 days following application, says Mark Loux, an Ohio State University weed scientist. He says drought can be a contributing factor to reduced glyphosate activity for several reasons:

1. Weeds actively growing in adequate soil moisture conditions are more easily controlled by herbicides. Drought-stressed weeds are generally more tolerant of herbicides.
2. Dry weather normally leads to slower plant growth, so it takes weeds longer to reach a size where glyphosate should be applied. As a result, older weeds found in dry fields are usually subject to drought-stressed conditions for a longer period of time, making them less sensitive to herbicides.
3. While more weeds are apparently developing a low level of resistance to glyphosate, this is not always apparent with small weeds growing under favorable weather conditions. Resistance appears to be more evident with larger weeds under drought conditions.

• Dry Weather Remedies. Loux suggests several strategies for getting the most out of post-emergence herbicide applications under drought conditions:
• He often recommends an increase in the application rate to 1.5 pounds of glyphosate acid with no-tilled soybeans. With corn, he suggests applying the maximum post-emergence range allowed in a single application based on the glyphosate label and the type of glyphosate-resistant corn being grown. “In any situation where…