Weed science odds and ends
"It's a bit challenging to write a single article that encompasses the possible weed management scenarios of a cropping season that currently ranges from emerged corn up to the five-leaf stage, with many fields not yet planted," said associate professor of weed science Aaron Hager. Here are a few items that might be of interest to weed management practitioners.
While conditions during March and early April were conducive to planting, these same conditions were NOT conducive to good performance of soil-residual herbicides. Many surface-applied herbicides received neither timely precipitation nor mechanical incorporation to move the herbicide into the soil solution.
"Herbicide effectiveness can be significantly reduced when a soil-applied herbicide is sprayed on a dry soil surface with no incorporation, either mechanical or by precipitation, for several days following application," said Hager. Surface-applied herbicides generally require one-half to one inch of precipitation within seven to 10 days after application for optimal incorporation. The time frame and amount of precipitation needed depend on factors such as soil condition, soil moisture content, residue cover, and the chemical properties of the herbicide.
Broadleaf and grass weeds have emerged in some fields where the soil-residual herbicide was applied to the surface with no subsequent precipitation or mechanical incorporation. Growers have asked whether the soil-residual herbicide will control these emerged weeds once the fields receive sufficient precipitation to move it into the soil.
"The simple answer is that there is no simple answer," said Hager. Sometimes the herbicide "reaches back" after precipitation to control small emerged weeds.
"For example, we have observed emerged velvetleaf up to about one inch tall turn white following precipitation in research plots treated with isoxaflutole," he said. However, in other instances, the emerged weeds survive the recently "activated" soil-residual herbicide.
For example, annual grass weeds that have emerged in fields treated with soil-applied chloroacetamide herbicides will not be controlled by that herbicide; a post-emergence herbicide is needed. "Take steps to immediately control weeds that are taller than one inch rather than waiting several additional days after precipitation to see whether the soil-applied herbicide will control them," Hager advised. Even though the soil-applied herbicide might not control emerged weeds following the next precipitation event, it could still provide residual weed control once the soil receives sufficient precipitation.