Though April showers should bring May flowers, or activated pre-emergence residual herbicides in this case, a dry spell has brought on herbicide doubt for many growers. In some areas, growers who used soil-applied herbicides this year are left wondering when showers will activate residuals. In an ideal world where weather would perfectly align itself to fit residual herbicide activation, precipitation would occur within seven to 10 days after herbicide application with exactly one-half to one inch of rain. Unfortunately, regions facing drought know this agricultural utopia is far from reality.
“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,” says Aaron Hager, associate professor of weed science. Influences that may affect an herbicide’s effectiveness include:
1) Soil condition
2) Soil moisture content
3) Residue cover
4) Chemical properties of the herbicide
Weeds Have Emerged…What Now?
Some are asking, “What if the residual is activated shortly after weeds emerge?” “The simple answer is that there is no simple answer,” says Hager. In some instances, the recently activated herbicide provides control over small emerged weeds. However, in other occurrences, emerged weeds exhibit minimal effects from the residual herbicide.
The Solution?
When in doubt, Hager recommends growers be proactive and make an early post-emergence application to control emerged weeds before they exceed 1 inch. While activation may yet occur for the pre-emergence herbicide and provide additional residual for subsequent weed flushes, a wait and see approach should be avoided.
For more information regarding herbicide activation or other weed management recommendations, contact your local Syngenta agronomy service representative.
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