June 5, 1999 | By: Cheddi Acham

Solving a problem well after it takes place is something we'd all like to do. Instead, we often concentrate on preventing a potential problem, or "nipping it in the bud."

Postemergence herbicides make it easier for corn and soybean producers to manage weeds after these pests become a problem. Producers can use these herbicides after both the crop and weeds emerge, making it possible to control weeds within a crop.

"It's important to consider several factors when using a postemergence herbicide to manage weeds in corn and soybean crops. These include the size and type of weed, crop growth stage, and interaction of the herbicide with the environment," said J.D. Green, Extension weed specialist at the University of Kentucky College of Agriculture.

Because timing is critical to the effectiveness of a postemergence herbicide application, producers need to scout fields to keep track of weed density and populations. Management is easier when weeds are small and actively growing because larger weeds are more difficult to control. Scouting gives producers the information necessary to apply a postemergence application at the right time for optimum effectiveness.

"Weather plays a role in timing as well," Green said. "Rainfall soon after application can rinse a herbicide off the crop, defeating its purpose. Herbicides usually need one to eight hours without rainfall to be effective. Windy conditions increase the risk of herbicide drift on to nearby plants. It's best to avoid herbicide applications when wind speed exceeds 10 miles per hour or when the temperature surpasses 85 degrees. "

Weeds stressed from hot, dry conditions are surprisingly stubborn and thus harder to kill. Conversely, drought-stressed crops are easily injured by herbicides.

"Several factors will influence the herbicide you use and the application rate," Green said. "These include the type and size of weeds, the tank mixture, and stage of crop growth. Depending on the herbicide used , you might have to alter the spray volume. Also, you might need to use an additive such as non-ionic surfactant or crop oil to improve leaf penetration or to serve as a spreader. Read the herbicide labels to be sure you pick the right additive. If you tank mix, be sure the herbicides are compatible; otherwise you will create an antagonism that neutralizes the herbicides. Using the right additives and tank mixtures will improve the herbicides' overall performance."

A liquid nitrogen fertilizer also might be necessary, according to Green.

Cleaning up usually is the most unpopular part of any job. However after spraying, it is imperative to flush, rinse and clean all tools. Some herbicide labels suggest a cleaning solution after using a product. Sprayer cleanup is important, especially when you use a herbicide for one crop and then a different herbicide for a second crop.

"Postemergence herbicides are an effective weed management tool provided you follow certain guidelines," Green said. "Remember that timing is very important along with attention to environment conditions. Following and adhering to label directions helps ensure safe and effective weed management within crops."


Writer: Cheddi Acham (606) 257-4590

Source: J.D. Green (606) 257-4898