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Rain or irrigation is necessary to activate herbicide residual and provide best weed control. Proper management of herbicides is a way Norder Supply agronomists help our customers maximize net return per acre.

For herbicides to provide residual weed control, they will need incorporation into the soil with rainfall or irrigation. When possible, apply an irrigation of at least 0.50” of water within one week of herbicide application when rainfall has not been adequate. Most herbicides will need a half inch of rainfall to properly incorporate herbicide in to the soil and activate residual. Some herbicides, like Valor and Zidua will require closer to one inch of water to activate residual. Group 15 herbicides are root and shoot inhibitors, so they need incorporated down to where seeds are germinating. You may need to follow up with additional irrigations every 7-10 days when weather conditions are hot and dry. Irrigation is most critical on one-pass applications, but is still essential with two-pass treatments. Repeated small rainfalls or irrigations will keep pre-emerge treatments’ residual active until an overlapping residual product can be applied post emerge.

Irrigations to keep herbicides active reduces weed emergence and growth, which improves the success rate of post emerge weed treatments. Without proper incorporation and activation of residuals, the need for added products and increased herbicide rates will be more likely with the post application.

With soybean pre-emerge herbicide treatments, proper herbicide incorporation will help limit possible plant injury. PPO herbicides are very effective in controlling tough to kill weeds including palmer amaranth species. The low solubility of the PPO herbicide means the herbicide stays concentrated very close to the soil surface. With a heavy rainfall, the herbicide can wash through cracks in the soil on to pre-emerge plants or splashed on to the leaves after soybeans emerge, leaving burn marks on plant tissue. Though this type of injury is often cosmetic, stand reductions and small yield losses have occurred.

Reducing competition of weeds directly improves corn and soybean yields, which is why timely application and activation of residual control is so important. Contact your local Norder Supply agronomist for specific product and rate recommendations for your farm.

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