Chemical Control of Weeds
Directed Postemergence Herbicides
Directed postemergence applications minimize crop injury because herbicide is placed on the weeds rather than on the crop. Precise application and a height differential between the crop and the weeds are required for directed applications. If the weeds are smaller than the crop, spray can be directed at the base of the crop so that little herbicide reaches the upper parts of the crop plant. Spray pressure should be set fairly low for this type of application, so that fine spray particles or mist are not produced. Safety of applications directed at the base of the crop will be increased through the use of shielded nozzles.
"Wipers" (sponge or rope wick applicators) operate above the crop canopy to control weeds growing taller than the crop. This type of application works best for soybeans, since weeds must generally be at least a foot taller than the crop. Control of johnsongrass, hemp dogbane, and volunteer corn is often achieved using concentrated solutions of Roundup Ultra, Touchdown, or postemergence grass herbicides applied in this manner.
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