Timing of weed control is key to victory in protecting yield

When it comes to postemergent weed control in corn, timing is everything. While the vast use of glyphosate herbicides and herbicide-tolerant seed has curbed weed pressure, the development of resistant weeds has made the timing of weed control more important than ever. If farmers wait too long in the growing season to treat, they can expect yield reductions.

“The level of crop yield loss will depend on environmental variables and weed species composition within a given field, weed density, and time of weed emergence relative to the crop growth stage,” says Stevan Knezevic, University of Nebraska Extension integrated weed management specialist, who conducted much of the timing research.

Knezevic determined that there is a need to understand if the weed infestation in question is likely to reduce corn yield to decide whether or not a treatment is economically worthwhile. He introduced the concept of critical period of weed control, or CPWC, a time period in the crop growth cycle when weeds must be controlled to prevent yield losses.

At a glance

CPWC is the period when weeds need to be controlled to prevent yield loss.

The less nitrogen fertilizer applied, the longer weed control must be maintained.

Treat RR corn with glyphosate plus a residual herbicide at beginning of CPWC.


“Weeds that emerge before or after this period may not present a threat to crop yields,” says Knezevic. “This information is essential in making decisions on the need for, or timing of, weed control and achieving an efficient use of herbicides.” His research found that nitrogen fertilization levels affected the CPWC in corn.

“A reduction in nitrogen fertilizer resulted in a longer CPWC, so corn was less tolerant to weed presence,” says Knezevic. “This suggests that when no nitrogen fertilizer is applied, the timing of a weed control measure should start early in the season, at the first leaf stage of corn, and needs to be maintained through the 11th leaf stage, approximately the time of crop canopy.”

An increase in nitrogen fertilizer delayed the need for weed control and increased the tolerance of the corn to weed pressure. “From a practical standpoint, insufficient nitrogen can reduce corn tolerance to weeds, and it can widen the window of a CPWC,” Knezevic says.

Delaying weed control treatment beyond the starting point of the CPWC costs yield, he says. “We conclude that delaying the time of weed removal after the CPWC will cost an average of 2% in yield loss per every leaf stage of delay,” says Knezevic. “When the weeds are taller than corn, they will shade the crop, so the control should be initiated four to five days prior to the beginning of CPWC.”

Knezevic says, “A generally sound strategy, for example, in Roundup Ready corn will be to apply Roundup tank mixed with a residual herbicide at the beginning of the critical period, which will provide adequate weed control the entire critical period.”

He recommends starting with a residual herbicide in fields where tough glyphosate-resistant weeds could be a problem. “It will also be good to start with the residual herbicide, because that will delay the start of the CPWC for a few leaf stages,” says Knezevic. Then a single follow-up treatment of glyphosate alone should do the job.

About N effect

Stevan Knezevic’s research on CPWC has helped farmers understand when treatment of weeds is most crucial to maintaining optimal crop yield in corn. He also found that nitrogen fertilizer applications affected the timing and length of CPWC.

When no fertilizer was applied, the CPWC ran from the V1 to the V11 leaf stage, or from about eight to 45 days after crop emergence. To ensure there would be no yield loss due to weed pressure, weeds needed to be controlled throughout this entire period.

If 55 pounds per acre of N fertilizer was applied to the field, the CPWC shrunk to between the V4 to V9 leaf stages, or about 15 to 39 days after crop emergence, because the corn plants were more capable of handling weed pressure without giving up yield.

The shortest CPWC was found if 210 pounds per acre of nitrogen was applied, shortening the CPWC to between the V6 and V9 leaf stages, or about 20 to 39 days after emergence.


This article published in the April, 2011 edition of NEBRASKA FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.