Weed Control and Corn Yield

The objective of weed management is to not only control weeds, but also to protect corn yield potential. Timely use of postemergence and preemergence herbicides, using a product mix that provides multiple sites of action, is key to weed management in corn.

Figure 1

Figure 1. The influence of time of weed removal on corn ear size, with herbicides applied PRE or at various corn vegetative stages compared to no herbicide treatment. Corn plants can detect the presence of weeds very early in their growth cycle and make reproductive adjustments that are not altered by subsequent ideal growing conditions.2 (Photo courtesy of Dr. Phil Westra, Colorado State University).


Weeds can compete with corn for space, light, water, and nutrients. A recent survey across North America concluded that weeds can cause more than 50% yield loss in corn without the use of herbicides in weed management.1 The timely use of herbicides is important to maximize corn yield potential (Figure 1).


Starting with a clean field, either through tillage or a burndown herbicide application, is the best approach to begin weed management in corn. This helps to reduce competition from weeds as corn emerges and becomes established. Weeds that germinate early in a corn crop can be the most competitive. Applying residual herbicides preplant or preemergence (PRE) in corn prior to postemergence (POST) in-crop treatments usually provides better weed control at the end of the season. Residual herbicides applied preplant or PRE can also provide more flexibility for timely in-crop POST herbicide applications.


Situations arise where total POST herbicide programs may be utilized. When making an in-crop POST application in glyphosate-tolerant corn, a Roundup® brand glyphosate-only agricultural herbicide should be applied in a tank-mixture with recommended herbicides when weeds are 2 to 4 inches tall. Delaying an application to weeds that are more than 4 inches tall can result in significant yield loss, especially when no residual herbicide is applied in a tank-mixture to control weeds that can emerge after application (Table 1).3

Table 1: Effect of application timing on weed control and corn yields in total postemergence herbicide programs.3
Application Timing (Weed Size)% Corn Yield Loss from Weed Competition
Early-Season*Early– and Late-Season**
2 inches07
4 inches36
6 inches67
9 inches1411
12 inches2221
* Represents what could happen if a residual herbicide were tank mixed with glyphosate and applied postemergence. ** Represents what could happen if only a single postemergence treatment of glyphosate were used without any residual herbicide.

There can be a greater risk of yield loss when using total POST herbicide application programs. Weeds grow rapidly, and weather can delay POST applications. Surveys have shown that when weeds are managed with only POST herbicides, that application timings often occur too late to protect the full yield potential of corn from early weed competition.4


Remember to control weeds when they are small (less than 4 inches) with POST applications in corn, and use residual herbicides in the program. Herbicide programs that provide multiple sites of action also help in the management of weed resistance. Visit www.RoundupReadyPLUS.com for more information on weed management solutions.