Protecting Corn Yield With Postemergence Programs

Mark VanGessel, Extension Weed Specialist;

Modified from an article by Bob Hartzler, Iowa State University.

Farmers have an array of products that allow weeds to be effectively controlled postemergence. These new technologies include herbicide resistant hybrids, such as Roundup Ready and Liberty Link corn, and several newer herbicides (Callisto, Status, etc.) While postemergence herbicides (2,4-D, Banvel) have been used successfully for more than 30 years to control weeds in corn, the new products offer greater flexibility in application timing, reduced risk of crop injury, and a broader spectrum of weed control. However, an understanding of weed/corn competition is needed to use these products most efficiently.

Most summer annual weeds (giant foxtail, pigweeds, common lambsquarters, etc.) begin to emerge near the time of corn planting, but significant numbers of weeds continue to emerge into late June and July. A temptation for many farmers relying on postemergence herbicides is to delay application until the crop canopy is large enough to shade out late-emerging weeds. Delaying application of postemergence herbicides may result in cleaner fields at the end of the growing season, but this approach may have serious economic consequences. A regional project investigated the effectiveness of using only glyphosate for weed control in Roundup Ready corn (Gower et al. 2003). Glyphosate was applied at several times during the growing season based on the size of the dominant weeds in the field. A total of 35 experiments were conducted in nine states, including Delaware. Most sites had high weed densities. In these studies, weed control continually improved as applications were delayed. For example, a single application when weeds were 12” tall resulted in 95% control, whereas spraying 2” weeds resulted in only 73% control. The reduced weed control was due to weeds that emerged after application, rather than an inability of glyphosate to kill the larger weeds. Looking only at weed control would suggest that delaying herbicide applications is an effective strategy to enhance weed control.

However, corn subjected to weed competition from emergence to postemergence application began to suffer yield losses when herbicide application was made to 4” weeds. Applying the herbicide when weeds were 4” tall resulted in a 3% yield loss, and each delay approximately doubled the yield loss. The reduction in corn yields due to competition prior to the postemergence application illustrates the risk of delaying treatment in hopes of minimizing problems with late emerging weeds.

Table 1. The effect of application timing on weed control and corn yields.
Adapted from Gower et al. 2003. Weed Technol. 17:821-828.

Application timing
(Weed Size)

Weed control Corn yield loss1
(Early-season competition only)

Corn yield loss2
(Early- and late season competition )



73 0 7


83 3



90 6



93 14


12″ 95 22


1 Weeds emerging after herbicide application controlled with hand weeding.
Weeds emerging after herbicide application allowed to compete with corn.

Reference: Gower, Loux, Cardina, Harrison, Sprankle, Probst, Bauman, Bugg, Curran, Currie, Harvey, Johnson, Kells, Owen, Regehr, Slack, Spaur, Sprague, VanGessel and Young. 2003. Effect of postemergence glyphosate application timing on weed control and grain yield in glyphosate-resistant corn: Results of a 2-year multistate study. Weed Technol. 17:821-828.

An efficient approach is an early application of glyphosate to protect the corn yield and in addition, include a herbicide with glyphosate that will provide residual control. Herbicides to consider include: atrazine, Callisto, Hornet, Resolve, Sandea, or Steadfast. Herbicide selection needs to be based on weeds present in the field. Be sure to consider corn height restrictions as well.


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