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Protecting corn yields with postemergence programs
Bob Hartzler

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February 20, 2004 Farmers have an array of new 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 new herbicides (Callisto, Steadfast, 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.  These tools provide an alternative to existing weed control tactics, but require a thorough understanding of weed-crop interactions to be used most efficiently.  This article will describe steps to successful use of these new control options.

The two primary objectives of weed management are: 1) minimize weed escapes in order to prevent an increase in weed populations over time, and 2) prevent corn yield losses associated with competition between weeds and the crop.  Due to the resilient and diverse nature of weeds, satisfying both objectives with simple weed management programs can be difficult.

Most summer annual weeds (giant foxtail, waterhemp, common lambsquarter, etc.) begin to emerge near the time of corn planting, but significant numbers of weeds continue to emerge into late June and July.   The developing crop canopy reduces the competitiveness of late-emerging weeds, thus it is not necessary to control weeds for the entire growing season.  Generally, weeds that emerge after the V6 stage of corn (six leaves with visible collars) should be of little economic consequence.  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 total postemergence weed control programs in Roundup Ready corn (Gower et al.  2003)Roundup 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 Midwest states.  Most sites had high weed densities.  The impact of late-emerging weeds on weed control is illustrated in Table 1.  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 glyphsate  to kill the larger weeds.   Looking only at weed control would suggest that delaying herbicide applications is an effective strategy to enhance weed control.

Most farmers realize that the objective of weed management is not only to control weeds, but also to protect yields.  A risk associated with the new herbicides that provide consistent control of large weeds is that treatment may be delayed until after weeds have already reduced corn yields.  Corn and soybean are able to tolerate weed competition for a limited time after emergence without yields being affected.  However, at some point of time weeds reduce the availability of water, sunlight or nutrients to the corn and therefore reduce the yield potential of the crop.  The specific time when this occurs varies from field to field, and is affected by weed infestations (species, densities), weather conditions, and many other factors.  In fields with moderate to heavy weed populations research has documented that weeds may begin to impact yields after reaching a height of only three to four inches.

In the regional study, treatments were included that determined yield loss associated with the weeds that emerged with the crop and also yield loss due to these weeds plus weeds that emerged after herbicice application.  Corn subjected only to weed competition from emergence to postemergence application began to suffer yield losses when herbicide application was made to 4 weeds (Table 1).  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.

Although early applications (2-4 weeds) minimized competition from weeds that emerged with the crop, weeds emerging after the early application were able to compete with the corn and reduce yields. Weeds emerging after the 4 application timing added an addition 3% yield loss to the 3% loss caused by early-season competition.  This study illustrates the difficulty in obtaining maximum yields with a single postemergence application.

 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 )
  ------------------------------------------------------  %  ---------------------------------------------------------
2" 73 0 7
4" 83 3 6
6" 90 6 7
9" 93 14 11
12" 95 22 21

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

New postemergence technology provides farmers greater flexibility to manage weeds in corn.  However, these tools require careful management to maximize productivity.  The nature of weeds makes it difficult to consistently achieve acceptable weed control and protect yields with a single herbicide application.  Farmers need to develop integrated weed management programs to consistently achieve high yields and good weed control.  This can be achieved by applying a preemergence herbicide to suppress early emerging weeds, therefore minimizing the risk of delaying postemergence applications until the crop canopy has developed to the point where it can suppress late-emerging weeds.  Where a broad-spectrum postemergence herbicide is part of the planned program, it usually is not needed to apply a full rate of the preemergence herbicide.  An alternative approach is to plan on two postemergence applications, with the first applied early in the season before there is any risk of early-season competition.  Successful weed management in corn is dependant upon knowing the characteristics of the weed infestations in individual fields, how the weeds interact with the crop, and understanding the strengths and weaknesses of the control tactics being used.

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.


Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.