Iowa State University

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Timeliness is critical   
Bob Hartzler

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June 19, 2008Weeds likely are emerging with corn in many fields due to the lack of a preemergence herbicide. Whether this was planned or due to weather constraints, it is critical to control weeds early in order to protect yields. The term critical period is used to define how long weeds can be allowed to compete with the crop before yields are impacted. To obtain maximum yields, weeds must be controlled before the critical period is reached. The difficulty in relying on total post programs is the variability in the critical period, making it impossible to predict the optimum time for postemergence herbicide application. The critical period is influenced by many factors, including: weed density, relative time of emergence of weeds and corn, weed species, and cultural and environmental factors.

A multi-state project evaluated the effect of time of weed removal in glyphosate resistant corn. In the 35 experiments, the average yield loss was 2% when the initial glyphosate application was made to 2.5 inch weeds (Table 1). Delaying the application until weeds were 5 inches tall doubled the yield loss. The variability in the critical period can be seen looking at the yield response at the sites with the least and most competitive environments. At sites with low levels of competition, corn yield loss was not affected when application was delayed until weeds were 7.5 inches. At the other end of the spectrum, corn yields were reduced 13% when weeds were only 2.5 inches at locations with high weed competition.

Table 1. Effect of early-season competition on corn yields. (Gower et al. 2003. Weed Technol.)

Weed height at application

% corn yield loss


Least competitive environments

Most competitive environments

2.5 inches




5.0 inches




7.5 inches




Due to our inability to accurately predict the critical period, it is wise to act conservatively when determining when to apply postemergence herbicides. Weed density is probably the most important factor influencing the critical period, and fields with heavy infestations should be treated as quickly as possible after weed emergence.

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
Submit questions or comments here.  

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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.