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Role of preemergence herbicides in Roundup Ready® crops. I.
Protecting yields.
by Bob Hartzler

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February 13, 2006 - One of the primary advantages of Roundup Ready® crops is glyphosate’s ability to consistently control larger weeds than other postemergence herbicides. This is a tremendous benefit for large farms and in situations when conditions prevent timely herbicide application. However, this benefit creates the risk of delaying application beyond the time when weeds have begun to compete with crops, therefore resulting in significant yield losses. One method of reducing the risk of early-season competition is the use of preemergence herbicides to reduce densities of weeds that emerge with the crop.

Experiments were conducted in conventional tillage corn and soybean to determine the effectiveness of low rates of preemergence herbicides at preventing early-season competition. Harness Xtra® 6L (0, 0.6 or 1.2 qt/A) was used in corn, whereas in soybean INTRRO® (alachlor) was applied at 0, 1 or 2 qt/A. Within each preemergence treatment, glyphosate (29 oz Roundup WeatherMax® /A) was applied at either the V2, V4 or V6 stage of crop development; a full-season competition treatment receiving no glyphosate was also included. A second glyphosate application was made in the V2 and V4 treatments to eliminate any late-emerging weeds. Weed biomass at the time of glyphosate application was determined by harvesting weeds within four arbitrarily selected 1 sq ft quadrats within each plot. End of season biomass was also determined in plots not treated with glyphosate.


Both rates of Harness Xtra® reduced weed biomass by at least 80% at the two experiments (Figure 1). Giant foxtail, velvetleaf and cocklebur were the primary weeds escaping control at Ames, whereas at Nashua velvetleaf was the species that most commonly escaped the preemergence treatment.


INTRRO® was less consistent than Harness Xtra® at reducing weed biomass present at the time of post applications (Figure 2). At Ames, both rates reduced weed biomass by approximately 50%. At Nashua only the 2 qt rate reduced weed biomass, whereas at Kanawha weed biomass was reduced 35 and 50% by 1 and 2 qt of INTRRO®, respectively.

Weed densities were relatively low (< 10 weeds / sq ft) at most experiments, thus yields were only affected by early-season competition at Kanawha. Soybean yields were reduced by 17% when glyphosate application was delayed until the V6 stage with no preemergence herbicide. Soybean yields were reduced only 1 and 17% with 1 and 2 qt/A INTRRO®, respectively. Thus, the preemergence herbicide provided greater application flexibility at the site with high weed densities, but at other sites with lower weed densities yields were not impacted when weeds were controlled as late as the V6 crop stage.

The relationship between weed biomass and full-season competition at the three soybean experiments is shown in Figure 3. As one would expect, yield losses increased with end-of-season weed biomass.

Preemergence herbicides can provide growers protection from early-season competition in Roundup Ready® crops. The preemergence herbicide does not need to provide complete weed control since a planned postemergence glyphosate application will be made. Thus, reduced rates can be used in order to reduce costs. In order to widen the application window for glyphosate, the preemergence herbicide must have good activity on the dominant weeds found in the field. In these experiments, INTRRO® reduced weed biomass less than Harness Xtra® since INTRRO® (alachlor) is not as broad-spectrum as the combination of acetochlor and atrazine found in Harness Xtra®. However, early weed growth was reduced sufficiently with both protects to protect yields when glyphosate was applied postemergence.

The role of preemergence herbicides in reducing the risk of selecting glyphosate resistant weeds is discussed in another article:

This article is also available in a printer friendly pdf format.

Roundup Ready, Roundup WeatherMax, INTRRO, and Harness Xtra are registered trademarks of Monsanto.

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.