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Power of the Crop Canopy   
Bob Hartzler

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February 01, 2007In today's agricultural system it is easy to assume that herbicides are the primary, if not only, tool used to control weeds. However, the crop canopy is as important as herbicides in maintaining weed-free* fields. The benefit of a competitive canopy was demonstrated in two different projects this past season.

In one study we investigated the effect of soybean planting density on weed management. Soybean was planted in 15" rows at densities ranging from 97,000 to 170,000 seeds per acre. Weeds were controlled with glyphosate applied at different soybean stages (V2, V4, V6), and a weedy control was included. The relationship between harvest soybean population and end-of-season weed biomass in three experiments is shown in Figure 1. Weed biomass was directly related to soybean population, with each increase of 100 soybean plants reducing weed biomass by 26 lbs/A. Soybean planting density only affected weed densities at one of the three locations, thus the primary effect of the soybean canopy was on the growth of weeds rather than weed survival. Weeds that emerged following glyphosate applications responded similarly to soybean population as did weeds in the no herbicide plots. The reduction in weed biomass associated with increases in soybean poplations would reduce the potential yield impact of weeds escaping control and reduce the number of weed seeds produced.


The second study was conducted on a cooperator field in northcentral Iowa in a project sponsored by the ISU Corn-Soybean Initiative. The purpose of the study was to evaluate the benefit of preemergence herbicides in glyphosate resistant soybean. No significant rains occurred within three weeks of planting, thus the preemergence herbicide was ineffective at controlling weeds that emerged shortly after planting. Weed populations at the time of the early glyphosate application (V3 soybean) were similar in plots with no preemergence and those treated with 2 pts Prefix. At harvest the plots treated with Prefix followed by glyphosate were clean (other than a few volunteer corn plants), whereas the glyphosate only plots were infested with a moderate infestation of giant foxtail. This shows that the Prefix was more effective than the soybean canopy at controlling weeds that emerged after the glyphosate application. However, as we walked the plots while the combine was harvesting, the agronomist from the dealership made an interesting observation. She had noticed that all of the foxtails were established in a narrow strip (approximatley 2 - 4" wide) directly between the soybean rows. Thus, the soybean canopy was as effective as the herbicide in the area within 12" of either side of the row, but the canopy had not developed sufficiently by mid-June to shade the areas in the row middles. Had this field been planted in 15" rows rather than 30", I suspect weed control would have been similar in the two treatments. In plots where the post application was made at the V6 stage, no differences in weed control were observed between the Prefix and total post plots. No yield differences were observed among any of the treatments.

These studies demonstrate the importance of the crop canopy in suppressing weeds, even in systems utilizing glyphosate resistant crops. Management practices that promote a uniform, competitive canopy include:

The crop canopy is essentially the only non-herbicide selection pressure placed on weed communities by most farmers. In order to preserve the effectiveness of herbicides, it is important to maximize this benefit by by taking steps to insure a crop stand that can effectively compete with weeds.

*Weed-free is a bit of a stretch since this is rarely, if ever, achieved. However, sufficient weed control is achieved in most Iowa fields to protect crop yields and prevent large increases in the soil weed seedbank.

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.