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Variable Herbicide Performance
by Bob Hartzler

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September 25, 2001 -  One problem that agronomists face each year is determining why certain herbicide applications fail while similar treatments in other fields provide excellent control.  The person investigating the complaint must sort through a myriad of factors, including application parameters (rate, spray volume, additives, coverage, etc.), weed factors (biotype, size, vigor, etc.) and environment (temperature, relative humidity, soil moisture, etc.), to determine the cause of the failure.  This complexity results in vague statements on herbicide labels regarding product performance, such as "The degree and duration of control depend on spray coverage, activating rainfall, weed spectrum, weed size, growing conditions before and after treatment, soil moisture, and adjuvant selection" 1.

A recent article in Weed Research 2 described research in which data from 59 experiments was analyzed to determine factors influencing clodinafop activity on wild oat.  Clodinafop is an ACC-ase inhibiting herbicide (same family as Poast and Assure) and is used for postemergence grass control in wheat.  The objective was to evaluate a statistical method for determining specific environmental or application factors that would explain differences in wild oat control among the different experiments.  The factors included in the statistical analysis are listed in Appendix A.

The research determined that the following factors were involved in the variable performance of clodinafop:

Included among factors that did not affect performance were:   weed growth stage, location of experiment in Australia, and weather events after application.  The authors of the paper stated that the findings indicate a potential for decision support tools that would allow applicators to adjust herbicide rates in response to the environment.  However, a huge database would be required in order to accurately define the contributions and interactions of all weather parameters.  Previous attempt to develop such tools have failed, most likely because research was not conducted under enough environments to sort out all of the factors influencing herbicide performance.

The environmental factors identified by the Australian researchers correspond with conditions we feel caused problems across Iowa this year in Roundup Ready soybeans.   Glyphosate performance, particularly on waterhemp,  was more variable in 2001 than in any year since the introduction of RR soybeans.  While each failure likely was due to a unique combination of factors, temperature and rainfall patterns prior to applications probably played an important role in many of these situations.  Most of the glyphosate performance problems occurred during the last week of June and first week of July.  This period was preceded by five days of below average temperatures (Figure 1).  In addition, most of the state received little precipitation between June 15 and July 6.  Shallow-rooted weeds in many fields would have experienced soil moisture deficits during this time period.  Cool temperatures and dry soil were two of the primary environmental factors identified by Australian researchers as significantly influencing herbicide performance.  It is likely that these two factors, rather than a shift in waterhemp populations or changes in Roundup formulations, led to the control failures observed in 2001.

In summary, the performance of herbicides is influenced by many factors.  While I'm skeptical that decision tools will be developed in the near future which allow fine tuning of herbicide rates in response to weather, this information can help determine why certain treatments fail to provide acceptable control.  In addition, the likelihood of performance failures can be reduced by monitoring weather conditions and adjusting application parameters accordingly.  While we are unable to predict the precise herbicide rate needed under specific conditions, we should be able to predict when weeds are less susceptible to control.  Under these conditions, herbicides rates should be increased or applications delayed until more favorable conditions occur.

1Steadfast Herbicide label, DuPont.

2R. W. Medd et al.  2001. Determination of environment-specific dose-response relationships for clodinafop-propargyl on Avena spp.  Weed Res. 41:351-368.

 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.