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Effects of Weather on Herbicide Performance
Bob Hartzler

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April 10, 2003Attempting to explain the cause of herbicide performance failures is a task most people involved with weed management have experienced.  In some situations the source of variation is obvious, whereas in other scenarios we leave the field shaking our heads wondering what went wrong.  A recent paper in Weed Research reports results of a project with the objective of better defining the environmental factors that influence herbicide efficacy (Collings, L.V., A. M. Blair, A. P. Gay, C. J. Dyer and N. MacKay.  2003.  The effect of weather factors on the performance of herbicides to control Alopecurus myosuroides.  Weed Res. 43:146-153).    Two herbicides, isoproturon and clodinafop, were used to control blackgrass in winter wheat.  Isoproturon is absorbed primarily by roots, whereas clodinafop is absorbed through the foliage of plants.  The herbicides were applied at 1/4, 1/2, 3/4 and 1 times the label rate on several dates during three growing seasons.  Experiments were conducted in the same field each year to minimize the effects of soil differences on herbicide performance.  Data was analyzed to determine which environmental factors in the two weeks prior to and after application influence herbicide performance.

As one would expect, blackgrass control varied among herbicides, application dates and years.  An example of the variation during one of the years is shown in Table 1.   The number of blackgrass seedheads present at the end of season was used to determine herbicide efficacy.  Isoproturon was more variable in activity than clodinafop, with reduction in seedheads ranging from 20 to 91%.  Clodinafop was much less variable, with reductions ranging from 50 to 100%.

Table 1.  Effect of application date on control of blackgrass with 1X rate
during the 1999/2000 growing season.1

Application date

Percent reduction in panicle number

isoproturon clodinafop
Nov 12 80 50
Nov 15 60 70
Dec 14 74 98
Dec 16 79 100
Jan 18 63 96
Jan 24 82 97
Feb 21 49 62
Feb 22 70 84
Mar 13 52 98
Mar 20 20 93
Mar 23 67 92
Apr 6 91 98

1Adapted from Collings et al.  2003.  Weed Res. 43:146-153.  Data points are estimate
of values in bar graph from original article (estimated accuracy of 3%).

As with previous efforts to correlate herbicide activity with environmental factors, the authors were unable to identify a simple relationship between these variables.  They reported that although recommendations state to apply the herbicides to small weeds, there was not a consistent relationship between blackgrass size and control.  Reduced activity with early applications was not due to late emerging weeds, but rather due to emerged plants that survived the application.   When evaluating weather factors, they were able to identify significant relationships for each of the individual years, but the factors were not consistent from year to year.   In 1997/98 they found that clodinafop activity increased when rain occurred following application.  They speculated that rain might wash herbicide from the leaf into the leaf axil where penetration and translocation might be greater.   Increases in isoproturon activity were associated with rainfall prior to application and increasing wind after application.   The response to wind is likely due to increased transpiration during windy conditions.  Water loss from leaves via transpiration is the driving force for absorption of herbicides from the soil, thus any factor than increases transpiration should increase the activity of isoproturon.

The authors concluded that with the herbicides and weed species studied, variations in weather conditions affected herbicide performance more than weed growth stage.  Thus, the statement that small weeds are easier to kill than large weeds is an oversimplification.  However, our current understanding of weed biology is not sufficient to accurately predict the precise susceptibility of a weed at any given time.  While application to small weeds may not guarantee optimum control, early applications do reduce the likelihood of early-season competition between weeds and crops, and also provide a better opportunity to implement secondary control tactics in situations with poor performance of the primary tactic.

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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.