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Farmer Perceptions of Herbicide Resistance   
by Bob Hartzler

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February 25, 2002 -  It is generally acknowledged that the world's worst herbicide resistant weed problems are found in the wheat producing areas of Western Australia.  Annual ryegrass is the predominant resistant weed in this region, and has developed resistance to most herbicides available for use in wheat.  In response to these problems the Australian government has initiated a large program to study the biology of resistance and the influence of agricultural practices on the problem.  A large effort is being made to understand why farmers make decisions that may influence the likelihood of resistance developing on their farms.  Rick Llewellyn and others recently summarized results of an extensive survey of farmer' attitudes toward resistance.  This report is posted on the web at:  http://www.general.uwa.edu.au/u/dpannell/dpap0113.htm.   In this article I will summarize a few of the findings of this study, readers are encouraged to go to the original article for complete results.

More than 130 farmers were surveyed in two regions of Western Australia, one in which resistance is widespread and the second where serious infestations of resistant weeds are sporadic.  The researchers wanted to determine the farmer's understanding of herbicide evolution in terms of herbicide use patterns.  Farmers were asked how many applications of an ACC-ase inhibitor would be required to select for resistant biotypes in ryegrass.   The mean response for both regions was six applications.  A higher percentage of farmers from the region with low resistance predicted greater than eight applications compared to farmers from the region with widespread resistance.   Research has shown that resistance is likely to develop with less than eight applications of these herbicides.  The prediction for number of glyphosate applications needed to select for resistance was 16.  Farmers from the area without severe resistance were more likely to suspect it would take the same number of glyphosate applications as ACC-ase inhibitors to develop resistance than farmers with first-hand experience with resistance.  Experience has shown that the likelihood of resistance is less with glyphosate than the ACC-ase inhibitors.  This finding indicates that farmers from regions experiencing resistant problems are more aware of current issues regarding herbicide resistance.

There has been debate on whether the development of herbicide resistance imposes a cost to the grower.  Australian farmers expected annual returns on land where resistance eliminated all herbicide options to decline by 37 to 50%.  To further evaluate the perceived cost of resistance, farmers were asked the price they would be willing to pay for land with different levels of herbicide resistance.  The presence of resistance to only ACC-ase inhibitors reduced land value by approximately 15%.   Resistance to both ACC-ase and ALS-inhibitors reduced the perceived value of land by 25%.  In the area with widespread resistance, farmers stated that the presence of resistance to the ACC-ase, ALS and glyphosate herbicides would reduce the value by more than 50%.  I am not aware of any similar studies conducted in Iowa or other states, but I suspect the results would indicate that herbicide resistance has little impact on land value.   This is probably due to the wider selection of herbicides available for use in corn and soybeans.

The farmers surveyed were aware of herbicide resistance and the likely number of herbicide applications required to select for resistance.   However, a significant percentage of farmers held misconceptions concerning resistance.  These included:

1)  Many growers expected that weed populations would revert to susceptible biotypes relatively quickly if use of the herbicide for which resistance was present was temporarily stopped.  (related article)

2)  Many growers felt that industry would be able to quickly develop new modes of action of herbicides to deal with resistant biotypes.   While this has been the case in most scenarios, it may not necessarily remain so in the future.

3)  A substantial number of growers suspected that resistance would develop in their fields regardless of how they managed the land.

The results of this survey suggest that Australian farmers believe that herbicide resistance has significant economic costs.  Because of this cost, adoption of integrated weed management programs that reduce the likelihood of resistance would be of value.  However, important misconceptions concerning resistance may hinder the implementation of these programs.  Although no similar in-depth surveys have been conduced in the U.S. cornbelt, I believe these same misconceptions are held by many of the farmers in this region.

 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
http://www.weeds.iastate.edu
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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.