Principles of Integrated Weed Management: Managing Weeds Resistant to Herbicides

Excerpt from 2008 Chapter 1, Publication 75, Guide to Weed Control, Order this publication

Table of Contents

1. Introduction
2. Delaying Herbicide Resistance
3. Diagnosing Herbicide-Resistant Weeds
   • Table E. Herbicide Resistant Weed Populations Confirmed in Ontario (as of Nov. 2005)
4. Testing for Herbicide-Resistant Weeds
5. Other topics in Integrated Weed Management
   • Introduction
   • Non-Chemical Weed Control
   • Critical Period of Weed Control
   • Use of Chemicals to Control Weeds
6. Related Links

Introduction

Resistance of weeds to herbicides is not a unique phenomenon. In fact, resistance to antibiotics, insecticides and fungicides is a world-wide problem that is not confined to any single pathogenic pest category.

In Ontario, resistance has been reported to Groups 2, 4, 5, 7, 9 and 22 as listed in Table E. In western Canada and the United States resistance has developed to Groups 1, 2, 3, 4, 5, 8, and 9
(refer to Table 4-5)

Note 1: For control of a particular triazine-resistant species, select an effective non-triazine (non-Group 5) herbicide, or select a tank mix or premix with an effective non-triazine (non-Group 5) herbicide component (see Table 4-5) .

Note 2: Group 2 ALS resistant weeds. For control of a resistant species select an effective non-Group 2 herbicide, or select a tank mix or premix with an effective non-Group 2 herbicide component (see Table 4-5).

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Delaying Herbicide Resistance

Herbicide resistance results from a change in a weed population over time after repeated application of the same group of herbicides. The result is reduced weed control in the field. Resistance to different types of herbicides develops at different rates among weed species and populations. To prevent or slow the development of resistant weeds on your farm, take the following common-sense approaches.

1. Identify and Monitor:
   Resistant weeds do not look different. Therefore, they are hard to identify. Furthermore, weed "escapes" are common after herbicide application. Resistance does not usually become obvious until 10% -30% of the population become resistant. Survey your fields regularly and apply the methods of diagnosing herbicide-resistant weeds to catch problems as they arise.
2. Prevent the Spread of Weeds:
   When entering or leaving a field, clean all implements. Do not allow resistant weeds to go to seed.
3. Use Alternatives to Chemical Weed Control:
   Combine mechanical weed control, such as rotary hoeing or cultivation, with chemical weed control where possible.
4. Rotate Crops and Herbicides:
   Rotate herbicide usage so that the same herbicide group is not used year after year. One of the easiest ways to rotate to a new herbicide group is to rotate to a new crop. Then use a herbicide from an alternative group. Refer to Table 4-5 for help selecting herbicides from different groups. This preplanning will prevent the same herbicide group from being used too often. When it is practical, use tank or formulated mixtures where both active ingredients within the tank act to kill the same weed in a completely different way.
5. Keep Records:
   Keep accurate records of crop rotations and herbicide use in all your fields. It will be easier to plan long-term weed management strategies if you have good records.
6. Communication:
   Stay in touch with farmer organizations, extension specialists, agribusiness, friends and neighbours about resistance problems they are encountering. Let your government or industry representative know about any cases of resistance so they can take the appropriate action to prevent spread of the weeds.

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Diagnosing Herbicide-Resistant Weeds

Before assuming that any weeds surviving a herbicide application are resistant, rule out other factors that might have affected herbicide performance. Those factors are misapplication, unfavourable weather conditions, improper timing of herbicide application, and weed flushes after application of a non-residual herbicide. If resistance appears to be likely, check for the following:

1. Are other weeds listed on the product label controlled satisfactorily? Usually, only one weed species will show herbicide resistance in any given field situation. Therefore, if several normally susceptible weed species are present, reconsider factors other than herbicide resistance.
2. Did the same herbicide or herbicides from the same group (see Table 4-5) with the same site of action fail in the same area of the field in the previous year?
3. Do your records show extensive use of the same herbicide or herbicides from the same group year after year?

If one or more of these three situations apply, it is possible that the weeds are resistant to the herbicide. If resistance is suspected, control the weeds with a labeled herbicide from another group or use appropriate non-chemical weed-control methods to prevent the weeds from going to seed. Next, contact your weed specialist, herbicide supplier and the appropriate chemical company to develop a comprehensive weed-control program to manage the problem.

For more information on herbicide resistance see:

• OMAFRA Factsheet, Herbicide Resistant Weeds (Order No. 01-123)
• International Survey of Herbicide Resistant Weeds at: http://weedscience.com/
• Herbiside Resistant Weeds in Ontario at http://www.plant.uoguelph.ca/resistant-weeds/

*The above weed populations have been found to be resistant to some or all of the herbicides in the indicated herbicide group. Refer to Table 4-5 for a list of herbicides associated with each herbicide group.

Testing for Herbicide-Resistant Weeds

The University of Guelph "Weeds Lab" can test for resistance of suspected weed species. For more information call 1-877-424-1300.

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Related Links

• Weeds in Ontario
• Pesticide Application and Safety
• Pesticide Companies and Agents
• OMAFRA Sprayer Calibration Calculator
• PMRA website
• OMAFRA Publication 505: Ontario Weeds
• OMAFRA Publication 75: Guide To Weed Control

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