Principles of Integrated Weed Management: Managing Weeds Resistant to Herbicides

Excerpt from 2010-11 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 1-5. 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 pest category.

In Ontario, resistance has been reported to Herbicide Groups 2, 4, 5, 7, 9 and 22 as listed in Table 1-5. Herbicide Resistant Weed Populations Confirmed in Ontario (as of Dec. 2009), page 6. In western Canada and the United States resistance has developed to Groups 1, 2, 3, 4, 5, 8, and 9
(refer to Table 4-6 Herbicide Groupings for Ontario, page 65)

Delaying Herbicide Resistance

Weed populations can change over time after repeated application of herbicides from the same group. This can result in reduced weed control due to resistant weed biotypes. Resistance to different types of herbicides develops at different rates among weed species and populations. To prevent or slow the development of herbicide resistant weeds on your farm, take the following 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 detect 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 and use a herbicide from an alternative group. Refer to Table 4-6 Herbicide Groups for Ontario, page 65, for help selecting herbicides from different groups. This preplanning will prevent overuse of a herbicide group. 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 herbicides used in all fields. It will be easier to plan long-term weed management strategies based on good records.
6. Communication:
   Connect with farmer organizations, weed and crop specialists, agribusiness, friends and neighbours about resistance problems they are encountering. Communicate to your government or industry representative about any cases of resistance to facilitate the appropriate action to prevent spread of the weeds.

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. 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 4-6. Herbicide Groupings for Ontario, page 65) with the same site of action fail in the same area of the field in the previous year?
3. Do field records show extensive use of the same herbicide or herbicides from the same group year after year?

If one or more of these 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 a 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 4-6 Herbicide Groupings for Ontario, page 65 for a list of herbicides associated with each herbicide group.

Testing for Herbicide-Resistant Weeds

The Department of Plant Agriculture at the University of Guelph can test for resistance of suspected weed species. For more information call 1-877-424-1300.

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