Don’t Overlook Group 2 Herbicide Resistance

You’ve probably read about media dubbed ‘superweeds’ like glyphosate-resistant palmer amaranth and giant ragweed. Glyphosate-resistant weeds often earn this distinction because they are viewed as a greater management hardship for producers than weeds resistant to other herbicide mechanisms of actions (MOAs).  And maybe rightly so.  Farmers dealing with glyphosate resistant weeds elsewhere in the world have been reduced to tillage and hand rouging for weed control in some crops.

But, while glyphosate use dominates the Roundup Ready corn, soybean and/or cotton rotation in the US, group 2 herbicides play an (equally?) important role in our more diversified cropping system. For example, group 2 herbicides are used in crops like alfalfa, corn, dry beans, field pea, potato, soybean, sunflower, and in Clearfield and other group 2-tolerant crops.  These herbicides are also a popular choice for group 1-resistant grassy weed control in cereals.

The point of this article isn’t to downplay the importance of glyphosate resistance but to elevate consideration of group 2 resistance. In Manitoba, over 10 weed species are known to have biotypes resistant to group 2 herbicides.  And herbicide-resistant weed surveys led by AAFC indicate that the prevalence of certain species is increasing (Figure 1).

Figure 1: Prevalence of group 2 resistance in Manitoba in 2003 and 2008 as a percent of the weed species population surveyed (Beckie et al).

 

The following practices can help reduce the risk of developing herbicide resistant weeds and/or managing existing resistant weed populations:

  • Diversifying your crop rotation;
  • Using multiple herbicide MOAs effective on target weeds (e.g. herbicide ‘layering’, tank mixing);
  • Practising good basic agronomy (variety selection, seeding rates, etc.);
  • Judicial use of tillage.

If you suspect group 2 resistance in a weed species on your farm, it’s best to verify this by herbicide-resistance testing. Unfortunately there‘s no quick method – seed from the suspect population needs to be allowed to mature and collected.  Samples can be submitted to AgQuest for testing in Manitoba.

In my opinion, knowing if you have group 2 resistance and assessing your risk factors is worth it. Because while glyphosate resistance is grabbing headlines, group 2 resistance may be quietly growing in your fields.

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Testing Weeds for Herbicide-Resistance

Do you have weeds that survived this year’s herbicide application(s)? Since there are many factors that can contribute to weed escapes, consider:

  • The distribution of escaped weeds. Herbicide-resistant weeds tend to occur in patches as opposed to geometric patterns (e.g. spray miss) or throughout the field (e.g. tolerant weeds).
  • Possibility of reduced herbicide efficacy. 2016 was a challenging year for weed management due to untimely and excessive rainfall. In many cases, weeds escaped because of herbicide application timing with respect to weed growth stage, limited herbicide choices because of crop growth stage (when producers finally could get on their fields) and product rainfastness.
  • Weed species. Annual weed species, like wild oat, green foxtail, cleavers, kochia, hemp-nettle, smartweeds, ragweeds and wild mustard, may be more likely to develop resistance compared with other weed species. Because the development of herbicide-resistance is based on chance, resistant weed patches are typically a single species, as opposed to non-resistant weed escapes, which may affect multiple weed species.

Suspect weed escapes can be confirmed as resistant or susceptible by herbicide-resistance testing. For most weeds, dry, mature seed is required for the analysis.  Although more is better, many labs require at least 100 g of small weed seeds (e.g. cleavers) and 200-250 g of large weed seeds (e.g. wild oat).  Weed seed samples should be submitted by December 31st, 2016 to either:

For suspected glyphosate-resistant kochia, a genetic-based tissue test is also available from the Pest Surveillance Initiative: http://www.mbpestlab.ca/field-testing/. In this case, about 5 to 10 g of green plant tissue (e.g. leaves and stems from plant tips) is needed for the analysis. Samples should be placed on ice and shipped immediately after collection. The advantage of the genetic test (vs. seed analysis) for kochia is the ability to determine resistance in-season.

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Submitted by: Jeanette Gaultier, Provincial Weed Specialist, Manitoba Agriculture

For more information on resistant weeds and weed management, visit the Manitoba Agriculture website: http://www.gov.mb.ca/agriculture/crops/weeds/

 

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Glyphosate Resistant (GR) Kochia Confirmed in Manitoba

Manitoba Agriculture, Food and Rural Development (MAFRD) jointly conducted a kochia survey across Manitoba in the fall of 2013 in with Agriculture and Agri-Food Canada (Saskatoon Research Centre) and the University of Manitoba, funded by the Western Grains Research Foundation and BASF Canada.
 
Plants from 283 different kochia populations were harvested, thrashed and planted over the winter. The resulting seedlings were tested for glyphosate resistance.  Kochia plants from two of the 283 sites were found to be glyphosate resistant (GR).  Both sites were in the Red River Valley.  Finding GR kochia was not unexpected as previous surveys in Alberta and Saskatchewan, North Dakota, South Dakota and Minnesota, have all identified GR kochia.
 
Herbicide resistant weeds are not a new issue in Manitoba, as Group 2 resistant kochia and Group 3 resistant green foxtail populations were identified in 1988.  However, resistance to glyphosate is new and it remains an important herbicide in Manitoba crop production systems.
 
If GR kochia populations become more common in the province, it will result in added management skills and expense for Manitoba farmers.  In-crop control of GR kochia can be difficult in broadleaf crops like canola, soybean or pulses and pre-seed or pre-emergent treatments may be necessary for adequate control.
 
As GR kochia has been found in less than one per cent of the sites sampled, Manitoba farmers have an opportunity to minimize the spread of this weed. Farmers should consider reducing the number of glyphosate applications in a single season and incorporate non-glyphosate herbicides in weed management programs when growing glyphosate-tolerant crops.  Farmers will also need to incorporate non-herbicidal measures like crop rotation, tillage and manual weeding if necessary to control populations.
 
Moving forward, MAFRD staff will continue to monitor the two sites where GR kochia was found and work with the affected and neighbouring producers this spring to discuss possible containment procedures. The department will also develop an extension program on herbicide-tolerant weeds, using GR kochia as an example.  The program will be aimed at industry agronomists, grower associations and producers and is expected to be ready later this spring.
 
For more information about GR kochia and related agronomic advice, please contact Nasir Shaikh, provincial weed specialist, at 204 750-2715 or Nasir Shaikh.
 

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