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Finding the right partner for your glyphosate application.

Strategies for managing glyphosate-resistant weeds

By: Andrew Reid


It’s no secret that crops grow best in a clean field, and the best way to achieve that is with a pre-seed/pre-emergent burndown. Since its introduction in 1974,1 glyphosate’s broad weed spectrum has made it the herbicide of choice for many growers. But with the continued spread of glyphosate resistance, an effective burndown calls for more than glyphosate alone.

Resistance impacts your yield early on.

Volunteer canola and kochia are known weeds with resistance to glyphosate in Western Canada. They compete for nutrients, sunlight and water during the early stages of crop development, robbing your crop of its yield. If not managed properly, resistant weeds can continue to spread and limit your cropping options. Repeated applications of a herbicide with a single mode of action can continually select for resistance – worsening the problem in the long run. Plus, weeds can harbour diseases even in non-crop years, diminishing the benefits of your crop rotations.

Volunteer canola

Not only do canola volunteers detract from your yield, they also emerge throughout the season. Without residual activity, a glyphosate-only burndown is unlikely to provide adequate control. And if canola volunteers happen to be of the glyphosate-tolerant variety, then a tank-mix partner is needed for your pre-seed/pre-emergent application. Note that canola volunteers can be outcrossed with other systems, leading to individuals with stacked herbicide-tolerant traits.

Glyphosate-resistant kochia

Research has shown that nearly 80% of kochia emerges before your crop does,2 hence the importance of a pre-seed/pre-emergent burndown. Due to overreliance on Group 2 chemistry, all kochia in Western Canada is now considered Group 2-resistant. Combined with the rise in glyphosate resistance, kochia management requires at least two additional modes of action during that season for effective control.

Source: 2014 IPSOS brochure

Wild oats

No instance of glyphosate-resistant wild oats has been reported yet, but wild oats has shown resistance to multiple Group chemistries already. Many are resistant to Groups 1, 2 and 8, and researchers say it could be the next weed to adapt to glyphosate.3

As both broadleaf weeds and grasses develop resistance to more and more chemistries, the cost of incorporating additional herbicides will only get higher.

How to choose an additional mode of action. Or two.

To reduce the impact of early emerging weeds and glyphosate-tolerant biotypes, we need to take an integrated approach that includes an additional mode of action in your pre-seed/pre-emergent burndown. This will help manage existing resistant biotypes, as well as prevent the onset of resistance in other weeds. Here’s what you should look for in a suitable tank-mix partner:

Residual activity

Glyphosate only works on emerged weeds. To control flushing weeds such as volunteer canola, cleavers or wild oats, choose a tank-mix partner with residual activity. If you’re using a herbicide-tolerant system, this will also help take the pressure off your in-crop herbicide later on.

Group chemistry differs from your in-crop herbicide

In-crop herbicide choices tend to be limited, so make sure you use a different mode of action for burndown. This will help manage weeds not targeted by your in-crop herbicide.

Overlapping activity on your weed spectrum

Know which weeds are growing in your fields and choose chemistries that target them. All kochia is now considered Group 2-resistant, so if you have glyphosate-resistant kochia, avoid tank mixing glyphosate with a Group 2 herbicide. Many wild oats are resistant to Groups 1 and 2. Use active ingredients with overlapping activity to avoid placing selection pressure on a single chemistry.

Suitability for your crop

Ensure that any tank-mix partners won’t cause harm to your crop. Some broadleaf herbicides have residual activity that can lead to canola damage. Double check the herbicide label to confirm.

Unique chemistries

Repeated use of some chemistries are more likely to lead to the onset of resistance than others, such as Groups 1 and 2. This is in part due to the complexity of resistance mechanisms involved. Wherever possible, use chemistries that have a lower risk for resistance and/or have fewer reported incidents of herbicide resistance such as Groups 14 and 15.

Source: Adapted from Beckie, 2006 as appeared in Grainews4

Speak to your retailer about solutions that fit your needs. Some good pre-seed tank-mix partners include Group 14 and Group 6 chemistries for managing glyphosate-tolerant volunteer canola, as well as Group 14 chemistry for managing kochia.5 Some Group 15 herbicides are also good options for application ahead of canola.

Once you’ve selected an appropriate tank-mix partner or two, take full advantage of your tank-mixed glyphosate application by applying these best practices:

  • Use full label rates to minimize weed escapes and the spread of weed seeds
  • Keep fields clean when the impact to yield is greatest
    • For cereals: emergence up to early jointing
    • For canola: emergence up to the 4-leaf stage
  • Apply when weeds are actively growing and when overnight lows are above 3°C to 5°C
  • If you miss the pre-seed application window, take note of environmental conditions to determine your timing for a pre-emergent application
  • Use a sprayer meant for that type of herbicide; double check the label or contact your local representative

Integrate non-herbicidal practices into your approach.

While using multiple modes of action is a good strategy, the best approach to managing resistance includes non-herbicidal practices as well. Increase the competitiveness of your crop by selecting varieties suitable to your field’s growing conditions. Plant at more shallow depths and sow at higher seeding rates to outcompete weeds. Rotate your crops to take advantage of different chemistries. Even rotating into a different herbicide-tolerant system will help manage volunteers and delay resistance. Be sure to use clean equipment to prevent the spread of resistant weed seeds. And most importantly, keep good records of your weeds, the management strategies taken and their outcomes.

References

  1. Benoit, L. (2015) Dicamba-tolerant soybeans part of the solution to glyphosate resistance. Farms.com. Accessed http://www.farms.com/ag-industry-news/dicamba-tolerant-soybeans-part-of-the-solution-to-glyphosate-resistance-084.aspx
  2. Canola Council of Canada. (2016) Weed management. Canola Encyclopedia. Accessed http://www.canolacouncil.org/canola-encyclopedia/weeds/weed-management/
  3. RealAgriculture. (2015) Glyphosate-resistant wild oats could be next. Accessed https://www.realagriculture.com/2015/07/glyphosate-resistant-wild-oats-could-be-next/
  4. Minogue, L. (2015) Practical ways to fight herbicide resistance. Grainews. Accessed http://www.grainews.ca/2015/01/14/practical-ways-to-fight-herbicide-resistance/
  5. Canola Council of Canada. (2015) Pre-seed burnoff: 8 top weeds and how to control them. CanolaWatch. Accessed http://www.canolawatch.org/2015/04/29/pre-seed-burnoff-8-top-weeds-and-how-to-control-them/

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