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Use whole toolbox on weeds

Waaay back Wednesday

First published March 2010 in Farming Smarter magazine

By Lee Hart

Bob Blackshaw
Bob Blackshaw

Technology improved crop production a lot over the past 50 years, but farmers need the whole toolbox rather than rely on the prospect of another new and improved herbicide as a solution to all weed control problems, says an Agriculture Canada researcher.

With more weeds showing herbicide resistance and only a few new chemistries (new modes of action) in the development pipeline, producers may have to re-enlist some older products and look at a using a toolbox of cultural practices to optimize weed control, says Bob Blackshaw, a researcher at the Lethridge Research Centre.

“It comes back to something I have advocated for many years and that is an integrated weed management system,” says Blackshaw. “There are no silver bullets out there. There isn’t going to be one chemical that solves weed problems. Crop protection products are important and effective, but they need to be used along with a number of weed management and crop production practices.”

Blackshaw, in a recent presentation called Blending Technology in Biological Systems, says increased use of the popular, effective herbicides causes more weeds to develop resistance.

Group 1 and 2 herbicides are the most commonly used in Western Canada. Products in these two groups are used on 50 to 60 percent of all crops and in many cases year after year. There is also increasing herbicide resistance to Group 3, 4, 5 and 8 products.

There are currently 30 species of grassy and broadleaf weeds in Canada with resistance to herbicides. On the grassy side they include wild oat, green foxtail, barnyardgrass, Persian darnel, witchgrass, yellow foxtail and giant foxtail. Among broadleaf weeds the much longer list includes, kochia, cleavers, chickweed, lambs-quarters, hempnettle, wild buckwheat, wild mustard and Russian thistle, to name a few.

Blackshaw adds that even the longstanding myth that no weed would develop resistance to glyphosate (Group 9)  has shattered. There are 16 weed species around the world now identified with resistance to glyphosate, including one in Canada – giant ragweed – that grows in Ontario. So far, there has been no glyphosate resistance found in Western Canada.

Crop protection companies are always looking to develop herbicides with new modes of action, but Blackshaw says there is a ‘minimal chance’ there will be many in the coming years. A couple of the newer herbicides with new modes of action recently released include Group 14 bleaching herbicides such as Authority, CleanStart and Kixor, and there is a Group 27 herbicide found in Velocity m3.

Bob Blackshaw
Bob Blackshaw presenting to a 2011 Farming Smarter crop walk.

Other weed control technology is under research. Australia is looking at technology that will destroy weed seeds as they exit the combine and there is also work underway to develop chemicals that can break weed seed dormancy, promoting all weeds to germinate at once.

Research and technology is working on a number of fronts, but Blackshaw urges producers not to just rely on technology, but also to include integrated weed management in their overall crop production system.

“Integrated weed management combines several methods of weed control, including the use of herbicides,” says Blackshaw. “Its focus is to inhibit weed emergence, reduce weed growth and seed set, which minimizes competition with crops. At the same time if we can lower weed populations over time, we can stop weeds from being there in the first place.”

Tools such as zero-till crop production practices benefit weed management. With minimum and zero till practices there is greater weed seed mortality with seeds left on the soil surface, the crop residue helps to suppress weed germination and growth and conserve soil moisture This results in healthier, more competitive crops.

Along with zero-till practices, Blackshaw offers these stratagies.

  • Control weeds earlier rather than later with herbicides.
  • Follow a proper three and four year crop rotation; which also promotes herbicide rotation.
  • Use higher crop seeding rates to produce a more competitive crop that chokes out weeds.
  • Select for more competitive crop cultivars.
  • Be strategic with fertilizer application to reduce the chance of fertilizing weeds.
  • Finally, use cover crops; which again can compete with weeds.

“Crop diversity is an important strategy for controling weeds,” says Blackshaw. “With different crops, producers use different seeding dates, different fertilizer practices, different herbicides and different harvest dates. Try to include more oilseeds and pulse crops in rotation with cereals. Look at using winter crops and forages in rotation too. All these strategies help break the weed cycle, reduce weed numbers and improve the effectiveness of herbicides.”

Blackshaw notes that weeds are genetically diverse and will continue to evolve to changing farming practices. So it is important that weed management and cropping practices also change in an effort to stay one step ahead of the weeds. “While it is unlikely that many new chemical tools will be available in the future, it is important that producers use cropping practices to manage weeds,” he says. “It is important that producers maintain and increase diversity in crop rotations and production practices wherever possible.”