Glyphosate resistant cotton has been grown without rotation for years in the deep southern United States.
Today, in the state of Georgia alone, $150 million is spent fighting herbicide resistant palmer amaranth through more and different herbicides, using rye crop coverage and pulling weeds by hand. Palmer amaranth can tower over a six-foot man. Imagine hand weeding that beast.
If this is not a cautionary tale for managing glyphosate resistance in Western Canada, nothing is.
In light of the rotation-free GM cotton story, it is surprising that the Canola Council of Canada is supporting shorter canola rotations of only two years.
“Growers have taught us that more intensive rotations can be managed sustainably and profitably in many soil zones and regions of the Prairies,” said the council’s document.
It would be interesting to see the full scientific rationale behind this recommendation, which may be based on switching canola systems within the rotation.
At CropSphere in Saskatoon last week, the scientific panel on herbicide-resistant weeds showed evidence that resistance in Western Canada is a growing problem, in large part because of chemical fallowing. The panel did not specifically address the council’s shorter rotation guideline and showed glyphosate resistance is not limited to canola systems. However, the herbicide remains in high use among canola growers.
Agriculture Canada scientist Hugh Beckie said glyphosate surpasses the total of the next 12 top herbicides in Western Canada in percentage of use. Glyphosate is used 52.7 percent of the time while the next 12 herbicides are used 51.7 percent.
“That’s a scary situation,” Beckie told reporters.
Furthermore, he said, there will not be a new herbicidal “silver bullet” in the future.
Eric Johnson, weed biologist at Agriculture Canada’s Scott Research Station, had the same view.
“The problem (of resistance) will not be resolved by chemistry alone,” he said.
Crop competition, crop rotation and managing weed seed production will all have to come into the battle against resistant weeds, Johnson said.
Philip Westra, professor of weed science at Colorado State University, said there is a serious problem of gene amplification in glyphosate-resistant weeds. In the case of palmer amaranth, the plant originally had one copy of the gene that produces the protein to which glyphosate attaches. Today, some palmer plants have 200 copies of the same gene.
You can not apply enough glyphosate to kill it, he said. Worryingly, the same thing is occurring in glyphosate resistant kochia, the first major resistant weed in Western Canada. Canadian kochia plants are showing up with three to 10 copies of the protein that accepts glyphosate.
When the canola council says rotations should be left up to individual producers, it does not adequately address the fact that there are not only environmental concerns but also neighbourly concerns. Glyphosate resistant kochia in one field can easily spread to nearby fields.
Furthermore, what works for some growers in some soil zones should not be the basis for a general recommendation.
Beckie said many options are left to manage weeds, and Western Canada is not yet in crisis. However, he also said two-year canola is not sustainable against weeds or disease.
Weed resistance is a powerful reason for using longer rotations, but there are many others: canola yields are much better in longer rotations and blackleg, clubroot and sclerotinia are easier to manage. In addition, biodiversity is always better than monoculture.
Hopefully, the canola council’s recommendation has good research behind it, but considering the evidence-based opinions provided by a long list of agricultural scientists at CropSphere and elsewhere, it may be worth revisiting.