If you’ve been focusing mainly on the CPWC, it may be time to switch strategy and add an “s.”
CPWC — the critical period for weed control — is the period of growth during which a crop must remain weed-free to prevent yield loss. Suppressing weeds in that period is the objective behind most control strategies.
But a Canadian weed ecology researcher has proposed a new concept — the critical period for weed seed control (CPWSC.) That’s the period of the growing season during which weed control would result in minimal seed production, thereby limiting the number returning to the seed bank.
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The global increase in herbicide resistance necessitates a shift in how we think about weed control, says Charles Geddes, a research scientist with Agriculture and Agri-Food Canada in Lethbridge, Alta.
“We’ve seen a growing body of research looking at the soil seed bank, and basically the reserves of those seeds in the soil are the source from which subsequent generations of weeds emerge. If you can drive down the herbicide-resistant biotypes in the seed bank, you have fewer emerging in subsequent years.”
One way to manage herbicide-resistant weeds is to limit the number of seeds they can produce. Geddes has tested the concept in an experiment with herbicide-resistant kochia. He did an experiment evaluating its phenology — in other words, the timing of its life cycle events — to determine when it starts to produce viable seed. He then sought input for developing the concept from University of Illinois plant scientist Adam Davis.
Together, Geddes and collaborators from across Western Canada have begun a new research project that will expand on Geddes’s kochia research to look at a range of weed species at five sites on the Canadian Prairies. The sites will be managed by Geddes in Lethbridge, Ag Canada researchers Breanne Tidemann in Lacombe, Alta., Shaun Sharpe in Saskatoon, Sask., Bill May in Indian Head, Sask., and University of Manitoba plant scientist Rob Gulden in Carman, Man.
The plots will contribute 10 site-years of data for six weed species: kochia, wild oat, wild mustard, green foxtail, lamb’s quarters and redroot pigweed, and four site-years for wild buckwheat.
Field research began in 2021 and will continue next year, followed by a year of analysis on seed biology and viability.
The research will consist of paired experiments. The first will involve planting weed seeds biweekly through the summer and harvesting and analyzing their viability at the end of the growing season. The second will involve planting weed seeds at the same date in April and harvesting every two weeks beginning in July to assess seed viability as the plants develop.
Geddes says the research is “100 per cent focused on weed plant phenology,” but the results could have major impacts on how producers approach weed control.
“Basically, most weed management to date focuses on herbicides. That weed management takes place at the seedling stage. What we’re seeing with kochia at least is that the critical period for weed seed control takes place much later — in mid- to late August. Management to reduce seed production will be most effective then.”
Zero seed threshold
Geddes says the concept of CPWSC pushes a shift in perspective but doesn’t necessarily translate into an overhaul of what farmers are already doing.
The idea is to tackle the weeds that escape herbicides. The most effective timing for that is likely to be later in the growing season, but that will depend on the weed species. For kochia, that’s late August, but for other plants, it could be earlier. That means weed phenology needs to come under the microscope in a new way.
“As of early July, wild mustard is already most of the way through flowering and has pods and I’m almost certain there’s currently some viable seed in those pods, whereas the other species have just started flowering,” he says.
For weeds that escape the first round of herbicide, a further application could be counterproductive, but there are a variety of other management strategies.
Farmers can’t harvest through dense patches of weeds anyway; Geddes suggests mowing those patches to control seed production. Other new research looks at “crop topping,” or applying non-selective herbicides to crops at later stages of growth and development. Another project led by Tidemann is looking at clipping wild oat seed heads in season.
“I think we’re talking about combinations of approaches. It doesn’t have to be the same method every time — when we’re talking about designing weed-management programs, the whole goal is to mitigate selection pressure. The greater number and diversity of weed-management tools would be of more benefit to a farmer,” Geddes says.
But the goal should be what he calls “zero seed threshold” — not allowing any weed seed to return to the seed bank. It’s a concept that was proposed by researchers in the 1990s, but Geddes thinks it’s worth a fresh look.
In a new research publication he co-authored with Davis, Geddes cites a 1980s Nebraska study in which weed seed bank reserves declined by 95 per cent after five years of weed-free corn. But after a single year without herbicide, the seed bank quickly replenished itself.
“Basically what that example shows is that effective management for multiple years in a row can deplete those reserves by a significant amount, but it’s important to continue trying to limit weed seed return as well.”
A growing body of research shows that the size of the weed seed bank has a correlation with the level of herbicide resistance that develops in an area’s weed population, Geddes adds.
This means better control of HR weeds has to start with a greater focus on weeds’ individual biology, ecology and the phenology of seed production.
It’s early days for Geddes’s research, but he says one of the most effective ways to tackle the CPWSC is to design cropping systems around it by including alternate crop life cycles — like winter annuals in the rotation, and pushing for greater diversity to choke out weed competition.
