Changing the conversation on weed control

Story written by Bruce Barker | The Canadian Agronomist

If the definition of insanity is doing the same thing over and over and expecting a different result, then current weed control strategies that solely rely on herbicides could qualify. As evidence, consider that some kochia populations now have multiple herbicide resistance to four herbicide Groups. Weed scientist are now looking beyond herbicides to shift the weed control paradigm.

“Given the proliferation of herbicide-resistant weeds, many researchers have suggested that a zero -- or minimal -- seed threshold may be necessary to prevent further spread. The idea is to limit weed seed production and return to the soil seedbank, which would decline herbicide-resistant weed populations over time,” says Charles Geddes, weed research scientist with Agriculture and Agri-Food Canada at Lethbridge, AB. “Allowing uncontrolled herbicide-resistant weeds to go to seed can result in substantial problems in subsequent years.”

Historically, weed control focused on the “critical period for weed control”, which is the period of the growing season where weed management will limit crop yield losses below a certain threshold says Geddes. In new research, Geddes and colleagues at AAFC and the University of Manitoba (U of M) are looking at a new concept called the Critical Period of Weed Seed Control (CPWSC). He says that CPWSC takes a similar approach but applies it to weed seed production.

“To prevent further spread of herbicide-resistant weeds, farmers must also consider how many weed seeds are returned to the soil seedbank. This is why the concept was developed,” Geddes says.

Geddes led a research project in 2021 and 2022, partially funded by Alberta Grains, to identify the CPWSC for seven common weed species in western Canada. The research was conducted to answer two fundamental questions related to the CPWSC. The first was when during the growing season do common weed species start to produce viable seed? This is called the Critical Period for Seed Production (CPSP). The second was how late can each weed species emerge and produce viable seed before the end of the growing season? This is called the Critical Period for Weed Emergence (CPWE).

Overlapping these two weed seed production timings identifies the CPWSC or the period of the growing season during which weed control can prevent weed seed return to the soil seedbank. For example, if Weed X starts to produce viable seed on July 24, and can germinate as late as July 10 and still produce viable seed in the fall, then the period between July 10 and July 24 is the CPWSC. This is the period when a grower would need to control the weed so weed seeds are not returned to the soil seedbank.

Ten site-years of research were conducted for green foxtail, wild oat, lamb’s-quarters, wild mustard, and redroot pigweed at AAFC Lethbridge, Lacombe, Saskatoon, and Indian Head, and the U of M at Carman, MB. Kochia research covered all sites except Lacombe for eight site-years. Wild buckwheat was covered at Lethbridge and Lacombe for four site-years. Weed emergence date, density, biomass, growth stage at harvest, seed retention at harvest, seed shatter over time and weed seed viability were measured.

The researchers analyzed the data based on reducing viable seed production by 80, 90 and 95 per cent. This corresponded to 20, 10 and 5 per cent seed thresholds – i.e. a 20 per cent seed threshold meant that there was an 80 per cent reduction in weed seeds returned to the soil seedbank. These three thresholds were calculated to allow farmers to make management decisions based on their individual risk aversion.

CPWSC was variable for weed species and locations

As would be expected, the CPWSC varied by site-year and weed species. However, the research found clear trends that identified the time frames when seed production and return to the soil occurred, and the timing for each weed species.

Kochia and wild buckwheat both had CPWSC at the five per cent threshold. For kochia, viable seed production was reduced by 95 per cent when controlled between August 8 and Sept 18 (1753 to 2444 growing degree days). For wild buckwheat, this period to reduce viable seed production by 95 per cent was during a 11-day period between August 5 and August 15 (1685 to 1879 GDD).

Green foxtail and wild oat did not have a five per cent seed threshold, but had a 10 per cent seed threshold. Green foxtail’s covering three days from July 29 to July 31 (1493 to 1534 GDD) and wild oat five days from July 20 to 24 (1321 to 1406 GDD).

Lamb’s quarters, wild mustard and redroot pigweed did not have CPWSC based on five or 10 per cent thresholds.

In cases where the CPWSC did not meet specific threshold levels, Geddes says separate management practices targeting early-emerging and late-emerging plants is required to prevent new seeds from entering the soil seedbank.

At the 20 per cent threshold level, all weeds had a CPWSC when weed management could be used to limit 80 per cent of seed from being returned to the soil seedbank.                 

The critical period for weed seed control (CPWSC) based on a 20% viable seed threshold for seven common weed species among all site-years of research in western Canada in 2021-2022. 

In the graph, the beginning of each line indicates how late the species can emerge and produce viable seed (20% of the maximum) before the end of the growing season. The end of each line indicates when the species begins to produce seed (20% of the maximum). Based on the CPWSC, effective management between these two time points can efficiently limit seed production and return to the soil seedbank by 80 per cent.

The research shows why relying on just pre-and post-emergent herbicide applications can be ineffective in preventing weed seed return to the seedbank. Geddes says that many summer-annual weeds can emerge later than the typical window for post-emergence weed control and produce seed before the end of the growing season.

“Certainly, PRE and POST weed control will help to manage weeds and limit the number of seeds that they produce. However, this project suggests that many weeds can emerge later than the POST application window and still produce a substantial number of seeds before the end of the growing season,” he says. “A typical PRE/POST weed control program may not be enough to prevent weed seed return to the soil seedbank. Some form of later-season weed control could help, but we have yet to answer what makes the most sense during this window.”

A pre-harvest glyphosate herbicide application is an option, but Geddes says that other research has found that it isn’t the complete answer, because control of smaller weeds beneath the mature crop canopy is difficult.

“In addition, it typically would not eliminate viable seed production. For example, our work on pre-harvest glyphosate + saflufenacil on kochia reduced viable seed production by 28 per cent compared with the untreated control. A significant, yet modest, reduction,” says Geddes.

Geddes says other integrated weed management strategies could be implemented based on this research. For wild oat and wild mustard, he says the CPWSC takes place before harvest of most crops grown in the region. Management strategies such as crop topping or clipping in short crops could be effective. For redroot pigweed, lamb’s-quarters and wild buckwheat with later CPWSC, growing an earlier maturing crop with earlier harvest, such as winter cereals could offer an opportunity for effective targeting of these weeds after harvest. Harvest weed seed control with an impact mill is an option for early maturing crops and weeds that retain their seeds later into the growing season.

“I think the CPWSC window could be targeted more effectively by growing crops with early harvest dates. For example, this is part of the reason why growing winter wheat works well for kochia management. It is harvested before most of the kochia produces seed and opens up a window for weed management post-harvest when the crop is not present,” explains Geddes.

Geddes says his intent with this new research was “to provide a framework that can apply across various weed species and effectively communicate that weed management must go beyond the critical period for weed control and consider weed seed production and return to the soil seedbank. Given that it is still “new”, further research is warranted to understand what management practices work best to target the CPWSC for different weed species, and it is likely that the CPWSC for winter-annuals will be entirely different.”

Funding was provided through the Governments of Manitoba and Canada through the Canadian Agricultural Partnership, and the Alberta Canola Producers Commission, Alberta Grains, Manitoba Crop Alliance, Manitoba Pulse & Soybean Growers, Saskatchewan Wheat Development Commission and Western Grains Research Foundation.