Seeding strategies benefit pre- and post-harvest weed control

The most effective treatments can involve decisions made at seeding

The early reports from a couple of ongoing field research projects are suggesting some of the most effective treatments for pre- and post-harvest weed control might actually involve decisions made at seeding, say weed specialists.

In Saskatchewan, Clark Brenzil, provincial weed specialist with Saskatchewan Agriculture, is looking at how narrower row spacing (10 inches) as well as higher seeding rates with wheat appear to have an impact in reducing the number of weeds and tillers present in the crop as harvest approaches.

Why it matters: Research projects indicate some of the most effective treatments for pre- and post-harvest weed control might actually involve decisions made at seeding.

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The combination of a higher seeding rate and narrower row spacing appears to produce an excellent crop stand, and may eliminate the need for a pre-harvest glyphosate application to “even out” maturity, which also helps reduce the risk of selecting for herbicide-resistant weeds.

In Alberta, Charles Geddes, a research scientist at Agriculture and Agri-Food Canada’s (AAFC) Lethbridge Research Centre (LRC), is leading a study with collaborators at various sites across Western Canada looking at the timing of pre- and post-harvest herbicides to effectively control kochia. Results from one year of trials at Lethbridge show one effective kochia treatment is a pre-harvest herbicide application in winter cereal crops in the early- to mid-August time frame, followed by a post-harvest herbicide if needed. The tentative recommendation — if kochia is an issue, seeding a winter cereal provides a good window with proper timing for a kochia control program.

Saskatchewan projects

Brenzil says the underlying message with pre- or post-harvest weed control is to use the right product for the right purpose. And he points out that unfortunately producers might view a pre-harvest glyphosate treatment as a harvest aid — helping to kill green weeds and green crop to mature, rather than the perennial weed control for which it was initially intended.

However, glyphosate is not a desiccant. And increased use increases the risk of selecting for weeds with herbicide resistance as well as raising public concerns about residues of harvest aids in raw grain and consumer products. Which leads back to the question, “Are there some changes in seeding practices that can reduce the need for pre-harvest herbicide treatments?”

Brenzil cited a study conducted by Chris Willenborg, weed scientist and associate professor at the University of Saskatchewan, that looked at seeding rates with oats.

That work produced similar results to a wheat study Brenzil was involved in with the Indian Head Agricultural Research Foundation (IHARF), as well as AgriARM sites — a network of producer-directed applied research and demonstration organizations across Saskatchewan. Willenborg’s work also showed that higher plant populations can reduce pre-harvest glyphosate residues in oat grain and improve several milling characteristics.

On the left is glyphosate-resistant kochia 10 days after pre-harvest application of glyphosate and saflufenacil. On the right is untreated kochia
photo: Charles Geddes

The AgriARM projects looked at the effect of various seed row widths and seeding rates within those row spacings with spring wheat and durum. The seed row widths at IHARF ranged from 16-, 14-, 12- and 10-inch spacings.

The base-seeding rate, considered the 100 per cent benchmark, was 270 seeds per square meter, or 25 seeds per square foot. So, on the various seed row widths, the wheat seeding rate varied from 75, 100, 150 and 200 per cent of the base rate of 25 seeds per square foot. To create a level playing field for the study, tame oats and mustard were cross-seeded to represent a consistent “weed” component in the wheat plots.

Brenzil says the research project measured several parameters of the wheat crop at the varying seed row widths and seeding rates. The parameters measured across the various treatments were the amount of weed biomass; the wheat head density; crop maturity; weed dockage (which was then converted to the ability of the crop to compete against weeds); as well as crop yield and test weight.

At the IHARF location, the 10-inch row spacing produced the best results compared with the wider row spacings of 12, 14 or 16 inches. For example, the early-season weed biomass was 60 per cent lower on 10-inch spacing compared with 16-inch row spacing. There was higher wheat head density at the narrower row spacing.

Crop maturity improved by one to 1.5 days earlier at the 10-inch row spacing and 150 per cent of the seeding rate. Dockage was reduced by two to three per cent at narrow row spacing and higher seeding rates. The narrower row spacing produced six per cent more yield and the test weight was also higher, compared with crop under wider row spacing. Similar trends were seen at the other AgriARM sites that had only two row spacings.

Brenzil says there wasn’t always a “significant” response to increasing seeding rate to 150 or 200 per cent, but there were penalties for cutting the seeding rate. “The main message is to stick with recommended or higher seeding rates, but resist the urge to cut the seeding rate for the slight savings that provides,” says Brenzil.

“Overall, what this research is showing is with the narrower seed row spacing and sometimes with higher seeding rates it increases crop competition against weeds,” he says. “It reduces tillering, so you get more seed heads with grain kernels, it helps to even out crop maturity and increases yield.

“If we can do this it produces a better crop stand, and hopefully reduces the need for a pre-harvest glyphosate treatment. One less glyphosate treatment reduces the risk of selecting for herbicide-resistant weeds and eases some of the public pressure on glyphosate.”

Pre-harvest control of kochia

At LRC, Charles Geddes says kochia can be a real challenge for producers because it often grows in dense patches that can stay green well into early or mid-October — it takes a killing frost ranging from minus five to eight Celsius for several hours to kill the plant. The plant will die, but by October viable seeds have already been deposited into the soil.

Geddes says the primary objective of a pre-harvest treatment is to eliminate or at least reduce production of viable weed seeds that could be added to the weed seed bank in the soil. The first year of research at Lethbridge suggests growing a winter cereal may provide an effective window for kochia control.

Kochia, an annual weed, begins producing viable seed in early- to mid-August and continues to produce viable seed for several weeks. The effectiveness of pre-harvest treatments in August will vary with timing.

Kochia in early August

Preliminary research indicates if a pre-harvest treatment is applied early — before August 15, for example — it will control the weed and eliminate or reduce production of viable seed. While there may be different herbicide options, Geddes used a tank mix combination of glyphosate and a saflufenacil, such as Heat herbicide, in 2018 and 2019 as the pre-harvest treatment at plot trials at Lethbridge. He noted that treatment was effective in managing both susceptible and glyphosate-resistant kochia.

With that early pre-harvest treatment and early harvest, Geddes says one thing to watch is that those kochia plants, although knocked back in early August, begin growing again after harvest. They will be in reproductive stages and determined to produce viable seed before freeze-up.

“The early pre-harvest treatment will knock the plants back and prevent some seeds from reaching maturity,” he says. “However, with a treatment at that time of year, there is a very good chance kochia plants will regrow after harvest.”

Brenzil says weed seed production will likely be reduced by 50 per cent on regrowth after harvest, but this can still be a considerable amount of seed entering the seedbank.

Geddes recommends assessing the field or kochia patches about 10 days after harvest. If it appears kochia is making a comeback, he recommends a post-harvest treatment to control the regrowth. The post-harvest application will help prevent the kochia from setting seed.

Kochia with later harvest

So that’s with an early pre-harvest treatment and harvest before or by mid-August. If the winter cereal is harvested later in August, kochia plants will have already begun producing viable seed. A pre-harvest treatment can still be effective in killing plants and reducing some viable seed production, but not all.

Similarly, with a spring-seeded cereal crop, for example, a pre-harvest treatment to control kochia in September will help reduce weed seed production, but won’t eliminate it. Geddes estimates the later pre-harvest treatments may reduce weed seed production by up to 25 per cent.

If the pre-harvest treatment is applied later — late-August or at some point in September, for example — the kochia plants have already produced viable seed. There will be minimal regrowth following these later harvest dates, regardless of herbicide application.

“We are looking at a number of management options, but any time we can change the timing of the crop and weed control as with a winter cereal, it helps,” says Geddes. “Anything we can do to limit the production of viable weed seed, it will be beneficial to weed control efforts in future growing seasons.”

He is planning to continue the collaborative research project involving AAFC, University of Saskatchewan and University of Manitoba this growing season with research trials at research centres at Lethbridge, Alta.; Saskatoon and Scott, Sask.; and Winnipeg and Carman, Man.

About the author

Field Editor

Lee Hart

Lee Hart is editor of Cattleman’s Corner based in Calgary.

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