Managing Herbicide Resistance on Sandy Soils

When it comes to managing herbicide resistance, growers do have tools that can help them gain the upper hand on troublesome weeds. Canada fleabane is now the most widespread glyphosate-resistant (Group 9) weed in Ontario. It’s been confirmed in 30 counties, stretching from Essex county in the southwest, all the way to Glengarry county in the east.

Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) weed specialist Mike Cowbrough fields questions from many growers looking for strategies to fight glyphosate-resistant Canada fleabane. He notes that growers looking to manage the weed on sandy soils need to be aware of crop safety and crop injury considerations when using one of the most effective control options — the pre-emergent chemistry metribuzin, the active in commonly used products such as Sencor® and Boundary LQD.

Consider organic matter and pH

Cowbrough says growers who farm sandy soils need to know the percentage organic matter in their soil to ensure they avoid potential crop injury. He notes that Sencor® is not recommended on soils with less than 2% organic matter; Boundary® LQD, which offers a lower rate of metribuzin, should not be applied on soils with less than 1% organic matter.

Growers working coarser sand soils also need to be mindful of pH levels. “We know pH influences the activity of metribuzin, so at super-low pHs (e.g. <6) it dissipates in the soil quickly, and is not taken up by plants as easily,” says Cowbrough. “That gives you better crop safety, but you also might get poor weed control.” On the other hand, high pH soil can present crop injury risk because metribuzin is then more readily taken up by the plant. He recommends ensuring soil pH sits in the 6.5 to 7.4 range to help optimize crop safety.

Fight resistance and protect yield

Cowbrough also reminds growers who use residual herbicides to fight glyphosate-resistant Canada fleabane that they are actually adding additional yield protection. He notes that residual chemistry will protect yield potential through early-season weed competition if a planned post-planting application cannot be timed properly or is applied late due to poor weather.

“There is definitely benefit to having residual activity on the ground compared to non-residual burndown treatments,” says Cowbrough. He saw evidence of this in trials he conducted on sand soils in Ontario’s Norfolk County in 2018.

Overall, 19 treatments were evaluated including pre-plant treatments on glyphosate-resistant Canada fleabane. He notes that the density of Canada fleabane was low at the time of application and the rosettes were small, but a significant amount of fleabane emerged later, demonstrating the benefits of residual chemistry. The farm cooperator then sprayed the entire trial with a post-emergent application that was generally timed late (at or slightly prior to early flowering).

2018 Norfolk County sand trials

In the research, Cowbrough identified three distinct yield tiers corresponding to different weed control programs:

• Tier 1 — the control plot. This included no pre-plant weed control, and a late post-emergent application. Average yield was 22 bu/ac.
• Tier 2 – pre-plant burndowns that were effective on small glyphosate-resistant Canada fleabane but provided no residual control, and a late post application (e.g. Eragon® LQ + glyphosate + Merge®) = Average yield was 35 bu/ac.
• Tier 3 – pre-plant burndowns with residual control (metribuzin-based products), late post application (e.g. Boundary LQD + Eragon LQ + glyphosate + Merge, as an example). Average yield 46 bu/ac.

“What this data indicates is that the addition of residual chemistry (like metribuzin) improved control of glyphosate-resistant Canada fleabane that was emerged at the time of application,” says Cowbrough. He notes that his findings are consistent with work done by weed scientist Dr. Peter Sikkema and his research team at the University of Guelph Ridgetown campus.

The views expressed in this article are those of the individual and do not necessarily reflect the official position of Syngenta Canada Inc.

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