Pest Patrol: Best practices for weed control in identity-preserved “IP” soybean – Part 2

Since 2010, the University of Guelph and OMAFRA have compared weed control programs in non-GMO soybeans. The key to picking the right herbicide program is simple in theory, but difficult in practice. A farmer needs to match the herbicide program with the spectrum of weed species in each field, which is easy if all they have is pigweed and green foxtail, but much more difficult with multiple species, herbicide-resistant biotypes and patches of perennial weeds. Let’s review the performance of soil-applied herbicides over the past two seasons at three different locations.

Head-to-head comparisons in 2020 and 2021

Trial locations: Elora, Woodstock and Winchester, Ontario.

Key notes from 2020 and 2021

  • The “top tier” herbicide programs have three modes of action.
  • The addition of metribuzin (Sencor) improves weed control, especially grassy weed control.

Lower yield with Conquest + Prowl H2O

Conquest + Prowl H2O, in general, provides very good control in soybeans. The Prowl label is very clear that it must be applied before planting. Application after planting can result in significant crop injury. At one trial location in 2021, Prowl H2O was accidentally applied after planting, resulting in significant stem brittleness and lodging (Figure 1 at top), which resulted in a 25 per cent yield reduction. When applied according to the label, soybean yields associated with a Conquest LQ + Prowl H2O application have been in the top tier.

The amount of metribuzin applied in Boundary LQD + Canopy Pro seems too high

The combination of Canopy Pro + Boundary LQD includes a rate of metribuzin (785 gal./ha) that is equivalent to applying Sencor 75DF at 420 gal./ac. This limits the use of this program to medium-textured soils with over 4 per cent organic matter or fine-textured (clay) soils, otherwise significant crop injury can occur.

* Standard deviation gives an indication in the variability of control. A low number indicates low variability while higher numbers indicate more variability in the control achieved with the herbicide across all trials.

What about water hemp?

dr Peter Sikkema’s team at the Ridgetown campus has done a lot of research evaluating waterhemp control. Of the herbicide programs in Table 1, Fierce provides the best control, while Triactor, Authority Supreme and Boundary LQD all provide over 80 per cent control.

How consistent are herbicide programs at controlling common ragweed and lamb’s quarters? Not very. That’s why looking only at the average level of control can be misleading. Every program tested has failed at some point. If little rainfall occurs in the week after application (15-25 mm is nice), this will reduce weed control. High weed population densities will also reduce herbicide effectiveness. Inconsisting rainfall and heavy weed pressure are big reasons why “two-pass” herbicide programs are best for consistent weed control.

Can Eragon LQ control glyphosate-resistant fleabane?

dr Sikkema has conducted a lot of research on Canada fleabane control. He has found the tank mixture of Eragon LQ + metribuzin has worked very well. Since Canopy Pro, Conquest LQ, TriActor and Commenza all contain metribuzin, it makes sense to tank mix Eragon LQ if glyphosate-resistant Canada fleabane is present. However, farmers have raised concerns about tank mixing Eragon LQ, a Group 14 mode of action herbicide, with TriActor, which also contains a Group 14 active ingredient. Does the risk of crop injury and yield loss increase if you add in two herbicides with the same mode of action? Based on several trials over multiple seasons, soybean yield and visible crop injury did not differ when Eragon LQ + Merge was added to TriActor.

Acknowledgments: These comparative trials could not be done without the help and support of Dr. François Tardif, Peter Smith, Holly Byker and Gilles Quesnel.

Have a question you want answered? Hashtag #PestPatrol on Twitter to @cowbrough or email Mike at [email protected].

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