Pest Patrol: Controlling weeds in non-GMO soybeans

#PestPatrol with Mike Cowbrough, OMAFRA

Weed control has always been a concern in non-GMO soybeans. Control of broadleaf weeds like lamb’s quarters and common ragweed has been inconsistent, and growers are always looking for ways to improve their results.

General principles for weed control in non-GMO soybeans

1. Budget $80 to $85 per acre for weed control and plan for two trips over the field (e.g. a pre-emergence herbicide followed by a post-emergence herbicide).
2. Avoid planting non-GMO soybeans in fields with significant perennial weed issues (e.g. perennial sow-thistle). No matter what herbicide you use for the perennial weed problem, you will be disappointed in the results.

3. Peak annual weed emergence in Ontario has ranged from two to eight weeks after planting, therefore:
a. Scout fields beginning at 14 days after planting and repeat every four to five days until soybeans have begun to flower.
b. If a soil-applied herbicide has failed, you should know within two to three weeks after application. If no soil-applied herbicide has been put down, post-emergence herbicides typically work best when timed at three weeks after planting as annual weeds are often at the two- to six-leaf stage, when they are most susceptible to herbicides.
4. Many common weeds (pigweed, ragweed, lamb’s quarters) are often resistant to two common herbicide groups used in soybean (e.g Conquest/Pursuit — Group 2 and Sencor — Group 5). When resistant weeds are prominent, soil-applied herbicides with three different modes of action perform better.

Since 2010, the University of Guelph and the Ontario Ministry of Agriculture, Food and Rural Affairs (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 weed spectrum in each of their fields. That’s easy to do if all you have in your fields is pigweed and green foxtail. It’s much more difficult with multiple species, herbicide-resistant biotypes and patches of perennial weeds.

In Table 1 (below), let’s review the performance of the four most consistent herbicide programs that went head to head at controlling the annual weed species that were present in eight field trials over the past four seasons.

Head-to-head comparisons

What about waterhemp?

Dr. Peter Sikkema’s team at the Ridgetown campus (University of Guelph) has done a lot of research evaluating the control of waterhemp. Of the four best performing herbicide programs in Table 2 (below), TriActor, provides the best control of waterhemp. Fierce and Authority Supreme are two herbicide programs that provide good control of waterhemp but have generally not been ranked in the top tier of non-GMO herbicide programs because of their inability to control common ragweed at our trial locations. If common ragweed is not a weed species of concern, then they are both good herbicide programs.

How consistent are herbicide programs at controlling common ragweed and lamb’s quarters?

The short answer is “not very” and that is why looking only at the average level of control can be misleading. Every program that has been tested has failed at some point. If little rainfall occurs in the week after application (15 to 25 millimetres is nice) then this will negatively affect weed control. High population densities of weeds will also reduce the effectiveness of herbicides. Inconsistency of rainfall and heavy weed pressure is a big reason why “two-pass” herbicide programs are best for consistent weed control.

Can I add Eragon LQ to control glyphosate resistant Canada fleabane?

Dr. Sikkema has conducted a lot of research on Canada fleabane control and 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 in your field. 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 eight trials conducted over the past four 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.

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