Soybeans & glyphosate-ready canola don’t mix

Most of those expanding soybean acres we see each year are on ground that had recently grown glyphosate-ready canola, and that presents a big problem.

Most desirable soybean varieties are glyphosate resistant. Seeds from those previous glyphosate-tolerant canola crops can remain viable in the soil for five years, surprising producers with volunteer canola in what they thought should be a profitable glyphosate-tolerant soybean year. Soybeans are already finicky when it comes to competing with weeds, and canola is notoriously stronger than soybeans, especially if it gets a jump on the beans.

It’s a real head head-scratcher, and this combination of factors could be robbing prairie farmers of millions of dollars, according to University of Saskatchewan researcher Chris Willenborg. He’s heading up research projects on different herbicide and cultural methods to find the best way to give the beans a break.

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“We started looking at this situation in 2014 when soybeans started to become more popular in Western Canada, especially in Saskatchewan where growers are already in very tight rotations with canola,” Willenborg said.

“We already knew in advance this was going to be a problem. We knew that most growers would try to grow Roundup Ready soybean on top of Roundup Ready canola land. And it didn’t look like many guys would be willing to rotate canola out of their system, so we were faced with trying to find ways to make the soybeans compete.

“So we took a four-pronged approach. Three of the projects focused on herbicides. The fourth focused on cultural methods, such as seeding rates and seeding dates. We ran experiments in 2014 and 2015, with three sites in Saskatchewan and one in Manitoba. We’ll be publishing the results shortly.”

One of the questions was whether a grower could get by with just a single pre-emergence product or in-crop with a post-emergent or a combination of the two. He said pre-emergent alone won’t do the job because some of that canola pops up later in the season. It was clear that a pre- and post-combination would be required.

The three pre-emergent products were 2,4-D ester; tribenuron, which is Express; and saflufenacil, which is Heat.

The five post-emergent products were bentazon which is Basagran; imazamox and bentazon, which is Viper; cloransulam-methyl, which is FirstRate, although it’s not registered in Saskatchewan; thifensulfuron, which is Pinnacle; and fomesafen, which is Reflex in Manitoba but not registered in Saskatchewan. There was also a glyphosate-only check.

“Treatments containing 2,4-D pre-emergence, thifensulfuron post-emergence or fomesafen post-emergence caused crop injury,” Allyson Mireau, Willenborg’s grad student on the project, wrote in the executive summary from her thesis.

“The remaining treatment combinations provided excellent volunteer canola control and low crop injury with similar soybean yield results.

“The optimal combinations are tribenuron, imazamox and bentazon and saflufenacil, imazamox and bentazon, as they contain the most herbicide groups and therefore will be most effective at delaying herbicide resistance.”

Willenborg said most of the combinations had better than 90 percent control with the range running from suppression to excellent control.

Researchers at the University of Saskatchewan have been looking at a number of different herbicide combinations to combat volunteer canola in soybean fields. Their findings are expected to be published within the next year. | Christian Willenborg photo

He said he also liked those combinations because they’re from groups 2, 6 and 14. Although this will help delay the onset of herbicide resistance, it’s only a matter of time before resistance shows up. This is especially important because there is already a significant amount of Group 2 resistance in weeds such as kochia.

“We ran some economic returns and the best overall net return turned out to be Express plus First Rate, but of course that’s not available in Western Canada,” he said.

“One of the things that surprised us was that the 2,4-D pre-emergence treatment didn’t work. I think that’s because in the years we ran the trials, we had cool wet soils. We probably made the application too early. If we’d been 10 to 14 days later we may have had better response. Even so, we did not get the crop safety we were looking for with 2,4-D.”

Willenborg’s team also looked into cultural methods with seeding rates and seeding dates.

Not surprisingly, higher soybean seeding rates resulted in better control of volunteer canola, but extremely high seeding rates become a cost factor.

“We found an optimal economic rate of about 40 plants per metre squared in years with below average soybean prices,” he said.

“With high prices, you could go up to 50 or 60 plants per metre squared to achieve even better control of the canola.”

Mireau’s summary concludes: “In the cultural control study, soybean was seeded at five different seeding rates targeted at 10, 20, 40, 80 and 160 plants (per sq. metre).

The three seeding dates targeted mid-May, late May and early June. Soybean yield consistently increased with higher seeding rates, while volunteer canola biomass decreased. However, seeding date results were inconsistent across site years. The optimal seeding date range is from May 22 to June 1.”

Willenborg said soybean seed should not go into the ground until soil temperature is 10 C. There’s a strong tendency to rush the seeding operation, he added, but soybean seed does not like cold soil.

“If soybean seed sits in cold soil, it won’t germinate,” he said.

“It’ll just sit there and rot. You’ll lose a lot of plants if the soil isn’t 10 C.”

The study was funded by Sask Pulse and the Saskatchewan government.


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