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The challenge of early weeds in soybeans

A two-pass weed control strategy makes a lot of economic sense

In an ideal world, it might not matter when a field of soybeans is sprayed, or whether it’s a soil-applied or post-emergence herbicide that’s used. But this isn’t an ideal world and application timing can be very important.

So is the crop.

In Country Guide’s spring Corn Guide, Dr. Peter Sikkema discussed the impacts of early-season weed control in corn, complete with yield loss and financial loss calculations. In this edition, he covers the same early-season scenario in soybeans, with some interesting differences between the two crops.

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For starters, compared to corn, soybeans are a more tolerant and resilient crop in terms of early weed interference and its impacts. It’s a concept that is somewhat counterintuitive when considering the stature of the two plants; corn grows to a height of three to four metres and would be expected to outcompete neighbouring weeds while soybeans grow to just one to 1.5 metres, and in some cases their growth lag behind quicker-growing weed species.

“Yield loss due to weed interference in soybean begins later and isn’t as dramatic as it is in corn,” says Sikkema, a professor of field crop weed management at the University of Guelph’s Ridgetown Campus. “Many Ontario farmers have gone to two-pass weed control in corn with an excellent soil-applied, pre-emergence herbicide, followed by a post herbicide. However, there hasn’t been the same adoption in soybeans and the farmers are right — soybean is just more tolerant to early weed interference than corn.”

Sikkema has a summary of 10 years of data, based on research conducted at the Ridgetown Campus and at the Huron Research Station near Exeter, Ontario. The findings indicate yield loss in corn due to weed interference is at 49 per cent, but in soybean, it’s 36 per cent. He then states that the studies have largely been carried out within the same experimental areas with a defined crop rotation, and by and large, the weed spectrum is the same, the weed density is the same and the soil type is the same.

But if a farmer is going to invest in a soil-applied, pre-emerge herbicide, they still get a bigger return on investment in corn than in soybeans, making that economic justification that much easier for corn. However Sikkema still recommends two-pass weed management programs in soybeans because in many cases, growers will maximize net returns.

Soybean in the middle

In the overall picture of weed interference and the impact of weeds on yield, soybeans are squarely in the middle, at least among those crops that Sikkema studies.

“We’ve been keeping this data in spring cereals and winter wheat, and just to appreciate how competitive some of these crops are, our yield loss in spring cereals is an average of 12 per cent,” says Sikkema. “In winter wheat, it’s three, and the crop that’s most sensitive to weed interference is dry beans, which is 55 per cent.”

Where the challenges for any crop become more substantial is in the critical weed-free period. As Sikkema notes, no one knows when that weed-free period occurs in each individual field. In some situations, a grower could rely on a total post program, hit the weed-free period perfectly and see no yield loss at all.

“In contrast to that, if you’re the unlucky guy and you get a couple of rain events, and it prevents you from applying that post-emergence herbicide when it should have been applied, our data says that in the most competitive environments, you can lose up to two bu./ac. per day,” he says.

At $12 per bushel, a grower could lose $24 per acre by spraying on Tuesday morning instead of Monday morning.

Although that’s a worst-case scenario, Sikkema says data from the Ridgetown Campus and Huron Research Station indicate an average soybean loss of one bu./ac. per day after the beginning of the critical weed-free period. That’s still a hit, especially when viewed through a week’s delay in spraying — a loss of $84 per acre, or $84,000 over 1,000 acres of soybeans.

“The potential yield losses due to delayed post-emergence herbicide application are tremendous,” says Sikkema. “That’s because, one, we don’t know when that critical weed-free period begins in each individual field, and two, we can’t control the weather, which can impact the timing of post-emergence herbicides. So I like a planned two-pass weed control in soybean of a soil-applied herbicide followed by a post-emergence herbicide.”

Plant type matters

Another factor that influences the benefits of the early-season weed management picture is a physiological consideration that actually makes control of the current glyphosate-resistant weeds easier in corn than in soybean.

It’s that corn is a monocotyledonous plant while soybeans are dicotyledons. Sikkema notes that all four of the weed species that are resistant to glyphosate — Canada fleabane, giant ragweed, common ragweed and waterhemp — are dicots, just like a soybean. And it is much easier to remove a dicot weed from a monocot crop like corn than it is to remove the dicot weed from a dicot crop such as soybeans.

“Herbicide selectivity is frequently due to differential metabolism,” says Sikkema. “For some herbicides a monocot such as corn can metabolize the herbicide and it is uninjured, while the dicot weeds cannot metabolize the herbicide and they are controlled. It’s harder to find herbicides that kill broadleaf weeds and not injure a dicot crop.”

The four weed species that are resistant to glyphosate in Eastern Canada — including waterhemp — are dicots, like soybeans, making their removal more challenging. photo: Dr. Peter Sikkema, University of Guelph (RC)

For the same reasons, it’s harder to remove a grassy weed from corn than from a broadleaf crop. In very general terms, the greater the similarity between crop species and weed species, the harder it is to get that selectivity. The most widespread and most difficult-to-control glyphosate-resistant weed species in Ontario is Canada fleabane in soybeans. However, because all four glyphosate-resistant weed species are broadleaf weeds, there are more effective herbicide options in corn.

As another interesting thought on monocot versus dicot glyphosate-resistant weeds — Sikkema points out that the situation in Ontario with respect to glyphosate-resistant weed species is an aberration. Globally, glyphosate resistance is in a virtual tie between grass and broadleaf weeds. In Ontario, they’re all broadleaf weed species.

New tool, new approach

Sikkema considers Roundup Ready Xtend soybean technology a valuable tool for weed management in soybeans in Ontario. He says that dicamba is an excellent option for the control of glyphosate-resistant giant ragweed, common ragweed and Canada fleabane. In contrast, the fit for this technology on glyphosate-resistant waterhemp is unclear, mainly due to dicamba’s relatively short residual capacity in soil, along with the full-season emergence pattern of waterhemp.

“The other big concern I have is that you get the best control of glyphosate-resistant waterhemp with dicamba (Engenia, XtendiMax, FeXapan or Roundup Xtend) when applied post-emergence, but that’s also when sensitive crops are up in adjacent fields,” cautions Sikkema. “You have the greatest potential for injury to those sensitive crops.”

Sikkema lauds the work of Mon­santo and BASF in 2017 in Ontario and their positioning dicamba as a pre-plant and pre-emergence herbicide in Roundup Ready Xtend soybeans. As a result, there wasn’t the same frequency of injury to sensitive crops in adjacent fields as there was in some areas of the U.S.

That’s primarily because most farmers applied it either pre-plant or pre-emerge, i.e. before those sensitive crops in adjacent fields had emerged, unlike the U.S. growers who sprayed later.

Sikkema believes the agricultural product companies should continue to position dicamba as a pre-plant or pre-emergence application in Roundup Ready Xtend soybeans, but that it’s strictly for crop safety, not efficacy, and for reducing risk in sensitive crops in adjacent fields.


Sikkema believes an undocumented yield loss that occurs in many fields across the province is due to reliance on post-emergence weed control. In soybeans, although losses do not begin as early, they can be substantial nonetheless.

“Farmers who use a total post-emergence weed control program in soybeans and get 55 bu./ac., they could be perfectly happy with that — it’s above the provincial average,” says Sikkema. “But what farmers will never know is that if they had used a two-pass program, would they have had, let’s say, 58 bu./ac.? That three bushels per acre is an undocumented yield loss that occurs in many fields across Ontario every year, and nobody can tell you how much it is, or what the value is, simply because we don’t track that.”

This article was originally published in the February 2018 issue of the Soybean Guide.

About the author

CG Production Editor

Ralph Pearce



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