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Glyphosate-resistant weeds

November 7, 2014 - Ontario has several weed species with herbicide resistance, but the ones with glyphosate resistance (GR) are a special challenge because of glyphosate’s pivotal role in the production systems on many farms. As well, some GR weeds have developed resistance to other herbicides, making the control challenge even tougher. For farmers, managing GR weeds could mean higher weed control costs and, in some cases, a loss of effective herbicide options in specific crops. So, Ontario researchers are evaluating the latest herbicide options as well as non-herbicide alternatives for managing GR weeds.

A quick look at the issue

Currently in Ontario, three weed species have been confirmed as having GR populations – biotypes that can survive and reproduce after exposure to what used to be a lethal dose of this herbicide. They are giant ragweed Ambrosia trifida, Canada fleabane Conyza Canadensis and common ragweed Ambrosia artemisiifolia.

Dr. Peter Sikkema of the University of Guelph-Ridgetown leads an annual survey of GR weeds in Ontario. The survey shows GR weeds are an increasing problem. GR giant ragweed biotypes were first confirmed in 2008 and have now been found in seven counties. GR Canada fleabane, first found in 2010, is now in 12 counties. And GR common ragweed biotypes were first confirmed in Essex County in 2011, but so far none have been found beyond that county. Some GR biotypes of these three weeds are also resistant to FirstRate, a Group 2 herbicide.

Herbicides are grouped are based on their mode of action – the way their active ingredients attack the plant. For example, Group 2 herbicides block the normal function of an enzyme referred to as ALS/AHAS. Group 2 includes a large number of active ingredients in several chemical families, such as the imidazolinones and sulfonylureas. Group 9 includes only one active ingredient – glyphosate. It inhibits the EPSPS enzyme.

When you apply a herbicide to control a weed species, you select for the few individual plants within the population that happen to be able to resist the herbicide’s mode of action. If that resistance is heritable, then those surviving plants can produce offspring with that resistance. If you use herbicides with the same mode of action year after year, then eventually the only weeds remaining in the field will be those that are resistant to that mode of action.

Sikkema’s surveys show the scope of Ontario’s GR weed problem, but they don’t necessarily catch every single GR weed in the province. So it’s very important for growers to scout for possible herbicide-resistant weeds in their fields so they can deal with the problem before it explodes. At the University of Guelph, Dr. François Tardif’s lab does testing to determine if suspect weed samples are indeed herbicide-resistant.

Assessing herbicide options

Sikkema’s research group is evaluating a wide range of herbicide options for each of the three GR weeds, with funding from Grain Farmers of Ontario, CanAdvance, and the herbicide manufacturers.

Sikkema outlines some highlights from his group’s recent studies in corn, soybeans and winter wheat.
For GR giant ragweed control in corn, they are testing preplant and post-emergence herbicide options in tank mixes with Roundup. “In the preplant programs in corn, the most effective treatments are Marksman and Callisto + atrazine,” he says.

“Similarly, for post-emergence herbicides in corn, the ones that contain dicamba have been most effective: Banvel, Distinct and Marksman. Pardner + atrazine has looked interesting as well.”

In winter wheat, most of the herbicides they’ve tested have given good control of GR giant ragweed, including 2,4-D ester, Target, Estaprop, Lontrel and Trophy.

For GR giant ragweed control in soybean, the researchers’ latest studies have involved Roundup Ready 2 Xtend soybean, which has tolerance to both glyphosate and dicamba; it is expected to be available to growers in the near future. Their experiments are showing very good control of GR giant ragweed – and also GR Canada fleabane – with tank mixes of dicamba and glyphosate applied preplant or in-crop.
Sikkema’s group has a large program on GR Canada fleabane control. He notes, “We can get quite good control in corn and wheat, but it’s a huge challenge in soybean.”

In corn, they are testing preplant and post-emergence options tank mixed with Roundup. In the preplant treatments, the dicamba-based herbicides Banvel, Marksman and Battalion gave greater than 90 per cent control. Callisto + atrazine and Integrity provided 80 to 90 per cent control.

Likewise, for post-emergence treatments in corn, the dicamba-based herbicides all gave greater than 90 per cent control. For Pardner + atrazine, control was between 80 and 90 per cent.

For GR Canada fleabane control in winter wheat, Infinity, Lontrel, Target, Dyvel, Banvel and 2,4-D ester were the most effective. However, Sikkema cautions, “Although Banvel, Dyvel and Target are very effective on GR Canada fleabane – and also on GR giant ragweed – I am reluctant to recommend them because I think the margin of crop safety in winter wheat is too narrow; you can incur yield losses due to crop injury.”

In soybean, the researchers have looked at a wide range of herbicides, but are finding it very hard to get consistent control of GR Canada fleabane. For example, in their enhanced burndown experiments, “The tank mixes of Roundup plus Eragon, Integrity, 2,4-D, Liberty, Gramoxone, FirstRate and Classic all have activity, but the range in control can be very wide from field to field,” says Sikkema.

“We’re continuing to do research on this. We’ve done over 20 experiments and haven’t yet found out what the contributing factors are causing this variability in control.”

In their burndown plus residual treatments in soybean, the most consistent control of GR Canada fleabane was with Roundup + Sencor applied at the highest label rate registered in Ontario. “However, at that rate you get crop injury on light-textured soils, you could have sensitive cultivars that contribute to crop injury, and it’s simply too expensive,” notes Sikkema. Tank mixes of Roundup plus FirstRate or Broadstrike RC are also effective, but, in some fields, there are multiple resistant Canada fleabane and these tank mixes will not control those biotypes.

In the post-emergence treatments in soybean, the best results were with Roundup + FirstRate. But Sikkema cautions, “From our work, we know that 12 per cent of the fleabane in Ontario is resistant to both glyphosate and FirstRate. We had from 20 to 90 per cent control with FirstRate, and I think that is a function of whether it’s GR Canada fleabane or if it’s multiple-resistant Canada fleabane.”

He adds, “We have no effective post-emergence options in soybean for multiple-resistant Canada fleabane.”

One of Sikkema’s graduate students, Annemarie Van Wely, has just finished a study of GR common ragweed that included herbicide treatments in soybean.

“In her enhanced burndown experiment, she looked at every possible tank mix that is registered [in Ontario]. She found that Roundup + Eragon was the most effective, but the control was only 72 per cent,” notes Sikkema.

“In her burndown plus residual treatments [using tank mixes with Roundup], Lorox and Sencor gave greater than 80 per cent control. Integrity and Optill provided between 70 and 80 per cent control. However, the Lorox and Sencor treatments were applied at the highest label rate registered in the province. I think farmers will be reluctant to use them at those rates because they are expensive, they can cause injury on light-textured soils, and there could be cultivars sensitive to Sencor.”

In Van Wely’s post-emergence treatments, Roundup + Reflex gave the best results with between 70 and 80 per cent control of GR common ragweed.

Based on these results, it appeared that a two-pass approach would likely be needed for effective GR common ragweed control. So Van Wely applied tank mixes of Roundup with one of Integrity, Lorox or Sencor, prior to soybean emergence, and then applied Roundup + Reflex post-emergence. That combination gave between 85 and 95 per cent control of GR common ragweed.

Exploring non-herbicide options

Non-chemical weed control options can be important for dealing with GR weeds, especially if they have resistance to multiple herbicides.

A number of Ontario studies are underway on such options. For example, Tardif is leading a project with Ontario Ministry of Agriculture and Food and Ministry of Rural Affairs weed specialist Mike Cowbrough to assess crop management approaches to reduce weed seed production in situations where weeds aren’t effectively controlled by a herbicide. “The weeds may be herbicide resistant, or the herbicide may not have lasting power in the soil to stop the weed seeds from germinating, or the weed’s biology may allow it to germinate later on,” explains Cowbrough.

The project, established in 2014, involves four cropping rotations. It is assessing the effects of crop type, seeding rate, fertility and cover crop practices on weed seed production and crop yields in corn and soybeans.

Cowbrough says, “We’ve coined the project ‘To Fight the Light’ because our goal with each cropping system is to cover the soil almost 100 per cent of the time so light isn’t touching the soil and stimulating new weed seed germination.”

He suspects it will take several years before the effects of the different treatments on weed seed production become clear. “It is something that really struck me recently when I spoke to three Australian producers [who were dealing with serious herbicide-resistant weed issues]. No matter what non-chemical strategy they used, they all said it took about five to six years before they saw meaningful differences in weed populations because there had been such high weed seed deposition in previous years.”
With such a long lag time until results appear, the researchers want to be sure the practices are cost-neutral. Cowbrough says, “For instance, let’s say I could demonstrate that a 10 per cent increase in your seeding rate in corn would dramatically reduce the amount of weed seeds and that the increased corn seed cost would be paid back in higher yields, but it will take five years to see a weed control benefit. There’s a higher probability of people adopting that practice than if you have to spend $35 per acre on a practice and you won’t see a weed control benefit from it for five years.”

According to Cowbrough, Dr. Eric Page with Agriculture and Agri-Food Canada (AAFC) at Harrow will be joining the To Fight the Light project in the future. Page is working on similar research on giant ragweed, including GR giant ragweed.

In another project, Page and AAFC’s Dr. Rob Nurse are testing a prototype of the Harrington Seed Destructor that AAFC has purchased. This machine was developed by Ray Harrington, a crop producer in Australia, as a tool for managing weeds with resistance to several herbicides (see ‘Australia’s fight against herbicide resistance’ on page 32). The Destructor crushes weed seeds that come out of the combine with the chaff. The researchers want to see how well it works in Ontario’s crops with Ontario’s weed spectrum.

Reducing the risk of resistance

Sikkema emphasizes that preventing glyphosate resistance is quite straightforward. “We have to apply glyphosate less frequently. It is as simple as that.”

To reduce reliance on glyphosate, he recommends adding diversity into your weed management programs. “Include multiple crops in your rotation, including some that are not Roundup Ready, so that you won’t be applying glyphosate post-emergence in the crop. You still could put it on preplant as your burndown or as a pre-harvest or post-harvest application,” says Sikkema.

“If you are growing Roundup Ready crops, then I suggest a two-pass weed control system. In the first pass apply a broad-spectrum, soil-applied herbicide that has activity on the primary grass and broadleaf weeds on your farm. Then watch the field and decide whether or not you need a post-emergence application of glyphosate.”
Sikkema also suggests using tillage to control weeds at strategic points in your crop production system, although it’s important to avoid excessive tillage which can lead to soil erosion, breakdown of soil structure and other problems. And he recommends, “Use excellent crop husbandry, whether that’s using well adapted cultivars or hybrids that emerge quickly and outcompete the weeds, using the right seeding rates, possibly reducing soybean row widths so the canopy closes earlier, controlling insects and diseases, using proper fertility – anything you can do to make the crop more competitive with weeds.”

Cowbrough has two pieces of advice for reducing the risk of herbicide resistance. First, focus on the most prominent weed species on your farm. “The bigger the weed’s population, the more likely it is that you will select for a resistant biotype if you apply that selection pressure. And if it’s a species that is more prone to being selected for herbicide resistance and its seed is mobile, then the risk will be higher.”

And second, use more than one strategy to attack those prominent species. “That could mean the addition of two or more herbicide modes of action, using some cover crops, doing some tillage, and so on.”

November 6, 2014  By Carolyn King

For GR Canada fleabane control in soybean Ontario has several weed species with herbicide resistance


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