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If you can’t beat ’em, clip ’em

New methods of mechanical weed control show promise, but don’t expect a yield increase the first year

With the high and growing percentage of Prairie cropland infested with herbicide-resistant weeds, researchers are turning their attention back to mechanical control, but that doesn’t necessarily mean tillage. The Alberta Wheat Commission and Saskatchewan Pulse Growers are partnering on a three-year research project at the University of Saskatchewan which is evaluating in-crop weed clipping as a method for weed control. The project has also received funding from the Saskatchewan Ministry of Agriculture through its Agriculture Development Fund.

The objective is to develop a strategy to reduce weed seed production, emptying seed banks to reduce future infestations.

“The main applications for weed clipping are to lower populations of herbicide-resistant weeds that have escaped herbicide application, and to reduce weed populations in organic systems,” says Lena Syrovy, a research assistant at the Agronomy and Weed Ecology Lab at the University of Saskatchewan and lead field researcher on the project.

Weed clipping is either used above or within the crop canopy to reduce weed seed production. The research team is using a machine called the CombCut, manufactured by a European company called Just Common Sense, but Syrovy says there are quite a few other clipping options out there. “I’ve talked to growers who are modifying their swathers to clip weeds above the crop canopy,” she says. “This can work well in lentil especially since it’s a shorter crop.” Bourgault has also recently launched its 50-foot BTT weed clipper.

Clipping cereals

For clipping to have an effect, the weed must be taller than the crop and produce most of its seeds above the crop canopy. Examples of excellent weed targets for clipping include wild mustard and wild oat. Shorter crops are well suited to clipping because a large part of the weed is growing above the canopy. “Crops like lentil are ideal, chickpea and field pea can be good too,” says Syrovy.

The team, which includes grad students Oleksandr Alba, is also trying the weed clipper in wheat, although both the technique and the timing is a little trickier.

“A feature of the clipper we are using is that you can selectively cut the stems of rigid weeds (like Canada thistle and wild mustard) within the canopy of a vegetative cereal crop by sinking the clipper into the cereal canopy and setting it so that the soft cereal leaves will move through unharmed,” says Syrovy. “Once the cereal starts jointing you have to move the clipper above the canopy, otherwise you will start cutting the cereal stems along with the weeds.”

Weeds need to start going reproductive so that they are bolting while the cereal is still vegetative, and that can give a narrow window for clipping. “We are finding that wheat starts booting shortly after wild mustard begins to bolt so the window where we can clip within the canopy is quite small,” says Syrovy. “However, there would be a longer window in a fall cereal, which stays vegetative for longer.”

Clipping can still work above the canopy in taller crops, but only if there is a period where the weeds are taller than the crop.

Clipping does not control weeds such as cleavers and wild buckwheat because they grow entirely within the crop canopy and don’t produce any seeds above the crop. “Weed clipping would reduce seed production in a weed like kochia, but not nearly to the same extent as wild mustard or wild oat, since kochia is bushy and produces seeds within the crop canopy as well as above,” says Syrovy.

Timing and frequency

Timing and frequency of clipping are also important. So far, the findings on wild mustard in lentils suggest it is most effective to clip three times in alternating weeks beginning the first week that most of the mustard bolts. “Clipping multiple times keeps the weed in the flowering stage rather than having it progress to forming pods and seeds,” says Syrovy. Delaying clipping till the mustard has been flowering for two to three weeks is also beneficial, but not as beneficial as clipping multiple times. “When clipping is delayed too late there is the risk that there are already some pods and viable seeds that will fall on the ground as the clipper goes through.”

The researchers are also looking at trying to limit Canada thistle patches with the clipper, but don’t have any data on the results yet. “We expect that the recommendations for Canada thistle management may be different, as Canada thistle patches spread mostly vegetatively rather than by seed,” says Syrovy.

Not a quick fix

Weed clipping will not increase yields in the current season. The goal is to reduce weed populations over time.

“By the time you can start clipping, when the weeds start bolting, the weeds have already been competing with the crop for resources for several weeks, during the time period when the crop is most sensitive to yield loss from weeds,” says Syrovy. “By driving down weed populations over time, a yield benefit may be seen several years in the future rather than immediately.”

Bearing in mind that the main targets for weed clipping are herbicide-resistant weeds, it may prove to be a valuable add-on technique to deal with weeds that aren’t otherwise controlled, rather than a stand-alone method.

Weed wiping

The same group of researchers, led by Steven Shirtliffe at the University of Saskatchewan, is also evaluating other chemical and mechanical weed control methods, including weed wiping — wiping herbicides directly onto weeds without touching the crop.

“The advantage is that you can use this technique to apply non-selective herbicides selectively to the weeds, so this broadens the options of which herbicides can be used while the crop is growing,” says Syrovy. “As with clipping, we are developing this technique primarily to prevent seed production of herbicide-resistant weeds that have survived herbicide application, and lower their populations over time. In some crops such as lentil there are limited herbicide options (mainly Group 2 herbicides), and there are weeds that are resistant to the few options that are available, so weed wiping allows different herbicides to be applied directly to the weeds that would normally harm the crop.”

There are several types of weed wipers out there, and Syrovy and her colleagues are using a rope wick, which is a plastic tube that contains the herbicide solution, and has a rope woven in and out of the tube. “The herbicide soaks into the rope, and then the rope wipes the solution onto the tops of the weeds as you drive along with the wick held above the crop,” she explains.

“This year we are just starting to work with a different type of wiper, which has a pressurized pad where you can control how saturated the pad is with herbicide solution. The advantage of this is that you have greater flexibility with how concentrated the herbicide solution is.”

Weed wiping uses the height difference between the crop and weeds to selectively apply herbicides to weeds, similar to clipping, so taller weeds and/or shorter crops are the best candidates. “Since we are looking at herbicides that have some systemic activity, we expect that there should be greater potential to reduce seed production in weeds such as kochia with wiping than with clipping as well, since the herbicide is taken up by the plant,” says Syrovy.

Some herbicides more suitable

The researchers have tested several herbicides and 2,4-D amine and glyphosate appear to work well for weed wiping. Dicamba, however, even with a low-volatility formulation, causes crop injury and yield loss, so it’s not suitable. “We found the same thing with aminopyralid and 2,4-D ester formulations,” says Syrovy. “We are currently looking at other herbicides as well to expand the active ingredients and modes of action for this purpose.”

Like weed clipping, weed wiping happens at a stage when the weed has already been competing with the crop for a month or two, so although it lowers weed biomass, it doesn’t increase the current crop’s yield, but should reduce weed seed populations over time. The team expects that weed wiping will be useful as part of an integrated weed control strategy.

“Weed wiping won’t have any effect on weeds that are shorter than the crop so those weeds need to be controlled by other means such as increased seeding rates, mechanical weed control and/or herbicides,” says Syrovy. “Because weed wiping won’t prevent crop yield loss in the current year, other techniques should be used to minimize weed competition earlier in crop growth.”

Mechanical techniques

A previous study at the University of Saskatchewan also evaluated three different mechanical weed control methods — rotary hoeing, harrowing and inter-row cultivation — as well as increased seeding rates to see which techniques or combinations of techniques were most effective to reduce weed biomass and increase crop yield.

The most effective practice for reducing weed biomass and increasing yield was combining rotary hoe with inter-row cultivation and increased seeding rate. Rotary hoe and inter-row cultivation reduced weed biomass by 75 to 85 per cent in field pea and lentil, and adding in increased seeding rate further reduced weed biomass by an average of 16 per cent in lentil.

“Rotary hoe application is done earlier, when the weeds are at the white thread to cotyledon stage and are just cracking the ground surface,” says Syrovy. “Then inter-row cultivation is done later to catch the surviving or newly emerged weeds between the crop rows, when the pulse crop is at the four- to 10-node stage.”

The increased seeding rate amounted to 135 targeted plants per square metre in pea, and 260 targeted plants per square metre in lentil. This is about one-and-a-half times the recommended conventional seeding rate for pea, and double the recommended conventional seeding rate for lentil.

Increasing the seeding rate increases crop tolerance to rotary hoe and harrow application, and it also increases yield and weed suppression on its own. “We found that going to the high seeding rates increased yield by 13 per cent in field pea, and 23 per cent in lentil, even if mechanical weed control was used,” says Syrovy. “This means that there is an additional yield increase on top of the best mechanical weed control treatments by also increasing seeding rate.”

Using any mechanical weed control at all was still beneficial, adds Syrovy. “Even the least effective mechanical weed control treatment, which was inter-row cultivation on its own, reduced weed biomass by about 40 per cent in both lentil and field pea,” she says.

Timing and type matter

The best weed candidates for control with a rotary hoe, spring-tine harrow or inter-row cultivator are small-seeded and shallow-seeded annual weeds like lamb’s quarters, redroot pigweed, wild mustard, kochia and green foxtail. “We also can thin the populations of weeds like wild oat and wild buckwheat,” says Syrovy. “The most important thing with the rotary hoe and spring tine harrow especially is to get the timing right and not wait until the weeds are too big and anchored.”

University of Saskatchewan masters student Colleen Redlick also found in 2015 that mechanical control and increased seeding rate work well to control Group 2-resistant wild mustard in lentil.

Short crops such as lentil are ideal for weed clipping because the the potential seeds are above the canopy.
photo: Lena Syrovy

“Combining rotary hoe with a high seeding rate allowed a lower (half) rate of herbicide to be used, while still getting the same weed control and yield as a full herbicide rate,” says Syrovy. “While the rotary hoe wasn’t as effective alone, layering it in with herbicides and increased seeding rate gave an extra mode of action to control weeds, and the economic returns were as good as relying completely on herbicides. The added benefit of this is that the herbicide we used, metribuzin, can cause crop injury at higher doses, and we were able to avoid that by using a lower rate.”

Weed stems after being mechanically trimmed.
photo: Lena Syrovy

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