Seed & Chemical
Manage cleavers in canola
By Donna Fleury
Field surveys across Western Canada are showing an increase in the presence of cleavers. Generally, the vast majority of populations have been identified in Saskatchewan; however in the 2010 weed survey in Alberta, cleavers ranked as the number three weed in canola and number one weed in pulses. Cleavers are difficult to control in many crops and can cause downgrading and reduced crop quality.
With funding from the Saskatchewan Canola Development Commission, Western Grains Research Foundation, Government of Saskatchewan Ag Development Fund, and multiple industry partners, researchers at the University of Saskatchewan (U of S) have just concluded a two-year study to help growers in managing cleavers in canola. They characterized the emergence and genetic characteristics of cleavers populations in Western Canada, which were believed to be two species: Galium aparine and G. spurium. Researchers also assessed the response of cleavers to potential new herbicides (in canola) such as quinclorac and clomazone, as well as their response to common canola herbicides such as glufosinate-ammonium and glyphosate to determine whether differences among populations existed.
In 2012 and 2013, field experiments were conducted at different locations in Saskatchewan, including Scott, Saskatoon and Rosthern (2014 only). Eight herbicide treatments were used in this experiment, including the herbicide standard for each canola system used alone and with the addition of quinclorac (tank-mix) and/or clomazone (preseed). At all sites, canola varieties (L130, 73-75 and 45H73), resistant to their respective herbicide system, were seeded into cereal stubble. Greenhouse dose-response experiments were also conducted to assess whether variability existed between populations in their response to herbicides.
“One of the most important findings from our research for management of cleavers in canola is that a portion of cleavers are emerging in both spring and fall, and that emergence timing of each of these fall and spring cohorts varied between years,” explains Christian Willenborg, assistant professor, department of plant sciences at the University of Saskatchewan. “Historically, cleavers were considered an obligate winter annual and generally emerged in the fall. However, our research confirms what growers and agronomists have been seeing: cleavers have largely responded to our cropping systems and, along with a shifting climate, are now emerging in both fall and spring. These differences suggest growers will need to pay close attention to emergence timing of this weed to ensure the small window for control is not missed.”
Another important outcome was that researchers successfully developed a molecular marker that could differentiate and characterize the cleavers species in the field. It has long been believed cleavers populations in fields across Western Canada are a mixture of both species. However, molecular analyses showed all sampled populations were in fact identified as G. spurium, or false cleavers. Although no G. aparine was found in the collected samples, it does not mean there is none present in fields across the Prairies. Willenborg adds that for growers, knowing populations are primarily one species, G. spurium, which is also the species that possesses resistance to Group 2 herbicides, is important because resistance will spread more quickly if all plants within the population are the same species.
One of the key recommendations resulting from the study is that growers will have to have a well-planned strategy for managing cleavers at different times in the rotation. “We found that in canola, spring emerging cleavers seem to emerge right after the crop is planted, and are often too large for some in-crop herbicide products, particularly those with a narrower application window such as the two-whorl or two- to four-whorl stage,” Willenborg says. “In some cases, such as the last couple of years, growers were unable to make a fall application because of either inclement weather or the timing of harvest. This can cause problems the following spring, as cleavers plants may be very large at this point and therefore difficult to control with pre-emergence herbicide applications. As well, some in-crop application timings have not been ideal because of the higher than usual moisture conditions.”
On the positive side, the results of the field study conducted over two years and at three sites showed that clomazone and quinclorac significantly reduced cleavers biomass and seed contamination and improved cleavers control in canola crops. The results consistently showed that applying clomazone prior to seeding (pre-plant) canola followed by an in-crop application of a herbicide standard provided acceptable control, usually greater than 85 to 90 per cent. The results also showed the tankmix of quinclorac with a herbicide standard applied in-crop brought control to at least 85 or 90 per cent, without a preseed clomozone application.
In fields where cleavers populations are a big problem, all three products could be used: preseed clomozone, and the in-crop tankmix of quinclorac and herbicide standard. “In the study, using all three products provided the best results, often with an additional five per cent increase over the other combinations,” Willenborg explains. “However, growers have to assess whether or not the use of all three herbicides will pay for itself.” The results of the greenhouse dose-response experiments appeared to suggest cleavers populations responded similarly to glufosinate-ammonium, imazapyr+imazamox, and quinclorac, despite being from different locations in Western Canada. However, further testing and statistical analysis is needed to confirm this.
Both herbicides, although new to Western Canada for cleavers control, are older technologies. Clomozone, which is not yet registered in Western Canada as of December 2015, is a Group 13 product that has been registered in Eastern Canada under the product name Command for several years. Quinclorac, a Group 4 product, is now registered, but growers are cautioned they cannot use quinclorac in canola until the industry addresses MRL (maximum residue limits) considerations in some markets. Registration and acceptance of these herbicides will significantly improve cleavers control in Western Canada.
To manage cleavers in canola, growers should start controlling cleavers in the year before growing canola in rotation, such as in cereals where good control options are available. There are some existing Group 4 products, along with two new products registered in 2015, Pixxaro and Paradigm. The active ingredient in these products, halauxifen-methyl branded as Arylex, also has activity on kochia and can be tank-mixed with a range of other products for control of broadleaf and grassy weeds.
“The most important recommendation is in addition to in-crop herbicide control strategies, the addition of a fall application is key to managing cleavers over the long-term,” Willenborg says. “Growers need to plan to control cleavers that emerge in the fall prior to growing canola. Glyphosate can be used for control, but growers need to recognize the higher risk of developing herbicide resistance in cleavers, and therefore tank-mixes and well-planned herbicide rotations are key. Avoiding tillage is also recommended as tillage can create situations that encourage germination and recruitment of cleavers seedlings.”
Although the addition of these new herbicide options will provide good options for canola growers when available, over the long term they are not a silver bullet. These herbicides, like all herbicide tools, will need to be carefully managed to reduce the risk of herbicide resistance. Cleavers resistance to Group 2 herbicides has already developed across Alberta and Saskatchewan, and cleavers rank second among weeds likely to develop glyphosate resistance in the Black soil zone.
“Our study showed that spring applied clomazone reduced the size and stage of cleavers found in-crop, and it is known that lower population numbers reduce the risk of developing herbicide resistance,” Willenborg adds. “Both clomazone and quinclorac, which can be tank-mixed with any of the in-crop herbicides, also provide alternative modes of action for control and by tank-mixing with herbicide standards, should delay the evolution of resistance to glyphosate and glufosinate.
“However, in the mid 1990s, some cleavers populations in Alberta were identified as resistant to quinclorac, which is Group 4, as well as some Group 2 products, so there are some multiple resistant populations that already exist in Western Canada. Although these resistant populations haven’t spread very much, the scale that quinclorac could be used in canola in the future could mean an increased selection pressure for this herbicide and this could result in the spread of these populations.”
Willenborg is building on this work with some new projects to address some other questions that impact management related to emergence timing and base temperatures. “A better understanding of both emergence timing of spring and fall cleavers, along with corresponding base temperatures, will help us to develop better models to more accurately predict emergence timing for growers,” he says.
“Another area we are looking at is as more growers move to straight cutting and the use of desiccants and harvest aids in harvesting canola, we need to have a better understanding of fall emergence timing of cleavers. Growers need to know what effect a pre-harvest application of desiccants or harvest aids might have on cleavers control and for reducing seed production. Moreover, a significant proportion of cleavers populations emerge in the fall and, therefore, management in the fall is key to the sustainable long-term management of cleavers in Western Canada.”