WEEDING OUT IN LENTIL
by Bruce Barker
Challenging times in weed control call for new approaches in lentils. As poor competitors and with the rise of herbicide-resistant weeds, researchers at the University of Saskatchewan investigated new herbicide options and approaches to help improve weed control in lentils.
“Reducing reliance on herbicides is important for the long-term sustainability of the agricultural sector, including pulse crops. It would also reduce the cost of weed control and lessen its impact on the environment. However, effective weed management in the short term will still require welltimed, efficacious herbicide applications,” says Chris Willenborg, professor at the University of Saskatchewan’s Plant Sciences Department. “Managing weeds using a variety of tactics must be incorporated into research programs to ensure the long-term viability of the pulse crops sector.”
Willenborg conducted five weed control projects from 2016 to 2021 funded by the Saskatch -
ewan Pulse Growers. The goal was to expand the number of active ingredients that might be registered in lentils and to see if existing herbicides could be applied at different times or with different application methods to enhance weed control.
In search of new herbicide options
In the first of three trials investigating alternative modes of action, Willenborg looked at lentil crop safety to a Group 5 active ingredient, amicarbazone, with the trade name Xonerate. A 2016 greenhouse study found potential to use amicarbazone for wild mustard and kochia control with adequate crop safety. Subsequently, field trials were conducted at Agriculture and Agri-Food Canada Scott, Sask., and the U of S research farm at Kernen in 2017 and 2018. The soil at Scott is loam, and the Kernen farm has heavy clay.
PHOTO BY BRUCE BARKER.
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Five increasing rates of amicarbazone were aplied pre- or post-emergent. Postemergent weed control was variable in 2017, so a surfactant was utilized in 2018. CDC Maxim lentil was the variety seeded.
The short story is that over the two years, weed control and crop safety were variable and unacceptable.
“The visual injury and NDVI reduction from amicarbazone application at Kernen, and the reduction in lentil yield at Scott, both indicate that the selectivity of amicarbazone is not sufficient to warrant further research for post-emergent application in lentil,” says Willenborg.
A second study investigating a new mode of action for lentil was conducted in 2018 at Kernen. Pyridate is a Group 6 herbicide sold under the trade name Tough 600EC. It targets broadleaf weeds such as redroot pigweed, lamb’s quarters, kochia and wild mustard and can be applied pre- or post-emergent on corn and chickpea.
Crop tolerance and weed control trials were conducted at Kernen in 2018. Rates applied to CDC Maxim lentil were 0.5x, 1x and 2x of registered rates, applied pre- or postemergent. None of the application rates applied pre-emergent caused unacceptable visual injury to lentil. There was a trend of increasing yield with higher pre-emergent application, but yields were not significantly different from the untreated check.
“The study in 2018 revealed the potential for the use of pyridate as a pre-seed burnoff treatment in lentil,” says Willenborg.
All of the post-emergent pyridate application rates caused unacceptable visual injury to lentil. Yields also trended downward with increasing rates of pyridate. Willenborg says that post-emergent application in CDC Maxim lentil resulted in unacceptable injury and reduced yields at rates that would be required to control broadleaf weeds.
A third trial was conducted looking at lentil tolerance to Group 14 herbicides pyraflufen-ethyl and fluthiacet-methyl when applied post-emergently. Pyraflufen-ethyl is an active ingredient found in several herbicides, including Goldwing, a tank mix with MCPA, which is registered on lentil when applied pre-emergently. Fluthiacet-methyl is registered as Cadet herbicide in the United States for post-emergent application to soybeans.
The active ingredients were applied when CDC Maxim lentil reached the two to
three-leaf stage. Two different formulations of fluthiacet-methyl were applied, while the pyraflufen-ethyl was an EC formulation. Three increasing rates were applied to assess crop tolerance and weed control.
All of the pyraflufen-ethyl rates applied caused unacceptable injury over 10 per cent and severe chlorosis over 30 per cent.
Treatments with fluthiacet-methyl at all rates had acceptable crop injury less than 10 per cent at 28 days after application. There was some unacceptable damage at 14 days after application, but it had subsided by 28 days after application.
Chart title: The effect of herbicide treatment on crop chlorosis at three separate timings after application.
Thresholds for acceptability were established based on the CWSS phytotoxicity rating scale and are indicated by the blue and red lines at 10 and 30% respectively.
Willenborg says that commercially acceptable control greater than 80 per cent of stinkweed and redroot pigweed occurred when higher rates of fluthiacet-methyl were used. Higher rates of EC formulation of Cadet provided good control of common lamb’s quarters, but the other SC formulation only suppressed it.
No herbicide treatment effectively controlled wild buckwheat or wild mustard.
“Overall, this study confirms that lentil cannot tolerate pyraflufen but fluthiacetmethyl can effectively control several weed species at normal and high rates without negatively impacting crop yield,” says Willenborg. He emphasizes, though, that fluthiacet-methyl was included for experimental purposes only, is not registered, and likely won’t be in lentil.
Layering herbicides improved weed control
A study was conducted in 2017, 2018 and 2021 at Kernen to evaluate fall pre- and spring pre-emergent combinations for managing ALS-resistant Group 2 kochia and wild mustard in lentil. Eighteen different herbicide layering treatments were applied. Herbicides applications included fall applications of Focus (carfentrazone + pyroxasulfone), Fierce (pyroxasulfone + flumioxazin), Valtera (flumioxazin) and PL1958, and a fall pyroxasulfone + Heat spring preemerge.
A post-emergent herbicide treatment of metribuzin + fluthiacet at one-half and full
rates followed the pre-emergent applications. For comparison, control treatments with no pre- or post-emergent treatments were also included.
PL1958 is a liquid flumioxazin premix herbicide that is registered and sold in the United States as Panther Pro and is not registered in Canada. Willenborg emphasizes that it was included for experimental comparisons and should not be used in Canada.
Treatments for layering fall and spring PRE- herbicides for managing group 2 resistant kochia and wild mustard in Lentil.
Crop injury was assessed in 2017 and 2018, and at 14 days after treatment, all the treatments, pre- and post-emergent, had phytotoxicity lower than the maximum tolerable level of 10 per cent. This indicated that a layering approach to weed control did not harm the crop.
Wild mustard control in 2017 at 14 days after treatment was almost 100 per cent when a half or full rate of metribuzin + fluthiacet was applied following any of the preemerge layering treatments. Control was substantially lower at less than 60 percent when no pre-emerge treatment was applied. Results in 2018 and 2021 were variable due to drought conditions.
The addition of a post-emergent herbicide on top of pre-emerge herbicides did not result in a significant increase in kochia control. In 2017, when no post-emergent herbicide was applied, all pre-emergent only treatments provided less than 60 per cent control of kochia. In 2018, all herbicide combinations had greater than 85 per cent control except Focus, which had lower efficacy even when a one-half post-emergent application was layered.
Crop yield was significantly greater when a full rate of metribuzin + fluthiacet was applied post-emergently than if a postemergent application was not made.
Overall, application of a pre-emergence herbicide alone was often as effective as applying both pre- and post-emergence herbicides in controlling weeds.
“While the trial showed pre-emergence herbicides were more effective than preplus post-emergence applications, layering is still important to mitigate resistance by exposing weeds to two modes of action. In addition, weeds may emerge after the residual impact of a pre-emerge is gone, and in these cases, the in-crop herbicide will become important,” says Willenborg.
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