Weed management is challenging in pulse crops because of limited herbicide options, poor competitiveness and now the emergence of herbicide-resistant weeds.
So to help develop integrated weed management recommendations for fababeans, Amanda Fedorchuk, a masters student at the University of Saskatchewan, has developed an experiment that uses residual herbicides with cultural and mechanical weed control.
She said the study is designed to help farmers reduce the selection pressure for the development of herbicide-resistant weeds.
“We’re hoping to achieve this through the effective use of integrated weed management strategies and residual herbicides that we are spraying pre-plant,” Fedorchuk said during a presentation at the University of Saskatchewan’s annual Soils and Crops workshop that was held online this year.
Integrated weed management uses multiple control tactics.
Small plot field experiments were conducted at the Kernen Crop Research Farm in Saskatoon to determine the optimal combination of pre-emergent herbicides with different levels of integrated weed management strategies meant to decrease the number of in-crop weeds.
There were 12 treatments containing two factors in the study that were replicated four times.
Trials were held in 2017, 2018 and twice in 2019.
The first treatment that was used as the control was a pre-planting glyphosate application that did not include a residual herbicide.
Further treatments included “a glyphosate and saflufenacil treatment has low residual activity and it’s been shown to have residual activity up to two weeks. Glyphosate and flumioxazin pre-planting has medium levels of residual activity up to four weeks. And then glyphosate with pyroxasulfone and sulfentrazone has been shown to have residual activity up to six weeks,” Fedorchuk said.
The level of integration was separated into three treatments; low, medium, and high.
Low integration used the recommended seeding rate of 45 seeds per meter squared, late seeding and 24-inch row spacing and no mechanical weeding.
Medium integration also used the seeding rate of 45 seeds per meter squared, moderately early seeding date, 12-inch row spacing and no mechanical weeding.
The high integration treatments had a very early seeding date, increased seeding rate of 90 seeds per meter squared, 12-inch row spacing and used a single pass with a rotary hoe for mechanical weeding.
Natural weed populations were used and no other herbicide applications were applied until the fall when Diquat was used to help with crop dry down.
CDC Snowdrop was used in 2017 and 2018 while Snowbird was used for both sites in 2019 because they were the most common varieties grown at the time in Saskatchewan.
Weed biomass and fababean yield in the plots were measured.
For weed biomass, there wasn’t any herbicide effect but the level of integration was considered significant in three out of four site years.
In 2018 there was, “over 70 percent decrease in that weed biomass from the low integration treatment to the medium and high, and in this year there was no difference between medium and high levels of integration,” Fedorchuk said.
In 2019, the same pattern appeared at the Saskatoon site with a 70 percent decrease in weed biomass from low to medium integration, but a further 60 percent decrease in weed biomass occurred between the high-integration treatment from the medium.
“We’re seeing a 95 percent decrease in weed biomass between that low and the high treatment,” Fedorchuk said.
She said at the Kernen site in 2019, there was a 95 percent decrease in weed biomass from the low to the medium treatment, and then a further 65 percent decrease from medium to high.
“I just also want to point out here to keep in mind that there were no post emergent herbicide applications in this experiment. So our weed biomass is getting pretty close to zero even without spraying anything in crop,” Fedorchuk said.
When it came to fababean yield, differences between the herbicide treatments were not observed and only the level of integration was significant.
“But this time, the same results were shown in all of our site years,” Fedorchuk said.
“Our fababean yield increased 60 percent from our low integration treatment to that medium and high, and there was no statistical increase between these two medium and high treatments.”
Even through direct effects of the different herbicide treatments were not observed, she said they likely did have an effect on the weed population.
“The benefit isn’t coming directly from using them. The benefits are coming from diversifying that herbicide portfolio, and therefore reducing our selection pressure just by using multiple modes of action in a season,” Fedorchuk said.
She said it may seem suspicious that a yield increase wasn’t observed between the medium and high integration treatments, because the seeding rate had been doubled in the high treatment.
However, she said if the law of constant yield is considered it makes sense.
“What happens is these plants start to compete for light, nutrients and water, and then they start to run out of these things and then self-thinning can happen. So your yield will eventually start to decrease with an increased seeding rate,” Fedorchuk said.