Dry weather exacerbates herbicide residue

Update: Please note this story has been corrected and updated since its original publication.

Another dry year on the Prairies will bring a higher risk of crop damage from imidazolinone herbicide carryover.

It takes moisture to generate conditions for herbicide breakdown in the soil, and although soil pH also plays an important role, under drought conditions a higher level of residue remains in the soil for a longer period of time.

Eric Johnson, a weed control researcher at the University of Saskatchewan, told those at the Jan. 15 Agronomy Update that precipitation between the start of the spraying season and Sept.1 is the biggest determinant for imidazolinone carryover. Areas at the highest risk see less than 100 millimeters of precipitation in that period.

The Group 2 herbicide includes, among others, such brand names as Pursuit, Viper (and outside of western Canada) Conquest, Meridian and Arsenal, as well as Solo and Odyssey designed for use on herbicide-resistant crops.

Johnson said herbicide carryover was fairly widespread in 2018, but crop potential was low because of drought, so the impact was not severe.

Farmers can have soil tested for chemical residue, but Johnson said the results aren’t always indicative of whether it is safe to seed a crop. If residue is suspected and rainfall is low in the coming season, he suggested farmers stick to a more tolerant crop to minimize risk.

“With the introduction of herbicide resistant crops, growers’ expectations of crop tolerance, and their willingness to accept some crop injury, has diminished,” said Johnson.

“That’s really different from when I started my career.”

When farmers had fewer options, they knew crops would recover from chemical applications. Now, as research into new herbicides continues and more resistance issues have appeared, “in order for growers to have some new alternatives, they may have to accept some crop injury,” said Johnson.

Most crops, not including those bred for herbicide tolerance, are tolerant but not resistant to injury from herbicide applications. Injury generally results from environmental or abiotic stresses on the plants, and herbicide labels provide advice on application in drought, cold, acidic soils and other variables.

Crops have to expend energy to metabolize herbicide. If environmental stress is also a factor, the crop has to use energy to respond to those conditions as well. The hope is that such stresses will be temporary and possibly reduce growth but not affect yield.

“That’s the situation in a lot of cases, but in some cases you certainly can have permanent stress and a yield reduction.”

Johnson said he was surprised to hear of imazamox carryover issues last year on wheat in some regions, as well as sulfentrazone (Authority) carryover symptoms on barley, which is usually very tolerant. Editor’s note:  While imazamox, Solo, is an imidazolinone herbicide, it generally has a much lower risk of carryover than others in this family.

He also saw clopyralid (Curtail M, Prestige) carryover in pulse crops in some areas. During a dry summer, the chemical binds to clay particles in the soil and remains there at seeding. If a heavy rain displaces the herbicide into the soil solution, it becomes active and crop injury can occur.

“It’s really difficult to research all the environments, and if we took into account all the situations where you might get carryover, you’d end up maybe not registering many herbicides at all,” said Johnson.

In most cases microbial degradation helps with herbicide breakdown so anything that promotes that is helpful to the process. In terms of soil properties, a higher level of organic matter, with attendant moisture-holding ability, means better conditions for herbicide breakdown.

Agriculture Canada researcher Charles Geddes, who studies weed ecology and cropping systems, has undertaken a study on glyphosate and aminomethylphosphonic acid (AMPA) residue in soils. AMPA is the primary metabolite of glyphosate in soil.

Reporting on a study designed by retired Agriculture Canada researcher Bob Blackshaw, Geddes said results showed low risk of yield impact from glyphosate and AMPA residue in soil.

“I’m confident to say that we’re not likely to see an impact on yield, but we can’t say that it wouldn’t happen,” said Geddes.

Studies showed impact only at much higher rates of glyphosate than are typically applied in Western Canada. However, he added that a recent prairie-wide weed survey with 600 respondents showed an average glyphosate use of 400 to 500 grams per acre, a figure that has tripled in the past decade.

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