If your flax isn’t looking as healthy as it should, you might consider the usual suspects — disease, insects or nutrient deficiency. But you might add another to the list — residual herbicides.
“The main concern is impact on the crop such as decreased emergence and reduced plant stand,” says Michelle Beaith, an agronomist with Sask Flax. “Plants may look abnormal with stunted or cupped leaves, twisted stems or be discoloured. The plants could be either chlorotic or a reddish-purple colour, depending on the chemical.”
Diagnosing the symptoms can be tricky because they are similar to damage from other causes. Beaith advises producers to consult provincial government specialists or to submit a plant tissue sample for laboratory analysis if they aren’t sure.
“Herbicide residues have become a hot topic because of the drought conditions we’ve been experiencing,” Beaith says. “Herbicides are either broken down by soil micro-organisms or chemical reactions in the soil, processes which are influenced by soil type, pH, organic matter content and lack of moisture, which slows breakdown.”
Clark Brenzil, provincial weed control specialist with the Saskatchewan Ministry of Agriculture, explains that the two main processes which degrade herbicides are microbial and hydrolysis. Both require water, warm temperatures and time.
“Microbes will not grow in dry soil and hydrolysis will not proceed without water, although hydrolysis can show some activity under situations where microbial breakdown does not.”
Brenzil says there are two other minor pathways of breakdown — photo-degradation and volatility — but they are largely insignificant with the exception of Group 3 herbicides, hence the reason they are worked into the soil.
Saskatchewan Agriculture posts maps on its website which provide an estimated risk of carryover based on rainfall the previous year. Brenzil says this year’s map dated June 5 to September 3, 2018, was most relevant to this season because last year’s early arrival of snow and cold weather shut down breakdown in the soil very quickly.
“We need both moisture and warm soils for breakdown. As a result, wet or dry conditions in the spring this year have little to no impact on the fate of the herbicide residue that was created last year. It is the in-season rainfall (early June to early September) that is the biggest predictor of injury risk.”
Go beyond the label
Brenzil adds that in some cases, rain the year after the residual herbicide is applied can liberate the herbicide carried over from the soil particles it has been stuck to and free it to be taken up by the crop. He suggests asking the herbicide manufacturer for advice on a safe cropping option.
Beaith says even if the product label says it’s okay to plant flax the following year, it might be wise to extend the re-cropping period by another year if it’s been dry.
“If producers are concerned that extended herbicide carryover might be a problem due to environmental conditions, they should at a minimum follow the best management practices for the crop, because stressed plants are more susceptible to damage by herbicide residue. Making sure the crop receives good fertility, planting into warm, moist soils and bumping up seeding rate can all help improve the outcome.”
Beaith says flax growers should especially look out for Group 2 herbicide residues.
“Last year, people were seeing a lot of damage from that outside of the normal window of what you’d expect because of the drought. Producers need to keep records of the cropping history of their fields and what chemicals they used last year because there is a lot of information to dig through and think about before they make cropping decisions for the season.”
Weed control and fertility
Since herbicide residue injury can reduce plant stand, it makes early-season weed control more difficult. “Good early-season weed control — with a pre-seed or pre-emergent herbicide — is critical because of the non-competitive nature of flax and limited options for in-crop herbicides,” Beaith says.
Dane Froese, oilseeds specialist with Manitoba Agriculture, says the only in-crop products registered for grass control in flax are Group 1 chemistries: sethoxydim (Poast Ultra), clethodim (e.g. Centurion) and quizalofop (e.g. Assure II), and broadleaf products such as clopyralid and MCPA (Curtail M).
“Agriculture and Agri-Food Canada is collecting data on a proposed minor-use label expansion for an in-crop herbicide for flax. Ongoing testing and evaluation is currently happening and it looks promising but still might be a year or two away.”
Beaith says many growers consider flax a low-input crop, but ensuring adequate fertility and increasing seeding rates can make it more competitive against weeds.
“When calculating seeding rate, producers need to consider the germination rate, and that flax typically only has a 50 per cent emergence. Because it varies from farm to farm, producers should go out and do emergence counts to see the results, and then modify the seeding rate for next year’s planting accordingly.”
Flax is fairly frost tolerant, so seeding early, which research has shown results in higher yields, is desirable, Beaith says. It can also help with straw management, which tends to be a problem with flax. “The earlier you seed it, the better the chance of the flax straw being drier and easier to harvest.”
“A lot of the labels for those herbicides with residual activity say that under certain conditions or before replanting, producers should do a laboratory or field bioassay, but it can get complicated,” Beaith says.
A laboratory bioassay (or soil bioassay) requires collecting soil samples from the field with potential herbicide residues as well as from a neighbouring untreated field (or perhaps a corner of the treated field where the sprayer missed), growing flax plants indoors and comparing differences between the crop grown in the treated and untreated soil.
A field assay involves planting strips of the intended crop in both a treated and untreated field and observing any differences in plant growth throughout the season.
It’s often not practical to try growing plants in soil samples at home where light conditions and temperatures may not be the same as in a field situation.
Brenzil says this type of test should be done by a laboratory offering these services since many of the symptoms of improper growth conditions can mimic herbicide residues.
“Improper sampling can be one of the biggest sources of error in these tests,” Brenzil says. “Consult with the lab to get proper sampling procedures.”
On the other hand, test strips are often not practical on the farm due to the size of today’s equipment and the need to grow treated and untreated strips in close proximity for comparison
Beaith says the aim of both tests is to look for any signs of herbicide injury. “Did the crop come out of the ground, is the emergence looking okay, is it growing as expected, is there anything abnormal about its growth like root clubbing, leaf cupping, twisting of stems, that sort of thing, and is the colour normal?”
Beaith says the strips need to be taken to harvest and compared to an untreated area to make sure there is no yield loss.
The agronomists say the issue of herbicides residues is here to stay.
Says Brenzil: “Because of the low rainfall experienced over the last couple of growing seasons, even those herbicides that are generally not considered ‘residual’ have created re-cropping problems.”