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Herbicide carryover may be high risk

If it was dry after last year’s application, there may be soil-residual herbicides

Crop selection for the 2019 growing season could prove challenging for growers in some parts of the Prairies, as a lack of rainfall means soil-residual herbicides could impact crop establishment. Moisture after application is critical for herbicide breakdown. In areas where rainfall was patchy at best, growers need to be conservative when selecting crops to follow potentially residual herbicides used in the 2017 and 2018 growing seasons.

Manitoba grain farmers Ron and his brother Robert Krahn have never had a herbicide carryover issue on their 5,300-acre farm and they want to keep it that way. That’s because they plan their rotations two years in advance. The Krahns grow wheat, canola, peas, soybeans, sunflowers and some corn in Rivers, Man.

The best way to avoid potential carryover issues, said Krahn, is to plan rotations one to two years in advance. “We don’t change our acres by crop much from year to year,” he said. “This reduces risk by not being ‘all in’ on a certain crop, both from a marketing perspective and also yield as well.”

By area, for example, Krahn never plants more than 6.5 per cent soybeans, and to keep disease at bay, sunflowers and peas are added to the rotation no more than once every eight to 10 years. In fact, Krahn plans every single acre in advance using a spreadsheet and knows what will go where up to 2020 and beyond.

“This is key, in my opinion, to managing chemical and carryover potential,” he said. “If we can avoid particular chemicals in a field in 2019 because we know what crop will be there in 2020, we have avoided potential problems.”

Consider canola

The main crops that concern Krahn are sunflowers, peas and soybeans. Canola can also be a concern when it comes to herbicide carryover, though, said Canola Council of Canada agronomy specialist Ian Epp. In canola, the herbicides of most concern tend to be the Group 2s, but sometimes Group 14s cause problems as well, Epp said.

“Under drier conditions they tend to persist longer,” he said. “Farmers who haven’t been very good at keeping records forget what they sprayed last year or potentially two years ago. Then under really dry conditions go to plant their canola and have poor emergence.”

“It’s generally earlier season that we’re seeing symptoms,” he said.

To avoid carryover issues, Epp suggests changing crops or changing varieties to compensate.

“Having that kind of plan and doing that kind of record checking in winter saves a lot of headaches in spring,” he concluded.

Saskatchewan Agriculture weed control specialist Clark Brenzil also expects carryover issues in 2019, which is why he’s been giving talks on the subject at ag conferences this winter.

“Rainfall in 2018 was very spotty, and a large localized rainfall event can influence a large region of the map beyond that point,” he said. “Crops affected will depend on the specific herbicide.”

“Any herbicide that has a ‘field bioassay’ recommendation on its label could be affected, indicating that it has some type of residual activity,” Brenzil continued.

Problems to watch for

Not all crops have been tested for re-crop safety under normal conditions either, he said. Herbicides with known re-cropping concerns that showed up in previous years where in-season rainfall was limited include the Group 2 herbicides for use in pulse crops on canola and flax in areas that saw less than four inches of precipitation. Re-cropping concerns are also possible in some sensitive cereals, like barley, canary seed and durum, which are at risk in areas that saw less than three inches of rain.

Clopyralid-containing herbicides, such as Lontrel, Curtail M and others, could present issues in legume crops and legume forages in areas where less than six inches of rain fell.

Pyrasulfotole-containing herbicides like Infinity, Velocity, Tundra and others have the potential to cause bleaching in pulse crops and canola, and flucarbazone in products such as Everest and Sierra or Inferno Duo on pulse crops.

Saflufenacil in Heat with late fallow uses on canola could cause problems in areas that saw less than two inches of precipitation.

“Unfortunately, producers cannot control the weather, so essentially they are in a response mode with this challenge,” said Brenzil. “We are talking about herbicides that under decent rainfall years do not pose a problem for re-cropping or would at least have a high confidence level of what crops were safe to replant following their use right from the outset.”

Concerns, he added, are not going to disappear overnight, even in a year of decent moisture. “If a herbicide has a re-cropping restriction of more than one year, the producer may have to begin the countdown on that once in-season rainfall reaches six inches,” he said. “All of the dry years before do not count as breakdown years.”

To help make more informed decisions, Brenzil suggests keeping rainfall records on each field. Growers can also send soil samples for chemical analysis.

“These can be in the range of $200 to $400 per sample,” said Brenzil, adding that producers may want to do at least three samples per field to get a good picture. Laboratory bioassay give a better answer on whether crops of interest will tolerate soil residues since they measure what is in the soil, but not what is biologically active.

With all of the lab tests, sampling can be the biggest source of error, said Brenzil. “It is important when sampling to take soil from the top two inches from the surface,” he said. “Sampling at too great a depth can result in the dilution of the herbicide and produce a false negative, giving the impression that all is well to plant that crop.”

Manitoba farmers may have concerns beyond those of Saskatchewan and Alberta growers. According to Tammy Jones, Manitoba Agriculture weed specialist, there are some soil residual herbicides that are specific to the province, mainly because growers there produce more soybeans and corn. In particular, she mentioned fomesafen, which is a Group 14, as well as atrazine (Group 5).

With herbicide carryover, precipitation is the driving force or your initial alert as to whether you may have a problem with a soil residual, she said.

“Total accumulation is important, but so is how that accumulation happens and when it happens,” she said. “For instance, with fomesafen if you apply it and you have a long dry spell, then there’s more chance of having carryover the next year because that breakdown didn’t start immediately.”

Brenzil and Jones both recommend looking at local rainfall records and cross-referencing them with field history records.

About the author

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Melanie Epp is a freelance farm writer.

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