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Unexpected carryover
Seeding non-Clearfield canola the year after an application of Odyssey or Pursuit is off-label because that isn’t enough time for the imazethapyr to break down in the soil. Carryover damage can be expected in this situation.
Unexpected carryover can occur when dry conditions in the year of application slow the expected pace of breakdown of herbicides. See the risk map. Carryover risk can increase even for residual products that are not usually a risk to the following crops. For Group 2 IMI herbicides, for example, lack of moisture and low soil pH stack the odds in favour of carryover. (Note, for some other Group 2 subgroups, high soil pH can increase carryover risk.)
Why do symptoms show up now and not earlier in the season? Symptoms for Group 2 damage can be slow to show up, and may not be obvious for a few weeks until rain washes herbicide into the root zone. Damage like stunted growth and curled leaves could also be confused with weather effects, including drought.
In cases of unexpected herbicide carryover damage, look back at herbicide notes for the field. What products were used? Do these products have any special notes about re-cropping restrictions? A field history of Group 2 use could mean that Group 2 residues are starting to stack up in a field. What was the rainfall situation for that field versus others in the area?
How to prove herbicide carryover damage: Plant tissue analysis and a soil bioassay could help, in addition to herbicide history for the field, weather since application of the product of concern, knowledge of soil texture, pH, organic matter, and observing symptoms. Another option is to plant a few Clearfield canola seeds in the same patch, along with some seeds from the same original lot, to see if they grow without symptoms.
Unexpected sprayer residues
Residues in the sprayer tank can affect just a few acres (if residues quickly wash out of boom ends and nozzles, for example) or large parts of a field (if residues get circulated throughout a whole tank of product). Residue damage is a risk any time a sprayer is left full while waiting for an opportunity to spray, or when sprayers are not effectively cleaned between crops and between products.
Gummy residues can occur with low-dose wettable granules. If clogged in nozzle screens, these residues can release damaging amounts of off-target herbicide for acres. Check nozzles and screens with every change in product or target crop. Another factor is oily adjuvants or tank-mix herbicide partners that allow layering of the low-dose herbicide within an oily buildup. The buildup results from infrequent clean-out over days of spraying.
Group 2 products can hide in and cling to sprayer tanks and plumbing. Liberty herbicide is very good at cleaning these residues off tank walls and hoses, bringing them into the spray solution.
One risk scenario for unexpected residues: A farmer sprays Group 2 herbicide on cereals or pulses then fills the sprayer with Liberty at the end of the day to be ready bright and early to spray canola the next morning. This is a high risk scenario for Group 2 damage to canola.
Another risk scenario: The only Group 2 used was in a pre-seed burnoff ahead of cereals. Some residues remain in the sprayer, even after various other applications, until a few weeks later when Liberty is loaded for in-crop application. Those residues come free, damaging the canola crop.
Further reading:
Sprayers101: Cleaning your sprayer (including the places where residues can hide)
Tips for spraying in the wind
Sprayer tank clean-out: Tips