Off-target herbicide damage – carryover or sprayer contamination

Canola plants are sensitive to off-target herbicide damage. Off-target herbicide can occur through herbicide carryover or contamination within the sprayer. This article has five sections to identify and avoid this damage.

Group 2 (Imazethapyr) damage in non-Clearfield canola
Group 2 (Imazethapyr) damage in non-Clearfield canola

Herbicide carryover damage

Canola injury from herbicide residue in the soil can occur in fields with a history of Group 2, 4, 5 and 14 herbicide applications. 

Typical herbicide carryover symptoms in canola

  • Group 2: Canola is highly sensitive to Group 2 carryover. Symptoms include purpling, chlorosis (yellowing) and stunting. Reduced leaf size and elongated and/or thickened leaf petioles are signs. In severe cases, these symptoms are combined with very slow plant growth and minimal recovery over time. Severe cases can also cause deformity at the growing point. Symptoms can persist for weeks. Symptoms may not be obvious for a few weeks until rain washes herbicide into the root zone.
  • Group 4: Canola growth may start normal, but as the plant takes up residual Group 4 herbicide, it can cause clear deformity of new leaves and stems. Abnormal growth can include twisted stems, callused stems, cupped deformed leaves and the development of deformed tissues and secondary roots.
  • Group 5: Symptoms can include wilting and yellowing of oldest leaves.
  • Group 14: While Group 14 carfentrazone can be applied pre-seed ahead of canola, Group 14 saflufenacil (Heat) cannot be. Symptoms of Heat damage to canola are yellow and desiccated leaves, and bleached seedlings. Seedlings usually die prior to or shortly after emergence.

Situations that increase carryover risk

  • Low soil pH can extend the breakdown period for some Group 2s.
  • In dry conditions, especially dry summers, products that would normally break down within a year can take longer. Podcast on carryover risk after dry 2021.
  • Growers don’t realize extended recropping intervals for certain products and crops. For example, Roundup Ready or Liberty Link canola cannot be seeded on fields that had Group 2 imazethapyr the previous year.
  • A field history of Group 2 use could mean that Group 2 residues are starting to stack up in a field. Canola Encyclopedia section on factors affecting carryover.

Confirming herbicide carryover damage

Labs can do plant tissue analysis and soil bioassays to herbicide issues.

You can try your own soil bioassay. (1) 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. (2) Gather soil from various parts of the field and put it into a pot. Seed canola or some extra sensitive crop like beets into that soil. Look for damage as plants emerge and grow. This will indicate the presence o herbicide residue, but it won’t tell you when residue entered the soil.

Will damaged plants die? 

In the case of Group 14 carryover, death can occur at the seedling stage. For Groups 2 and 4, plants may start off looking healthy but deteriorate over time. Herbicide carryover damage can be spotty, even showing differences plant to plant, so if enough plants survive, patiently waiting may be the best move. However if large areas are dead or stunted or delayed, reseeding may be required. If the result is fewer than 2 healthy plants per square foot, reseeding may be the best choice in May. In mid June, crop insurance reseeding options may be limited and leaving the crop may be the best choice.

Note that crops reseeded to the same crop and same HT system, could be damaged as well. The safe bet would be to reseed to a crop with tolerance to the herbicide group in question.

Group 2 herbicide carryover damage
Group 2 herbicide carryover damage.

How sprayer contamination occurs

Potential sprayer mis-applications and contamination issues lurk behind every weather delay and every switch of target crop or timing (pre-seed to post emergent). It helps to understand what risk each product and each tank mix presents.

Common sources of contamination

  • Canola is particularly vulnerable to Group 2 contamination. Liberty and its surfactants are very good at removing these residue deposits from tank walls and sprayer plumbing, causing them to mix in with the spray solution. The longer the sprayer sits full waiting for wind or rain to go away, the greater the concentration of scrubbed-free herbicide residue in the tank and booms.
  • Pre-seed burnoff programs often include three or four active ingredients, including Group 2 and Group 14 products ahead of cereal crops. You want to make sure these Group 2 and 14 products are completely cleaned from the sprayer tank, plumbing and nozzles before spraying canola.
  • 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.
  • Oily adjuvants or tank-mix herbicides can create a buildup.
  • Herbicide residue can build up in sumps, filters and boom ends. New sprayers often have features to avoid these catchment areas and to improve clean out. Retrofits, such as boom end clean-out valves, are available.

Identifying a spray contamination issue

Labs can test damaged tissue for herbicide residue. Growers and agronomists can also identify sprayer contamination with some investigation of the evidence:

Misshapen growth

Look for stunted growth on leaves and roots, leathery leaves, twisted stems, clumpy root hairs or otherwise odd root development. These symptoms can also result from herbicide carryover in the field. 

Dead plants on one side of the field only

This is a sign of drift from a neighbouring field.

Patterns in the field

  • Look for damage that starts severe then gets progressively less and finally ends after a few sprayer passes. This indicates something left over in the booms and filters that was sprayed out within the first few passes.
  • If damage is across the whole field but is lower in corners where the boom moved faster and deposited less product or is lower (or non existent) in sprayer misses, this indicates a spray tank contamination that affected the whole load. (Note, with pulse width modulation and GPS guidance, these patterns do not usually occur.)
  • Patterns that follow field topography – hilltops are worse than lower high-moisture areas, for example – are more typical of carryover.

What else could it be?

A scenario: Contamination worse with the second fill

Saskatchewan Ministry of Agriculture extension specialists have heard cases where the first tank of Liberty applied to a Liberty Link causes no damage, but the second tank leads to Group 2 contamination. Mixing is the likely cause.

With a contaminated sprayer, removal of the contamination in any significant amount would not occur until the operator introduces a strong cleansing agent (i.e. Liberty or an adjuvant like Merge). If for the first tank load, the operator adds the cleansing agent at the last minute before spraying, not enough of the contaminant is removed to result in significant injury. But when the operator turns off agitation while the second tank fills, the relatively concentrated cleansing agent has an extended period of contact with the main circulatory system. This extended contact removes a significant amount of contaminant from the plumbing. When the operator resumes agitation, it spreads the contaminant throughout the entire load.

In another case, the operator shuts down for the night and leaves a solution containing a cleansing agent in the tank. When spraying resumes the next morning, all of the contaminant had been liberated from the interior of the sprayer. This leads to crop damage, even though no damage was reported from the application the night before. Subsequent tanks showed no injury, as the cleansing agent had removed the majority of the contaminant form the sprayer.

Sprayer clean out to avoid contamination

Spray immediately after filling and spray until the tank is empty

Do not leave herbicide solution sitting in the tank for an extended period. Do not park an empty sprayer without at least initiating the clean-out process. All herbicides must be crop-safe at twice the label rate to be registered, so going over previously sprayed crop to empty the tank should not damage the crop. While out in the field add 10x the sump’s remnant of clean water, circulate, and spray it out in the field as well. Flush boom ends. Repeat.

Apply direct pressurized spray to all parts of the tank wall

Many small washes are better than one big one. For example, a single 600 gallon wash is as effective as two washes with 70 gallons each, and three with 30 gallons each, assuming a 10 gallon sump remainder. Even better would be continuous clean-water rinsing, when it becomes available.

Empty the sump completely

The only way to remove the remaining herbicide is through dilution by repeatedly adding water and each time draining the sump as much as possible.

Look for solid herbicide residue

Some herbicides may precipitate out of solution and many dry herbicides use clay as a carrier. Nozzle screens and in-line filters can be a significant reservoir for undiluted or undissolved herbicide. Remove all filters and nozzle screens and thoroughly clean these with fresh water. Run clean water through plumbing leading to the screens. Consider having a second set of screens on hand to allow soaking and cleaning time with the first set.

Pump clean water through the boom

Check that all return and agitation lines also receive clean water. Flush all residue. This may require opening and closing various valves several times and repeating the process with new batches of clean water.

Clean all nozzle bodies

When rinsing the boom, rotate through all nozzles in a multi-outlet nozzle body. Remove screens that may have been used with herbicide, even if just for a short while.

Consider tank cleaning additives

Check herbicide product labels to see which cleaner is recommended. If this is not on all labels, call the chem company. Some farmers and sprayer operators keep a list of clean out products specific to each herbicide they use. Specific herbicides benefit from ammonia alone, detergent (surfactant) alone or a combination. All-Clear is one example of a detergent product. Finish and Flush are examples of ammonia-based products.

Safely dispose rinsate

Always spray out the tank in the field. Do not drain the tank while stationary unless you are certain it is free of pesticide and that you are away from sensitive areas and waterways. Another option is to build a biobed for rinsate disposal near the yard. A biobed is a clay-lined hole filled with a mixture of topsoil, peat or compost, and straw. This rich microbial environment can speed the breakdown of pesticides. Biobeds are a new concept in Canada but are considered a best management practice in Europe. More on biobeds, including research from AAFC.

Spring cleaning – the “sprayer spa day”

Chemical residue can last over the winter and cause crop damage, so you want to give the sprayer a complete pre-season cleaning. Here are the steps:

  1. Fill the sprayer tank full of water and run the water through the system. Make sure the only place water comes out is through the nozzles.
  2. While at it, run through a cleaning cycle using a multi-function sprayer cleaner that will raise pH and provide a detergent function. Let the cleaning solution sit in the system for a day or so.
  3. While the solution sits in the system, check and remove all screens and filters, clearing them of debris. Clean them by hand using a scrub brush and detergent.
  4. Remove boom ends and flush them. Consider installing boom end caps to make clean out easier. 
  5. With everything back together, run rinsing water through the sprayer and check nozzle flow rates. Check that they’re within 5% of new and of each other by timing and measuring their output. Replace any nozzles that are faulty. If a lot of them are faulty, you may opt to replace the whole lot with new low-drift nozzles.


Spray drift management – the other off-target issue