Herbicide residue and drift injury

Canola injury can occur from exposure to low soil concentrations of some herbicides, in particular several Group 2 herbicides, plus some Group 4, 5 and 14 herbicides. Herbicide carryover can cause crop injury ranging from minimal to complete crop loss.

Therefore it is vital to show canola injury symptoms associated with herbicide residues, and assist in differentiating between symptoms due to herbicide residue and due to other problems.

Best tips for Herbicide and residue injury management:

  • Be familiar with herbicide labels and follow crop restrictions.
  • Mild symptoms of herbicide injury may be confused with symptoms caused by cold temperatures or nutrient deficiency.

Introduction

Increased acreage of canola and new herbicide options in other rotation crops have created more opportunities for residue injury. Injury problems have typically arisen where normal breakdown of herbicides has been inhibited by factors such as drought, low organic matter and pH, sometimes in conjunction with increased frequency of use of residual herbicides in the rotation.

Table 1. Recropping restrictions for canola

Factors affecting herbicide carryover

Field history

Residual herbicide injury can only occur in fields with a history of Group 2, 4, 5 or 14 herbicide applications. Not all of the Group 2, 4, 5 or 14 herbicides have residual action and only some affect canola. Refer to specific herbicide labels for restrictions on recropping to canola.

Table 1 provide information on the number of cropping seasons before each crop can be grown (“1” means that the crop can be grown the year following application). Products with preseeding restriction in months or days are labeled as such. A blank space means that there are no recommendations given on the product label and a field bioassay is recommended by many product manufacturers to determine if these crops are safe to plant. Note that the minimum re-cropping intervals are listed. Safe intervals may be longer than those listed depending on the use rates, region, province, soil types, environment, time of application and crop variety. Always refer to product label for more information.

Soil characteristics

Interactions between soil factors are complex and may either slow the rate of herbicide breakdown or increase the availability of any remaining herbicide residue to the crop.

Organic matter and soil texture

As organic matter decreases, microbial degradation of the herbicide decreases, increasing potential carryover. Soils with low clay content have decreased adsorption of residual herbicides, thereby increasing potential carryover. Therefore, potential for injury on subsequent canola crops increases as organic matter decreases, and clay content decreases (except where soluble herbicides are leached by lots of rainfall).

pH

Soil pH affects herbicide decomposition and availability to the subsequent canola crop.

Climate

Patches with variation in development
Patches with variation in development

Under drought conditions, microbial and hydrolytic breakdown of herbicides is decreased and adsorption of herbicide to soil particles is increased. The influence of drought on soil may override any previously favourable pH or organic matter conditions.

When microbial decomposition is an important mechanism (e.g. imidazolinones), decomposition is reduced by cool soil temperatures.

Purpling caused by cold temperatures
Purpling caused by cold temperatures

Field scouting

Uneven plant stands
Uneven plant stands

Injury can occur anywhere in the field and may be patchy. Patches that appear bare have normal emergence but there is considerable variation in plant development. Areas of low organic matter, headlands, corners or overspray may have more injury.


Severe patching
Severe patching

A severely affected area (red arrow) and the effect of slope (yellow arrow) are shown.

Carryover can have considerable field variation in acreage affected and severity of plant injury. Uneven plant stands can affect crop maturity and weed pressure.

Symptoms can vary in a small area. All plants in this picture emerged at the same time, however they showed different levels of impact. Plant number one was unaffected, two had mild injury, three had moderate injury and four had severe injury.

Varied symptom severity
Varied symptom severity

Symptoms similar to herbicide carryover injury

Mild symptoms of herbicide injury may be confused with symptoms caused by cold temperatures or nutrient-deficient soil. Cold stress symptoms can arise only after a cold temperature event. Recovery will be rapid as temperatures increase. Nutrient stress symptoms are extremely unlikely to occur at the cotyledon stage as nutrient demands are low. A soil test can determine nutrient availability.

Cold temperature symptoms

Cupping caused by cold temperatures
Cupping caused by cold temperatures

Since the first and second leaves are of normal size, the purpling observed is not herbicide injury. The purpling is a result of anthocyanin production caused by cold temperatures. Purpling may be towards the base, on the leaf margins or may cover entire young leaves of the plant. This symptom will diminish as temperatures increase.

In this case, cupping was caused by cold temperatures and symptoms quickly diminished as temperatures increased.

Cupping was caused by a low level herbicide residue. Variation in herbicide carryover means uninjured (red arrow) and injured (yellow arrow) plants may be found in close proximity. Cold stress generally causes more uniform damage.


Nutrient deficiency symptoms

Cupping caused by herbicide residue
Cupping caused by herbicide residue

These plants were grown under wet conditions from the two-leaf stage and show classic purpling, cupping and chlorosis. Poor aeration to the roots has created nutrient deficiencies.

Nutrient deficiencies caused by wet conditions
Nutrient deficiencies caused by wet conditions
Can result in death in seven to 10 days
Can result in death in seven to 10 days

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