Herbicide Residue & Drift Injury

Table of contents

    Herbicide Residue Injury

    Recropping Restrictions for Residual Herbicides

    Source: Guide to Crop Protection 2012. Saskatchewan.

    Table 1 Soil Factors That Effect the Degradation of Some Residual Herbicides

    Source: Factors Affecting Herbicide Residue: Impact of a Dry Year. Eric Johnson, M.Sc., P.Ag., Agriculture and Agri-Food Canada, Scott Research Farm, Scott, SK. 2006

    Introduction

    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. In recent years, 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.

    This information is intended to:

    1) show canola injury symptoms associated with herbicide residues, and

    2) assist in differentiating between symptoms due to herbicide residue and due to other problems.


    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.

    Figures listed are 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.

    *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. 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 (Table 1).

    Climate

    Drought
    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.

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

    Field Scouting

    Patches with variation in development

    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

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

    Uneven plant stands

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

    Varied symptom severity

    Symptoms can vary in a small area. All plants in this picture emerged at the same time.

    1) Unaffected   2) Mild injury   3) Moderate injury   4) Severe injury

    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

    Purpling 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.

    Cupping caused by cold temperatures

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

    Cupping caused by herbicide residue

    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

    Nutrient deficiencies caused by wet conditions

    These plants were grown under wet conditions from the 2-leaf stage and show classic purpling, cupping and chlorosis. Poor aeration to the roots has created nutrient deficiencies.

    severe sulphur deficiency

    These plants were grown in severely sulphur deficient soil and have typical purpling and leaf cupping symptoms. Plants improved with the addition of magnesium sulphate.

    Symptoms of sulphur deficiency

    These plants also exhibit symptoms of sulphur deficiency. Other symptoms include; interveinal chlorosis, purpling of the leaf margins, and necrosis.

    Cupping of sideshoot leaves

    Sulphur deficiency on older plants may result in purpling and cupping of axillary or sideshoot leaves. Herbicide carryover symptoms seldom appear at later leaf stages.

    The information in this section shows canola injury symptoms associated with Group 2 herbicide residues. Photographs were generated from field plots and bioassays. Field plots were sprayed with reduced rates of Group 2 herbicides prior to planting the crop to simulate herbicide carryover.

    Group 2 Residue Symptoms on Cotyledons

    purpling, chlorosis, size reduction and thickening

    Severe injury symptoms such as purpling or chlorosis, severe size reduction or thickening of cotyledon leaves and petioles often appear at the cotyledon stage. However, cotyledon symptoms must be verified by damage to true leaves and/or meristems to eliminate cold stress as a cause for symptoms. Plants with mild injury to true leaves do not show injury to cotyledons.

    Plants with this severity of cotyledon injury are unlikely to form true leaves, mature and flower.

    A) 2-leaf seedling with normal cotyledonsB) severely affected seedling

    A 2-leaf seedling with normal cotyledons (A) and a severely affected seedling (B) at the same age.

    A) unaffected (left) and severely affected cotyledon (right)

    B) injured plant (right) healthy plant (left)

    An unaffected (left) and severely affected cotyledon (right) (A). The affected plant had purpling on the back of the cotyledons, but no size reduction. Inspection of the true leaves of these same plants (B) reveals symptoms consistent with Group 2 herbicide injury. The injured plant (right) has reduced leaf area, is chlorotic and is more cupped than the check plant (left). While short term cold events produce cotyledon injury, they do not reduce true leaf area.

    Group 2 Residue Symptoms on True Leaves

    Mild Symptoms

    Mild injury involves minimal or no damage to the meristem, but can interfere with early leaf development. Mild injury may not produce sufficient symptoms for definite diagnosis until the 3-leaf stage since symptoms may mimic nutrient deficiency or cold stress. Maturity delay can occur with mild injury.

    A) Mild symptomsB) two weeks later leaf size was normal

    Mild symptoms (A) can include mild chlorosis noticeable on the first and second leaves, reduced leaf area and mild cupping of the third leaf. After two weeks, (B) leaf size was normal and no other symptoms were evident. Plant development was delayed by two leaf stages relative to nearby plants.

    unaffected plant (left) chlorosis and reduced leaf size (right)

    An unaffected plant (left) and one with chlorosis and reduced leaf size (right). The first and second leaves show the initial injury through reduced leaf area and elongated petioles. Petiole elongation is typical of mild injury.

    uninjured (left) and injured (right)

    Differences between uninjured (left) and injured (right) plants could not be seen until the 3-leaf stage.

    Severe Symptoms

    Severe symptoms are characterized by meristematic damage which may result in;

    1) poor recovery and subsequent maturity delay,

    2) yield loss, or

    3) plant death

    Other symptoms may include early and long lasting purpling or chlorosis, cupping, reduction in leaf area and very slow growth. Severe injury is not mimicked by nutrient, insect or disease injury symptoms.

    Severe symptoms

    Severe chlorosis, purpling and cupping (1) and a normal plant (2).

    A) 3-leaf stage unaffected (left) and severely affected (right)

    Severe Symptoms at the 3-leaf stage (A), unaffected (left) and severely affected (right) plants. Affected plants have chlorotic cotyledons with minimal first leaf growth.

    B) after four weeks of growth

    After four weeks of growth (B), there was minimal leaf recovery.

    A) 2-leaf stageB) after three weeks

    Severe Symptoms at the 2-leaf stage (A), severe symptoms include growth reduction, purpling and cupping. After three weeks (B), symptoms were still present.

    C) after six weeks

     After six weeks (C), symptoms have diminished, but leaves are chlorotic and small.

    Classic injury symptoms

    Leaf purpling and cupping on the first (yellow arrow) and second leaf (red arrow) are classic injury symptoms. Although plants can recover, this level of injury slows plant development.

    Leaf chlorosis

    Another classic symptom is leaf chlorosis. Both the first and second leaves have overall chlorosis. Chlorosis was observed through the 4-leaf stage.

    This plant is unlikely to flower

    The first leaf is almost unrecognizable (red arrow). The production of the colourless leaf buds (yellow arrow) indicates a residue effect. This plant is unlikely to flower.

    Severe chlorosis

    An example of severe chlorosis. Leaves can appear almost transparent.

    A) severe chlorosis and significant leaf area reduction

    The combination of severe chlorosis and significant leaf area reduction indicates substantial herbicide carryover (A). Inset: an unaffected plant the same age.

    B) after six weeks

    After six weeks, plants still exhibit symptoms: elongation and thickening of leaf petioles, mottled leaf chlorosis and reduced leaf area (B).

    A) meristem damage

    This level of meristem damage causes premature growth of side branches (A). Inset: an unaffected plant the same age.

    Can result in death in seven to 10 days