Herbicide Injury
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Herbicides can be very effective in reducing weed competition. However, if herbicides are used improperly, or applied accidentally, they can cause injury to crops in several different ways. Symptoms of herbicide injury may look similar or may be very different depending on which kind of herbicide injury is present:
- Direct Application: Most crops have good tolerance for herbicides that are registered for use on that crop. However, certain weather conditions or timing of application may result in crop injury. Suspect herbicide injury from direct application if:
- Herbicide is known to be “hot” on this crop
- Injury is common throughout the field and/or injury is obvious where one tank of spray has been applied
- Weather conditions have been extreme eg. high relative humidity(above 80%), high temperatures (above 25ºC, 77°F), frost, days of cloudy weather
- Surfactants, adjuvants or fertilizer were included in the tank-mix
- Problems have been observed with:
- Brassica: glyphosate, trifluralin
- Peppers: s-metolachlor
- Strawberries: terbacil
- Sweet corn: nicosulfuron
- Tomatoes: thifensulfuron-methyl, metribuzin, trifluralin
- How to avoid injury from Direct Application:
- Choose herbicides safe for the crop
- Follow the directions on the label about crop stage
- Wait for moderate weather conditions
- Use only registered surfactants, adjuvants, or tank mixes according to label directions
- Herbicide Carryover or Residue: Some herbicides may remain at low levels in the soil for several years after application. Subsequent crops may show injury even if levels of remaining herbicide are very low if the subsequent crop is sensitive to that herbicide. Suspect herbicide injury from herbicide carryover if:
- Long residual herbicides were used in preceding crops eg. triazines (Atrazine, Princep, Simadex), Callisto, Converge, Pursuit, Classic, Devrinol, etc.
- Injury is spotty throughout field, or the general vigour of the crop is poor
- Headlands or overlapping spray strips show more injury
- Soil pH is above 7.5 or below 6.0
- Problems have been observed with:
- Brassica: imazethapyr, s-metolachlor
- Cucurbits: imazethapyr, s-metolachlor, atrazine, isoxaflutole
- Peppers: imazethapyr, s-metolachlor
- Strawberries:
- Sweet corn:
- Tomatoes: imazethapyr, atrazine,
- How to avoid injury from herbicide carryover:
- Keep accurate, written records; determine what herbicides have been used in the previous 5 years
- Maintain soil pH in a neutral range (6.0 to 7.5)
- Avoid overlaps when applying herbicides especially around headlands
- Avoid long residual herbicides in rotations with sensitive crops
- Herbicide Drift: Herbicides that drift onto adjacent fields can cause serious injury to sensitive crops, even if very low doses actually reach the field. Suspect injury from herbicide drift where:
- Injury is more severe along field edges, or adjacent to crops that have been sprayed.
- Injury is less as you move into the field, or may show a wavy pattern if wind gusts carried the spray into the field.
- Weeds along field edges show symptoms of herbicide injury.
- Records from date of application show wind direction and speed arriving in the damaged field from the source.
- No technology to reduce drift was used by the applicator.
- Problems have been observed with:
- Brassica: glyphosate,
- Cucurbits: glyphosate,
- Peppers: glyphosate, dicamba, 2,4-D
- Strawberries: glyphosate, imazethapyr
- Sweet corn: glyphosate
- Tomatoes: glyphosate, dicamba, 2,4-D
- How to avoid injury from herbicide drift:
- Spray when winds are blowing away from sensitive crops
- Use drift reducing application technology (eg. shields, air induction nozzles, adjuvants where labeled)
- Plant windbreaks to prevent drift from neighbouring fields
- Choose herbicides that do not volatilize
