Soybeans: Herbicide Injury

Excerpt from Agronomy Guide for Field Crops (Chapter 4)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Introduction
2. Specific Types of Herbicide Injury
   • Triazine Herbicides
   • Chloroacetamide Herbicides
   • Auxinic Herbicides
   • Diphenylether (PPO) Herbicides
   • Bentazon
   • Acetolactase Synthase (ALS, Group 2)
3. Updates on Soybeans: Herbicide Injury
4. Related links...

Introduction

All herbicides have the potential to cause crop injury. Some soil-applied (preplant or premergent) herbicides can be splashed up on the leaves to cause injury, or the herbicide can be taken up through the roots. With postemergent herbicide products, most injuries occur at the time of application or shortly after. Spray from the target crop can also cause injuries to a neighbouring, non-target crop susceptible to the herbicide applied. Spray overlaps are also common places to find crop injury from herbicides.

Stage of growth, crop stress, weather conditions, variety, tank-mix partners and adjuvants will all affect the amount and severity of crop injury. When the target crop is under stress, it is sometimes not able to metabolize the herbicide fast enough to avoid injury. The type of herbicide (mode of action) is a major factor in how herbicides affect crops. In general, while contact herbicides injury may look worse, systemic herbicides will cause longer-lasting injuries, which may be more severe. Always read the label for information on how to reduce herbicide injury.

See also the OMAFRA Factsheet, Herbicide Mode of Action Categories, to order Order No. 00-061, available through the OMAFRA Web site at www.omafra.gov.on.ca/english/crops/index.html

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Specific Types of Herbicide Injury

| Triazine Herbicides | Chloroacetamide Herbicides | Auxinic Herbicides |
| Diphenylether (PPO) Herbicides | Bentazon | Acetolactase Synthase (ALS, Group 2) |

Refer to the OMAFRA Factsheet Herbicide Mode of Action Categories, or order Order No. 00-061, or OMAFRA Publication 75, Guide to Weed Control, for herbicide trade names.

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Triazine Herbicides (Examples are atrazine, metribuzin, simazine and other products containing these herbicides.)

Plate 80 shows triazine injury on the lower leaves of soybeans. Leaves will show interveinal chlorosis and a yellowing of leaf margins and tips followed by browning. Lower leaves are affected most. There may also be reduced root growth. Triazine injury usually results from carry-over in the soil of residues from applications of herbicides containing atrazine in previous years.

Plate 80. Triazine injury. Injury on lower leaves of soybeans showing yellowing and dead tissue between the veins.

Metribuzin can injury soybeans through root uptake or more commonly by splash-up on the leaves. Root growth may be reduced. Heavy rains can splash herbicide from the soil surface on the leaf, causing injury. Fields with high soil pH or low organic matter soils will show more injuries. In some cases, older leaves may die and fall off while newer leaves are not injured. Yield effects are minimal unless the plants are killed. Some varieties are sensitive to metribuzin.

Plate 81. Triazine injury on soybeans through root uptake or more commonly by splash-up on the leaves.

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Chloroacetamide Herbicides (e.g., dimethenamid, flufenacet, s-metolachlor)

Plate 82 illustrates chloroacetamide injury on soybeans. Symptoms are puckered leaves, and shortened midrib ("drawstring"); leaves sometimes appear to be heart-shaped.

Plate 82. Chloroacetamide injury on soybeans showing shortened midrib causing "heart-shaped" leaves.

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Auxinic Herbicides (Examples are dicamba and other products containing these herbicides or similar herbicides.)

Auxinic (hormone-type) herbicides such as dicamba can drift onto soybeans and cause leaf and stem malformations, such as leaf-cupping, twisting, parallel venation or leaf strapping. Low-level drift that occurs before flowering can cause significant visual injury symptoms but in many cases, this does not have a significant impact on yield. Injury can come from herbicide drift from adjacent fields or from contaminated spray tanks that were not properly cleaned out.

Plate 83. Auxinic herbicides such as dicamba or 2, 4-D cause leaf puckering and twisting of stems.

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Diphenylether (PPO) Herbicides (e.g., aciflurofen, fomesafen)

PPO herbicides cause contact burning injury on the leaves, but newer leaves will not be affected. New leaves that are not fully expanded at the time of spraying may appear twisted or crinkled without having the necrotic spotting. Injury symptoms may occur within a few hours after application. The canopy greens up again as new leaves emerge.

Plate 84. Diphenylether herbicides cause contact burning injury on the leaves, but newer leaves will not be affected.

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Bentazon

Bentazon injury on soybeans is shown in Plate 85. Leaves may show yellowing, bronzing, speckling or burn. Broxoynil is a herbicide with the same mode of action that is used in corn. Surfactants enhance the injury.

Plate 85. Bentazone injury on soybeans.

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Acetolactase Synthase (ALS, Group 2)

Plate 86 shows ALS injury - reddish veins on the underside of the soybean leaf (red-to-purple veins on the lower veins on the lower side of the leaf). Some ALS herbicides may also cause yellow leaf margin, crinkled leaves, yellowing near growing point, stunting and death of the growing point.

Plate 86. ALS herbicide injury showing reddish veins on underside of soybean leaf.

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Updates on Soybeans: Herbicide Injury

No updates available at this time.

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Related links...

• Soybeans
• Weeds
• Publication 75, Guide to Weed Control
• Field Crop Protection Guide - Publication 812

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