Herbicide and Nonherbicide Injury Symptoms on Spring Wheat and Barley

Weed control with soil-applied herbicides depends on three conditions: 1. the herbicide must be in the soil solution and in contact with an actively growing plant (dormant weed seeds are not affected), 2. the herbicide must be available to the plant in adequate quantities, and 3. sufficient soil moisture must be present. Many weeds have small seeds that germinate primarily within the top two inches of the soil. For soil-applied herbicides to be effective, they must be positioned from the soil surface to two inches deep. Herbicide placement within the germinating weed zone can be accomplished by mechanical incorporation or rainfall. Soil-applied herbicides are absorbed when roots or shoots of germinating weed seedlings come in contact with the herbicide in the soil solution.

Plant roots continue to take up herbicide as long as root hairs remain in contact with the treated soil. As the roots penetrate deeper in the soil and move farther away from the treated soil, the herbicide absorption declines. If root tips of weeds grow out of the herbicide-treated soil, they are likely to survive.

Many soil-applied herbicides are absorbed by the plant shoot as it emerges through the soil. Shoot-absorbed herbicides often injure or kill plants before emergence. Volatile herbicides such as the thiocarbamates (e.g., triallate [Far-Go]) and the dinitroanilines (e.g., trifluralin [Treflan]) can penetrate the shoot as a gas.

The conditions that affect herbicide efficacy can also affect crop injury. For example, trifluralin can severely inhibit root growth of wheat if it is incorporated in the spring so deeply that the herbicide comes in contact with the developing root system. High herbicide application rates can also injure crops. In addition, excess moisture from heavy rainfall can cause herbicides to leach or concentrate in sufficient quantities in the crop germination zone to damage the crop. Physical and environmental factors that enhance rapid crop emergence and reduce the time that a plant is exposed to the treated soil will reduce the potential for crop injury.

Herbicides vary in their ability to translocate within a plant. Some soil-applied herbicides, such as trifluralin (Treflan), are not mobile within the plant. Plant injury from this type of soil-applied herbicide would be confined to the site of uptake. Other herbicides are mobile and move within the plant. For example, soil-applied atrazine is absorbed by plant roots and moves upward in the xylem (water transport system) and becomes concentrated in the leaves. Generally, plant injury symptoms associated with mobile herbicides will be most conspicuous at the location where the herbicides concentrate.

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