Herbicide and Nonherbicide Injury Symptoms on Spring Wheat and Barley

The susceptibility or tolerance of different plants to a herbicide is called herbicide selectivity. Many plants can deactivate or metabolize a herbicide before plant injury occurs. Wheat and barley tolerate growth regulator herbicides, such as 2,4-D and MCPA, because they can metabolize these herbicides faster than broadleaf plants. The genetics of a plant also affect a plant's tolerance to a herbicide. Some cultivars of wheat are tolerant while others are susceptible to the herbicide difenzoquat (Avenge). This difference is due to specific genes within the plants.

Many plant-related factors determine herbicide selectivity. Plant age and height differences between crops and weeds can be used successfully to kill some weeds. Other factors include differences in herbicide retention, penetration, translocation, and metabolism. It is also important to remember that selectivity depends upon herbicide dosage. A herbicide applied at a high dosage may kill all the weeds, but also increases the risk of crop injury. In contrast, a low dosage is safer for the crop but may not kill any weeds.

Situations may occur in which a crop may be injured by a herbicide to which it is normally tolerant. This often occurs because environmental stresses such as hot or cold temperatures, high relative humidity, or hail decrease a plant's natural ability to reduce uptake or deactivate a herbicide. Postemergence tribenuron + thifensulfuron (Harmony Extra) injury to wheat or barley under cold and wet weather conditions is a good example of environmentally induced herbicide injury. An excessive application of a herbicide, due to misapplication, can also injure a tolerant crop by overwhelming the crop's herbicide metabolism and deactivation systems.

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