Herbicide and Nonherbicide Injury Symptoms on Spring Wheat and Barley

Effective postemergence herbicide application is dependent on an adequate amount of the herbicide making contact with aerial plant parts. Thorough spray coverage of the plants can be achieved through proper selection of the spray tips, spray pressure, and spray volume. Flat fan nozzles are generally recommended for postemergence applications. Always read herbicide labels carefully for application details.

For most postemergence applications, the chemistry of a compound determines how it interacts with the biological and physical systems of the plant. This interaction often determines the rate and amount of the herbicide uptake. Factors such as plant size and age, water stress, air temperature, relative humidity, and herbicide additives also influence the rate and amount of herbicide uptake. Additives such as crop oil concentrates, surfactants, or liquid fertilizer solutions (e.g., 28% UAN) can enhance the performance of the herbicide. These same factors can also increase the risk of crop injury. Herbicide uptake may decrease under hot dry conditions or when applied to older, larger weeds under water stress. Environmental conditions at application can affect the rate and amount of uptake. For example, wild oat control with difenzoquat (Avenge) increases with rising temperature after treatment. These differences in herbicide uptake help to explain how weed control effectiveness and potential for crop injury can fluctuate. Rapid herbicide absorption by the plant helps ensure that the compound enters the plant and minimizes potential wash off from rain.

As with soil-applied herbicides, postemergence herbicides differ in their ability to move within a plant. Nonmobile postemergence herbicides are called contact herbicides. It is essential that contact herbicides thoroughly cover the plant. Contact herbicides include the bipyridylium, diphenylether, benzothiadiazole, and nitrile families. Other herbicides are mobile and can move from where they enter the plant to their site of chemical activity. Growth regulator herbicides such as 2,4-D, dicamba (Banvel), or clopyralid (Stinger), move up and down within the plant's transport system (phloem) to the growing points of the shoots and roots. With systemic herbicides it is most likely that crop injury will be prominent where the herbicides concentrate, in the roots and growing points.

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