By Rui Liu, Washington State University, Irrigated Agriculture Research and Extension Center, January 4, 2024.
Rachel Bomberger, WSU Pesticide Publication Review.
Pre-emergence herbicides
Pre-emergence herbicides are herbicides that take effect prior to weed seed germination/emergence. They inhibit the growth of the roots and/or shoots of weed seedlings. They don’t kill the weed seeds directly but can prevent the weed seedlings from establishing in the field.
Pre-emergence herbicides are also called “residual herbicides” or “soil-active herbicides.” They stay in the soil for a period of time (usually 8 to 12 weeks) and are gradually degraded by the soil microbes. Pre-emergence herbicides need rainfall or irrigation for activation. The rates of pre-emergence herbicides need to be adjusted based on the soil type indicated by the label. If the soil type is sandy or gravelly, lower rates are needed. Higher rates are required if the soil type is clay or has more organic matter. Pre-emergence herbicides are often applied in the fall and/or early spring for vineyards and orchards. Pre-emergence herbicides can control both grass and broadleaf weeds.
Post-emergence herbicides
Post-emergence herbicides are herbicides that work after weed seeds emerge from the soil. Post-emergence herbicides can be grouped into non-selective and selective herbicides based on the weeds they control. Non-selective herbicides—for example, glyphosate, paraquat, glufosinate, etc.—can damage or kill any plants that they come into contact with. Selective herbicides injure some plants, but not others. For example, the grass herbicide clethodim kills grass, but does not damage broadleaf plants. Therefore, they can be used selectively for grass control in broadleaf crops.
Post-emergence herbicides usually work best when weed seedlings are small (less than four inches tall) and actively growing. The optimum timing of applying post-emergence herbicides is when the majority of weed seedlings have emerged from the soil and the biggest seedling is not too big to control. Refer to the herbicide label for the maximum seedling size and adjust the rates accordingly. Weather conditions may affect weed seedling growth and impact herbicide efficacy. Some post-emergence herbicides require surfactants (e.g., crop oil, non-ionic surfactant, etc.) and/or nitrogen (e.g., urea ammonium nitrate [UAN], ammonium sulfate [AMS]) to improve weed control.
Herbicide resistance
Herbicides are classified into different groups based on their Mode of Action (MOA). MOA is the way in which herbicide affects a plant and controls the susceptible plant. Site of action (SOA) is the specific biochemical site that is affected by the herbicide. The herbicide disrupts the process and interferes with plant growth. Both MOA and SOA are often used to describe the different groups of herbicides. Please visit Weed Science of America for a list of herbicide groups and their MOA. The group numbers are displayed on the first page of herbicide labels. You can find it next time you read the herbicide label.
Herbicide resistance is the consequence of repeated use of the same herbicide SOA. Resistant biotypes exist at very low frequencies within a population, carry the resistance trait to the next generations, and expand over time.
To help slow down the development of herbicide resistance, it’s recommended to: 1) Use multiple MOA, and tank-mixing herbicides with different SOAs; 2) Always follow the label and apply herbicides at the labeled weed growth stage and rates; 3) Include non-chemical control methods in the program, such as cultivation, mowing, crop rotation, etc.; 4) Prevent weed seed production.
Contact
Rui Liu
Washington State University – IAREC, Prosser, WA
Rui.liu@wsu.edu
509-786-9354
Additional information
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Use pesticides with care. Apply them only to plants, animals, or sites listed on the labels. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock.
YOU ARE REQUIRED BY LAW TO FOLLOW THE LABEL. It is a legal document. Always read the label before using any pesticide. You, the grower, are responsible for safe pesticide use. Trade (brand) names are provided for your reference only. No discrimination is intended, and other pesticides with the same active ingredient may be suitable. No endorsement is implied.
