Submitted to: Extension Service Bulletins
Publication Type: Other
Publication Acceptance Date: 3/1/2005
Publication Date: 4/1/2005
Citation: Parker, R., Boydston, R.A. 2005. Weed management and herbicide performance during drought conditions. Washington State University Website Extension Bulletin. 2 pp. Interpretive Summary: Weed control is important in all cropping systems, but may be even more critical in dry years. Low soil moisture increases the competition for water between the weed and the crop. Drought tolerant weeds, such as kochia, Russian thistle, and field bindweed develop extensive root systems early and take advantage of limited water making them more competitive and difficult to control then when soil moisture is adequate. Performance of herbicides is compromised under dry conditions due to poor uptake and movement of translocated herbicides within water stressed plants and due to tight binding of soil applied herbicides to soil particles, making the herbicide unavailable. Use of proper herbicide rates and adjuvants is critical to good herbicide performance in dry conditions. Cultivation during dry years may increase the loss of critical soil moisture resulting in poor crop stands and increasing the susceptibility to erosion. However, mechanical activation of soil applied herbicides with a rotary hoe, tine harrow, or other implement may be required in dry years. The importance of weed management and how to best manage weeds with limited moisture is discussed.
Technical Abstract: Low soil moisture increases the competition for water between weeds and the crop making weed control even more important when water is scarce. Drought tolerant weeds develop extensive root systems early and take advantage of limited water making them more competitive and difficult to control then when soil moisture is adequate. Weed control is often more difficult under dry conditions. Cultivation for weed control may be detrimental due to the further loss of critical soil moisture resulting in poor crop stands and increasing the susceptibility to erosion. Efficacy of postemergence or foliage applied herbicides, particularly those that are translocated within the plant, is reduced when plants are water stressed. Drought stressed plants produce thicker leaf cuticles resulting in less herbicide absorption into the plant. Drought stress affects many plant processes, resulting in less translocation of herbicide within the plant to the site of action. Generally, more herbicide is needed to control drought stressed weeds than nonstressed weeds. Herbicide formulations can affect efficacy on drought stressed weeds. Ester formulations of 2,4-D generally perform better on stressed weeds than amine formulations. Effectiveness of postemergence grass herbicides is reduced when grass weeds are water stressed. The addition of the proper adjuvant can improve control of drought stressed weeds by improving herbicide coverage, retention, and uptake. Performance of contact foliage applied herbicides are usually not affected as much by drought stress as performance of translocated herbicides. Activation of soil applied herbicides is often best achieved by rainfall or irrigation. However, mechanical activation with a rotary hoe, tine harrow, or other implement may be required in dry years. Herbicides that are mechanically incorporated are not as dependent on overhead moisture since the shallow tillage for incorporation will place the herbicide in the zone of weed seed germination. Herbicides applied to dry soils may be so tightly bound that they become unavailable for uptake by weeds and less likely to be leached, whereas herbicides applied to moist soil are available for uptake by plants and for leaching. Dry soil conditions can also prevent chemical and biological processes that degrade herbicides making them more likely to persist and injure subsequent crops.