Impact of Drought on Weed Management

Drought complicates weed control in most crops. Prolonged dry weather has a direct bearing on several aspects of weed management including (1) weed germination, growth, and hardiness, (2) weed and crop interactions, (3) mechanical weed control, and (4) chemical weed control.

Weed germination is inhibited under dry conditions. Thus early season drought may actually reduce weed infestations. Drought that occurs after weed emergence toughens or hardens plants. Weed response to severe drought stress includes leaf cuticle thickening, reduced vegetative growth, and rapid flowering. Drought-stressed weeds are more difficult to eliminate with postemergence weed control efforts.

Weeds compete with crop plants for moisture, nutrients, and light. Many weeds are highly efficient at using available soil water. For example, cocklebur can extract moisture four to five feet around each plant and crabgrass two to three feet around each plant. Both species are capable of drawing moisture from up to four feet deep in the soil. When rainfall is limited, effects of weed competition on crop yield may be even greater than during years of adequate moisture. The combined effects of drought and weed competition limit yield potential considerably.

Tillage is an important means of weed management before and during the cropping season. Seedbed preparation is critical for elimination of established weeds before planting as well as for incorporation of residual herbicides. However, preplant tillage depletes soil moisture, and multiple tillage operations may reduce available soil water to marginal levels, resulting in erratic crop stands. Eliminating preplant tillage trips across the field--whether using conventional or conservation tillage systems--conserves moisture and improves conditions for stand establishment. Reduced tillage, however, generally allows proliferation of perennial weed species.

Chemical weed control can be significantly affected by dry weather. Greatest effects are observed on preemergence and postemergence herbicides. Preemergence herbicides are highly dependent upon rainfall or overhead irrigation for "activation" or movement into the zone of weed seed germination. Sunlight degrades preemergence herbicides on the soil surface, and if rainfall or irrigation does not follow within seven to ten days after application, poor weed control often results. Even for highly persistent herbicides, failure to move the compound into the soil due to the lack of rainfall allows weeds to germinate just after planting. With subsequent rainfall, these persistent compounds usually provide residual weed control of later-germinating weeds.

Shallow incorporation can provide some weatherproofing of preemergence herbicides. In other words, incorporation with implements such as a rotary hoe or shallow-rolling cultivator can substitute partially for rain to activate herbicides. The key is limiting depths of soil disturbance to the upper inch of the seedbed to avoid damage to the planted crop.

To a lesser extent, dry conditions also influence the performance of soil-incorporated herbicides. Uniform herbicide incorporation in excessively dry soils is difficult to achieve. In extreme dry soils, some herbicides are tightly bound to soil particles and become unavailable for weed uptake.

Postemergence herbicides can be dramatically affected by drought. Efficacy of postemergence herbicides, particularly those that are translocated within the target weed, is highly dependent upon active plant growth. Typically, the better the growing conditions, the better the performance of postemergence herbicides. Good soil moisture, moderate temperatures, and high relative humidity are conditions favorable for maximum growth and herbicide activity. If possible, postemergence herbicide applications should be made during periods of favorable conditions.

For some weeds, delaying postemergence control efforts until moisture stress is relieved is not feasible. Many postemergence treatments are effective only on small weeds. In the presence of slight stress, you should opt for higher rates (assuming product labels allow for a range of rates) and possibly alter adjuvant. Where product labels permit, addition of crop oil concentrate rather than nonionic surfactant usually results in greater herbicide activity. Some products also allow for N-based spray additives which tend to improve efficacy of certain products during periods of slight stress.

Some postemergence herbicides have a temporary negative effect on crop growth. Under prolonged drought or heat stress, herbicide injury may reduce crop yields.

Drought may also influence herbicide carryover. Soil microorganisms play a significant role in degradation of many pesticides. Activity of soil microbes is favored by warm, moist conditions. Under dry conditions, microbial degradation slows and herbicide persistence in the soil is extended. For long-residual products which have specific restrictions relating to carryover, persistence is greater for incorporated rather than surface applications.

Prepared by

Steven M. Brown, Extension Weed Scientist, University of Georgia

Douglas A. Worsham, Professor Emeritus, North Carolina State University.

This file is one in a series of electronically available drought information publications produced with support from the U.S. Department of Agriculture, Extension Service, under special project number 93-EFRA-1-0013. The Drought Disaster Recovery Project was a joint effort of the Extension Services in Delaware, Georgia, North Carolina, South Carolina, and Virginia. It is distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. State land-grant institutions, U.S. Department of Agriculture, and local governments cooperating.