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. 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.