The ongoing drought of 2007 and 2008 has brought on many challenges to those working in agriculture and agribusiness such as limited water for crops, livestock wells running dry, and limited hay and feed production. However, another challenge faced by farmers also is further complicated by drought — weed control.

Prolonged dry weather has a direct bearing on several aspects of weed management, including weed germination, growth and hardiness, weed and crop interactions and mechanical and chemical weed control.

Weed germination is inhibited under dry conditions, thus early season drought actually may reduce weed infestations. After growth begins, weed response to severe drought stress includes leaf cuticle thickening, reduced vegetative growth and rapid flowering. Most importantly, herbicide control of drought-stressed weeds is much more difficult.

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 4 to 5 feet around each plant and crabgrass 2 to 3 feet around each plant. Both species are capable of drawing moisture from up to four feet deep in the soil.

When rainfall is limited, the 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 crop yields considerably.

Chemical weed control can be affected significantly by dry weather. Greatest effects are observed on pre-emergence and post-emergence herbicides. Pre-emergence herbicides are highly dependent upon rainfall or overhead irrigation for "activation" or movement into the soil where the weed seeds germinate. Sunlight degrades pre-emergence herbicides on the soil surface, and if rainfall or irrigation does not follow within seven to 10 days after application, poor weed control often results.

Post-emergence herbicides also can be affected dramatically by drought. The effectiveness of post-emergence herbicides (such as 2,4-D, Grazon, Remedy and others) is highly dependent upon active plant growth. Typically, the better the growing conditions, the better the performance of post-emergence herbicides.

Good soil moisture, moderate temperatures and high relative humidity are conditions favorable for maximum growth and herbicide activity. If possible, post-emergence herbicide applications should be made during periods of favorable conditions.

Some post-emergence herbicides have a temporary negative effect on crop growth. Under prolonged drought or heat stress, herbicide injury can reduce crop yields.

Drought also may influence herbicide persistence (or carryover). Soil microorganisms play a significant role in the breakdown of many pesticides. Activity of soil microbes is favored by warm, moist conditions. Under dry conditions, microbial degradation slows, and herbicides may persist in the soil for longer periods.

Weed control is a crucial component in farm management, and when farmers’ control options are limited by drought conditions, a difficult task becomes next to impossible.