Many growers are moving away from preplant-incorporated herbicides. This is due in part to the substantial increase in conservation tillage, strip-tillage, etc., and the relatively large acreage of Roundup Ready soybeans and cotton.
Preemergence herbicides require rainfall or irrigation to move them from the surface into the zone where weed seeds are germinating. This relationship is simple: no rainfall= little or no weed control.
If preemergence herbicides fail, decisions regarding selection and application of postemergence herbicides are more critical. Timing of the postemergence application is especially important to ensure successful weed control.
Postemergence Herbicides
Essentially all postemergence herbicides have a statement on the label regarding weed growth and environmental conditions. A typical example is Do not apply to grasses or crops under stress such as stress due to lack of moisture...as unsatisfactory control may result. To attain adequate control, weeds must be actively growing and must not exceed the maximum growth stage.
The problem is making the decision of delaying herbicide and waiting for a rain or going ahead and spraying drought-stressed weeds. Control is generally unsatisfactory when weeds are drought-stressed, but delaying an application often allows the weeds to get too big. Making this decision is difficult, and must be done on a field-by-field basis.
We have all received many complaints regarding poor weed control this year. In many cases, the complaints have followed a postemergence herbicide application to drought-stressed weeds.
Herbicide Carryover
Risk of herbicide carryover is generally greater in seasons when prolonged drought conditions occur. Lack of soil moisture reduces the rate of chemical and microbial herbicide degradation. Presumably, we will likely see more carryover symptoms in rotational crops next spring.
CONTACT: Dr. E.C. Murdock, Extension Weed Specialist