The adverse environmental conditions common across much of Illinois are challenging the performance of many foliar-applied soybean herbicides.

Weeds that survive an initial herbicide application are often resprayed later in the season. However, according to University of Illinois associate professor of weed science Aaron Hager, the likelihood of controlling larger, moisture-stressed weeds continues to decline.

Moreover, herbicides applied late in the season are more likely to persist long enough to injure sensitive rotational crops. Nearly all herbicide labels (soil-applied or postemergence) specify the interval between herbicide application and planting a rotational crop. Some of these restrictions are based solely on time, while others take into account factors such as soil pH and the amount of precipitation received after herbicide application when determining interval length.

"Soil moisture is often the most critical factor governing the efficacy and persistence of soil-residual herbicides," Hager said. "Many herbicides are degraded in soil by the activity of soil microorganisms, and populations of these microorganisms can be greatly depressed when soil moisture is limited."

Dry soils can also enhance herbicide adsorption to soil colloids, thus rendering the herbicide unavailable for plant uptake and degradation by soil microbial populations. Some herbicide rotational intervals are increased if a specified amount of precipitation is not received by a certain calendar date.

The intervals are established to prevent herbicide residues from reaching levels that will adversely affect the rotational crop. Respecting these intervals becomes particularly important with late-season herbicide applications and when soil moisture is limited.

"Please keep in mind that the labels of almost all postemergence soybean herbicides indicate a preharvest interval or a soybean developmental stage beyond which applications cannot be made," said Hager.

Labels of some products may indicate both a developmental stage (before soybean bloom, for example) and a preharvest interval. Preharvest intervals indicate the amount of time that must elapse between the herbicide application and crop harvest.

Failure to observe the preharvest interval may result in herbicide residue levels in the harvested portion of the crop in excess of established limits. Moreover, labels on many postemergence soybean herbicides specify that foraging of, or grazing livestock on, treated soybean is not allowed.