Weeds that survive an initial herbicide application are often
re-sprayed 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 post-emergence) 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 says. “Many herbicides are degraded
in soil by the activity of soil microorganisms, and populations
of these microorganisms can be greatly depressed