Ideally, soil-residual herbicides should provide several weeks of weed control but not persist long enough in the soil environment to cause damage to rotational crops. Dry soil conditions, similar to what most of Illinois experienced during the 2012 growing season, often slows the rate of herbicide degradation and increase the potential for damage to rotational crops from herbicide carryover. Some remember the dry growing season of 1998 and the problems encountered in 1989 due to the persistence of one or more soil-residual herbicides that did not adequately degrade in the dry soil conditions of 1988. Will the dry soil conditions during much of the 2012 growing season lead to similar problems with herbicide persistence in 2013? Will the precipitation that was received later in the growing season and throughout the fall eliminate the potential for herbicide carryover? Many factors interact to determine how long a herbicide remains active in the soil environment, including factors related to the herbicide, the soil and climactic conditions.
Herbicides vary in their persistence in the soil; some have very little soil activity (such as thifensulfuron) while others can persist for several months (such as picloram). Variation in soil persistence even can exist among herbicides within a particular chemical family. For example, within the imidazolinone herbicide family, soil persistence of imazamox (Raptor) is much shorter than persistence of imazethapyr (Pursuit). An indication of a herbicide’s soil persistence can sometimes be inferred from the crop rotation intervals listed on the respective product label. Herbicides that tend to persist longer in the soil generally have longer crop rotational intervals compared with herbicides that don’t persist long. For example, labeled crops may be planted anytime following the application of carfentrazone (Aim), while 10 months must elapse between application of fomesafen (Flexstar) and planting corn. Rotational intervals for a particular herbicide sometimes vary by rotational crop, which also provides an indication of which rotational crops are more sensitive to herbicide residues remaining in the soil. The chemical characteristics of the herbicide molecule that contribute to long soil persistence are inherent properties of the molecule, and there is little that can be done to shorten their persistence once they have been applied. However, variations in certain soil physical and chemical properties can influence the persistence of a particular herbicide apart from its chemical composition.