Late-Season Herbicide Applications

Nearly all herbicide labels (soil-applied or postemergence) have rotational crop intervals that specify the amount of time that must elapse between herbicide application and planting a rotational crop.  This becomes particularly important with late-season herbicide applications.  These intervals are established to reduce the likelihood that herbicide residues will persist in sufficient quantities to adversely affect the rotational crop.  Some herbicide rotational restrictions are based solely on time, while other factors, such as soil pH and the amount of precipitation received after herbicide application, can influence the length of the crop rotational intervals.

Soil moisture is often the most critical factor governing the efficacy and persistence of soil-residual herbicides.  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.  Additionally, dry soils can 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.

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.  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.  Also, livestock grazing or foraging treated soybean is not allowed on the labels of many postemergence soybean herbicides.  Table 1 contains information regarding preharvest intervals and grazing restrictions for a number of postemergence soybean herbicides.

Table 1.  Preharvest intervals and grazing restrictions for postemergence herbicides used in soybean.

¹Data, taken from the Roundup PowerMax label, are for broadcast applications in glyphosate-resistant soybean varieties.  Intervals change for applications (spot treatment and preharvest) made to nonglyphosate-resistant soybean varieties.  Forage and grazing allowances can vary among glyphosate-containing products.  Consult the respective glyphosate product label for specific information on forage and grazing restrictions.