Many corn and soybean fields have weed escapes or weeds that were left uncontrolled after the postemergence application. Field wet holes are also well populated with weeds.  Pre-harvest herbicide applications are possible in some of these fields to limit the amount of weed plant tissue being run through combines at harvest.  In some cases, these pre-harvest herbicide applications may also limit production of weed seeds, although it is already too late to accomplish this for many weed species.  So, the first question is: Is this application needed to increase harvest efficiency, or is this application needed to reduce the amount of weed seed produced?  

Preharvest herbicide applications often are good tools to increase harvest efficiency but doing this while reducing production of weed seed is very difficult.  Many weeds will flower before maturity of the crop and are capable of producing mature viable seed, even when the weeds are killed before harvest.  For example, research with late applications of 2,4-D on corn found that late season applications were more effective at reducing seed production in cocklebur than in velvetleaf.  Cocklebur flowers later in the season, so 2,4-D application results in greater limitation of production of viable seed.  Velvetleaf flowers over a longer period of time than does cocklebur, so preharvest 2,4-D application still allows some viable seed to be produced.  Effective pre-harvest weed control requires that we identify the weed and determine the stage of weed growth, to estimate the level of weed seed produced at the time of application.

One of the most troubling weed escapes in fields at this time is waterhemp.  Waterhemp has discontinuous seed germination, so some plants have clearly had time to develop mature seed, while seed production of later emerging waterhemp can still be reduced with harvest aids.  Large female waterhemp plants are capable of producing in excess of 250,000 seeds per plant.  Even small waterhemp plants may produce hundreds to thousands of seeds. Choosing herbicides for pre-harvest control of waterhemp must be done carefully due to the prevalence of multiple resistance.

Be sure to make herbicide applications on the basis of the crop growth stage, not the weed growth stage.  Significant yield reduction can occur if applications are made before the labeled application timing.  So, the first step is to identify the growth stage of the crop and be certain that the application is made after the recommended application timing.  Also, it is important that the preharvest interval (PHI) is followed.  As always, read and follow the label directions.