• Weed escapes are easily visible in many fields this year.
• Harvest Aid / Desiccation applications of certain herbicides may help limit green plant tissue that must be run through combines at harvest, but these applications are not likely to significantly reduce weed seed production.
• Waterhemp is a prolific seed producer and plants mature at different times during the season, making it difficult to reduce seed production with harvest aid herbicides.
• Consider resistance profile in waterhemp populations when choosing chemistries for pre-harvest herbicide applications.
• Schedule harvest aid herbicide applications on the basis of crop maturity, not weed maturity.

Because of excessive early rainfall this season, many corn and soybean fields have weed escapes or weeds that couldn’t be controlled with timely sprays.Field wet holes are also well populated with weeds. Preharvest 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 preharvest 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 allow the weed to produce mature, viable seed even though 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 preharvest 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 later emerging waterhemp plants might still have seed production curtailed with harvest aid herbicide applications. Large, female, waterhemp plants are capable of producing in excess of 250,000 seeds per plant. Even small waterhemp plants may produce from hundreds, to many thousands of seeds. Choosing herbicides for pre-harvest control of waterhemp must be done carefully due to the prevalence of multiple resistances.

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.