Many Kansas fields were affected by flooding in 2019 (Figure 1). These fields will likely need some special considerations as weed management plans are finalized for the 2020 field season.
Figure 1. Partially flooded corn field near Manhattan, KS on May 8, 2019. Photo by K-State Research and Extension.
Expect larger weed populations if weeds were not managed before producing seed last year. Early pre-plant or burndown herbicide applications may be especially beneficial in these fields. Be sure to include a product that has some residual activity in this application. Also, consider that many of the seeds produced last year will remain dormant this year and will be present in the field for years to come.
Remember that flood waters leave a variety of items as they recede. Be on the lookout for newly-introduced weed species and adjust your weed management plan accordingly. New soil with different chemical and physical characteristics may have also been deposited. Soil properties, such as texture and CEC, influence the activity and application rate of many residual herbicides.
Flooding may have affected the breakdown of some soil-active herbicides. Most herbicides are broken down by soil microorganisms, which are generally less active in saturated soils. Some herbicides that may degrade slower and cause concern for carryover include: metribuzin, imazethapyr (Pursuit), and dimethenamid (Outlook).
Sarah Lancaster, Extension Weed Science Specialist