Postemerge control of emerged waterhemp in soybeans
Waterhemp Competitive, Genetically Diverse
Waterhemp is an important weed species, well adapted to cropping systems throughout the Midwest. In Nebraska, waterhemp is a problematic species predominantly in the eastern half of the state. This species is very competitive and yield losses greater than 50% have been reported when waterhemp was not controlled in soybeans.
Waterhemp seedlings are known to emerge over an extended period (May through August), with major flushes in late spring or early summer. Waterhemp cotyledons are egg-shaped and mature plant leaves are alternate, narrowly elongated, often waxy, and shiny in appearance. Stems and leaf surfaces of waterhemp are hairless. These characteristics help to differentiate waterhemp from other pigweed species (e.g., redroot pigweed, palmer amaranth) that are also found in Nebraska. It also makes waterhemp a formidable weed in cropping systems because spray droplets have been shown to bounce off the waxy leaf surface.
Waterhemp is a dioecious species (male and female flowers occur on separate plants) and prolific seed producer (a single female plant can produce more than 100,000 seeds); thus, high genetic diversity is common within a waterhemp population. The genetic diversity in waterhemp makes this weed species prone to evolve herbicide resistance when exposed to high selection pressure (exposure to a same herbicide multiple times within and/or across growing seasons).
Waterhemp populations resistant to atrazine (Photosystem II inhibitors), imazethapyr, chlorimuron-ethyl (ALS inhibitors), 2,4-D (synthetic auxins), mesotrione (HPPD inhibitors), and recently to glyphosate (glycines) have been reported in Nebraska. Populations of waterhemp in other states such as Illinois have been found to be resistant to as many of four herbicide groups including resistance to some herbicide groups not currently found in Nebraska (such as Group 14).
Sustainable waterhemp management should be a priority for Nebraska farmers, given waterhemp’s well-documented record of resistance evolution to most herbicide modes-of-action available for postemergence control.
To learn more how to manage herbicide-resistant waterhemp populations and how to avoid or postpone this problem, the Weed Science Extension group will hold a Resistance Management Field Day on August 7 near Fremont.