Waterhemp is annually one of the most challenging weeds to control in corn and soybean fields. Adding to that challenge, glyphosate-resistant waterhemp populations have been confirmed in seven Nebraska counties. At our research sites waterhemp emergence has been occurring in earnest for approximately three weeks. These plants will start to grow rapidly as we move into the warmer portion of the growing season.
In Roundup Ready soybeans there are three primary herbicide modes-of-action that can be applied for postemergence control of broadleaf weeds. These are
- glyphosate (group 9),
- ALS-inhibiting herbicides such as Pursuit, Classic, or FirstRate (Group 2), and
- PPO-inhibiting herbicides such as Cobra, Flexstar, Cadet, or Ultra Blazer (Group 14).
Growers who suspect they have a glyphosate-resistant waterhemp problem should use a tank-mixture of glyphosate and a herbicide from Group 14 and/or Group 2 for effective postemergence control. To have success using Group 14 or Group 2 herbicides, waterhemp height should be less than 4 inches. Soybean fields with emerged waterhemp where glyphosate has struggled in the past should get top priority for spraying. Ideally, a grower would use both a group 14 and a group 2 herbicide if the waterhemp is glyphosate-resistant to slow the selection pressure for multiple-resistant waterhemp.
Group 14 herbicides are more likely the best choice for postemergence control of glyphosate-resistant waterhemp. Examples include Cobra, Flexstar, Ultra Blazer, or Cadet. Growers must be aware that they may need to adjust their application practices to maximize the performance of herbicides in this group. Group 14 herbicides are contact, cell membrane disruptors that require good spray coverage for best control. It is important to read the product label to know the required carrier volume and adjuvant selection for a given product. Some labels require a minimum of 15 GPA carrier volume, while some may require as high as 30 GPA.
The use of Group 2 herbicides (e.g., Pursuit, Classic, FirstRate, etc.) for postemergence waterhemp control may not result in expected control because ALS-resistant waterhemp populations have been documented for many years. Many Nebraska farmers may have experienced problems with ALS-resistant waterhemp in soybeans prior to the introduction of Roundup Ready technology when herbicides such as Pursuit were extensively used for postemergence weed control in soybean. Unfortunately, resistance to this class of herbicide is likely still present in those populations. In cases where Group 2 resistance does not exist, it is likely to evolve where this herbicide class is used for two to three years in a row.
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.