Op-ed: Herbicide Resistant Waterhemp found in Portage County

By Seth Kannberg, Agriculture and Natural Resources Extension Educator, Portage County 

Back in the fall I was conducting my annual harvest survey in which I drive down as many backroads as possible throughout Portage County and scout soybean fields for current weed pressure before the beans are harvested. It’s always my favorite time of the year and gets me out of the office for a few days which is never a bad thing!

For the last three years I’ve generally seen the same weeds over and over; giant ragweed, foxtail, marestail, the usual. This time however, while driving southeast of Edinburg not far from Route 14, I spotted a group of weeds in a field that made me slam on my brakes. Each plant had these distinct seed head clumps, some red, some purple, heavily packed along the branches as soon as the weed broke above the soybean canopy.

Upon closer inspection I found the stems to be hairless and noted oar-shaped oval leaves scattered throughout. That’s when I knew: waterhemp was here in Portage. I long suspected and assumed it had to be but this was the first time I could actually confirm it with my own eyes. 

Why is this weed so troublesome? For starters, waterhemp is dioecious meaning it has completely separate male and female plants coexisting which allows for extreme genetic diversity within field populations during cross pollination. In the particular field I was standing in I saw waterhemp stems that were red, purple, yellow, and khaki brown. The seed heads as well were red, purple, green, and every shade in between. This genetic variability coupled with the fact that one plant can produce up to one million seeds in a season means just a few weed escapes can quickly pass down their herbicide resistant traits to all their offspring. The next season, your preferred herbicides for weed control may be entirely ineffective. 

What kind of herbicide resistance are we actually seeing? At the moment, Ohio doesn’t have it quite as bad as many of our sister states in the western half of the Midwest, though that isn’t much reason to celebrate. Missouri by all accounts has it the worst with some waterhemp populations there resistant to seven different herbicide modes of action. Here in Ohio, we’re operating under the assumption that all waterhemp has resistance to Group 2 herbicides (our ALS inhibitors) and Group 9 (glyphosate) but it’s likely some populations additionally have Group 5 (the photosystem II inhibitors, namely atrazine) and Group 14 (PPO inhibitors) resistance. 

The best way to control waterhemp is by starting with a clean field. If we start clean, we can stay clean. Waterhemp is tricky because of how long it can emerge for. Although initial emergence typically occurs later than other summer annuals such as giant ragweed, waterhemp will continue to emerge all season long. This makes utilizing residual herbicides crucial since their long-lasting nature provides weeks or even months of weed suppression. Current trials have shown the pre-emergence herbicide pyroxasulfone as being very effective for long-term residual control of waterhemp. In order to stay clean a post-emergence spray mix with glufosinate and/or 2,4-D and an effective Group 15 residual should provide adequate season long management. These herbicide recommendations are by no means a silver bullet, however, and resistance cannot be prevented by herbicides alone. Any individual plants that survive our herbicide treatments must be removed by hand before they can produce more seed. A significant amount of research is currently examining the effects of controlling pigweed like waterhemp when planting soybean green into a living cereal rye cover crop. Studies are also analyzing cover crop termination timing to allow enough biomass to accumulate without impeding the growth and ultimate yield of the soybean crop. 

The good news is that there’s one chink in waterhemp’s armor: its seeds only last for about three to five years in the soil. This means diligent weed control can eradicate the seed from the cropping system in just a few seasons. Contrast this with a weed like common lambsquarters which can survive in the soil seedbank for decades. When it comes to waterhemp, I’ll take a win where I can get one! If you have any questions or suspect you may have waterhemp in your fields later this spring, give the Portage County Extension Office and myself a call at (330) 235-7649 or email me at kannberg.1@osu.edu, I’m always happy to come and investigate further.