Bryan Brown, New York State Integrated Pest Management
Collaborators: Antonio DiTommaso, Kathleen Howard, Mike Hunter, Jeff Miller, Scott Morris, Jodi Putman, Peter Sikkema, Mike Stanyard
Last summer, several populations of waterhemp (Amaranthus tuberculatus) survived herbicide applications in western NY corn and soybeans.
Growers asked if these weeds are actually resistant to certain herbicides. If so, which ones? And are all populations of western NY waterhemp resistant to the same herbicides, or do they differ?
To answer these questions, we collected seed from these surviving weeds at three locations in NY, grew them in a Cornell University greenhouse alongside a population of waterhemp that we know is susceptible to herbicides, and then used a spray chamber to apply a range of herbicides and rates.
The herbicides we used were glyphosate (i.e. Roundup, WSSA Group 9), atrazine (i.e. Aatrex, WSSA Group 5), lactofen (i.e. Cobra, WSSA Group 14), and imazethapyr (i.e. Pursuit, WSSA Group 2). The WSSA groups represented here are the ones waterhemp has developed the most resistance to in other states. For each herbicide, we used five different rates. Each rate was applied to five waterhemp plants from each population. Following the methods of other studies, plants were sprayed when they were around 5” tall. BASF Agricultural Solutions and Valent USA LLC supplied some of the materials for this study.
We’ll be doing a final analysis three weeks after spraying. But here’s how the NY populations look after only one week. At the full labelled rates, glyphosate resulted in 50% control and lactofen resulted in 99% control. Atrazine and imazethapyr resulted in poor control, but waterhemp plants were larger than the maximum size stated on the label.
But it’s the comparison of our three NY populations to the susceptible population that determines resistance. Final control ratings will be done in two weeks, but initial results indicate that two NY populations are potentially resistant to glyphosate, three are potentially resistant to atrazine, none are likely resistant to lactofen, and two are potentially resistant to imazethapyr. So herbicides in WSSA Groups 2, 9, and 5 shouldn’t be solely relied upon to control this weed.
Since there were some differences between NY populations, we’ve shared each farm’s results with the participating growers so they can make the necessary changes to their management plans.
So if you haven’t seen it already, keep an eye out for waterhemp this year. It looks similar to other NY pigweeds, except that it’s completely hairless and it has separate male (pollen-producing) and female (seed-producing) flowering heads. (A video comparison may be found from American Agriculturalist.) Since it can travel in seed, feed, and equipment – make sure they’re clean. And think about trying out some new weed control options.