Soybean School: Patrolling for herbicide-resistant waterhemp and other pigweeds

Those green plants sticking out of the crop — are they regular redroot pigweed, or a much nastier pigweed species, such as waterhemp, or even Palmer amaranth?

This has become an increasingly common question for farmers and agronomists on the eastern side of the Canadian Prairies as waterhemp that’s resistant to multiple herbicide groups continues marching north and west.

As soybean fields move toward maturity, Kim Brown-Livingston, weed specialist with Manitoba Agriculture, joins us for this Soybean School episode to discuss the latest on waterhemp and Palmer amaranth findings in Manitoba, the keys to managing against these Tier 1 noxious weeds, and advice when out on “pigweed patrol” prior to harvest.

A small patch of waterhemp — ideal for hand-picking and destroying. (courtesy Tammy Jones)

“My August, I call it pigweed patrol — looking for it everywhere,” she says, noting waterhemp has been found through all regions of agro-Manitoba since it was first confirmed in 2019. “It’s in western Manitoba. It’s in all through south-central Manitoba. We’re finding it up in the Interlake, the Eastern Interlake region.” (see map below)

While pigweed escapes become evident as the crop dries down, scouting should begin much earlier, after every herbicide application, she stresses. “We need to be doing more and more post-spray scouting to see ‘did we have some type of failure and why?’ It’s not necessarily herbicide resistance, but that has to be definitely something that we’re watching for.”

When suspicious plants are found during scouting or at harvest, she recommends they be bagged for testing, while taking all possible measures to prevent seed spread. “You need a shovel, and get big garbage bags. We recommend to carry garbage bags on the combine when we get into combine season. If you see something funny, that shouldn’t be there, do not put that through the combine.”

Check out the video above for more on the spread of new prolific pigweed species and best practices in preventing and managing them, filmed at the 2023 Manitoba Crop Diagnostic School at Carman, Manitoba.