Waterhemp isn’t a new problem in agriculture, and managing the weed isn’t a new item on the checklist. However, new University of Illinois research shows controlling waterhemp may be even harder than everyone thought.

Up until 2009, growers could use 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor herbicides such as Callisto on corn because the plant could naturally tolerate it, says Dean Riechers, U of I crop scientist and co-author of the recent study. However, that year, waterhemp showed signs of developing corn’s ability to resist HPPD-inhibitor herbicides.

That’s when Riechers and his team of crop scientists set out to better understand the ways waterhemp works to resist certain herbicides.

Corn and weeds such as waterhemp can tolerate specific herbicides through metabolism — a detoxification process that allows the weed to break down the herbicide before reaching the site of action in the plant.

“We started out with Callisto, because at the time its metabolic process was known in corn,” Riechers says. “We then shifted to another HPPD-inhibiting herbicide called Impact, which has a little different structure. And sure enough, waterhemp could metabolize and detoxify it — not exactly the same as corn, but very similar.”

Riechers says the team was curious to see how far they could push waterhemp’s biological capabilities.

Syncarpic acid-3 (SA3), the great-great-grandfather of Callisto, sat on the shelves unused because it killed everything, including corn, he says. Even though SA3 can’t be used as a corn herbicide, the team wondered if it would kill waterhemp, too.

But it didn’t.

“SA3 had never been examined before, so we wanted to figure it out,” says Riechers, adding it took eight years to do so. “We unfortunately discovered a mechanism we didn’t know was there. So, Syngenta [which helped fund the research] and other companies coming out with new herbicides need to be aware of these findings.”

The biological mechanism they found was metabolism, which means waterhemp was able to break down and tolerate SA3, and SA3 didn’t kill it. Riechers says the finding was unpredicted because corn is not able to stop SA3 and waterhemp often evolves corn’s metabolic system.

So, what should farmers do?

Riechers, who grew up on a farm near Grant Park, Ill., knows his job as a crop scientist is about performing research that can be applicable to the average farmer, not just to help companies to develop new herbicides.

“We want to know everything about waterhemp,” he says. “The biology and everything about it. We need to understand it, because if not, then we might implement a new tool that is ineffective.”

While crop scientists continue to learn about waterhemp’s capabilities, Riechers reminds farmers and custom applicators to scout often.

Get out in the field twice a week or more to observe where the field is at in growth stage and identify any waterhemp, or other weeds, he says. Scouting can help assure you are applying the appropriate herbicides at the right time.

And control weed seeds like your job depends on it.

If applied chemicals don’t work, remove waterhemp seeds with equipment such as modified harvest machinery before they fall back into the soil, Riechers suggests. Seed destruction attachments are an option, like the Seed Terminator. It destroys weed seeds that pass through the combine at harvest before they return to the soil.

Also, keep practices diverse.

“You don’t want to just rely on one herbicide to control waterhemp, because if it is ineffective, then you’re in trouble. Remember to use crop rotation, tillage and other sites of action,” Riechers says. “Waterhemp has an incredibly diverse toolkit to go against us, so farmers need to be diverse, too.”