Weed resistance is increasing “at an alarming rate,” and with no new herbicide sites of action becoming available in the immediate future, “we need to follow stewardship programs to the letter to preserve the herbicide products we have,” warns Michael Owen, Iowa State University Extension weed specialist. Common waterhemp, marestail (horseweed) and giant ragweed are likely to be the biggest weed challenges for Corn Belt farmers in 2013, and Palmer amaranth will be the next big weed threat, Owen says.

“The big story in Iowa will be waterhemp,” he says, noting tests show almost 30% of Iowa’s waterhemp is resistant to ALS inhibitors, atrazine and glyphosate.

“Marestail and giant ragweed are not as common. Marestail is primarily widespread in no-till fields, and ragweed is kind of where you find it, but it’s important for the growers who have it,” he says.

“As you go from West to East in the Corn Belt, the frequency of issues with marestail and ragweed increases. In Ohio, marestail is a bigger issue than waterhemp.”

Owen offers these tips for managing an increasingly herbicide-resistant weed population:

  1. Diversify your management approach. “Forget the siren song of simplicity. Growers are used to diversifying hybrid choices and fertility treatments to mitigate risk. The same approach for weeds means knowing what you’ve done historically, knowing if you’re seeing issues in specific fields, scouting and managing weeds for as small a unit as possible. Don’t treat all fields alike.”
  2. Use herbicide group numbers to diversify weed management. “We still have too many growers relying solely on one post-emergent herbicide for control,” he explains. “Even if you’re applying a herbicide with multiple numbers, you aren’t diversified if the weeds already are resistant to some of those sites of action.”
  3. No herbicide will provide consistent, season-long control. An herbicide that lasts that long could present a carryover problem. Meanwhile, “don’t get too paranoid on carryover from 2012,” Owen says. “The best thing is to know what was applied in each field, when it was applied, and when it got rain, then make a good business risk decision.” The highest risk of carryover is associated with atrazine, chlorimuron, inazaquin and simazine.
  4. Use residual soil herbicides with multiple effective sites of action. “We are leaving a lot of money on the table in lost yield potential from early weed competition,” Owen argues. “There’s lots of grower field data on the economic viability of using residuals.”
  5. Scout, scout, scout. “Better weed management begins with including more diverse tactics, scouting, and using multiple herbicides with alternative effective sites of action,” he concludes.