Weed control accounts for the biggest expenditure in peanut pest control in Oklahoma and the difficulty is magnified by the limited number of management tools available. Potential for herbicide resistant weeds makes the chore even more complex.

“Tillage practices also have changed,” said Joe Armstrong, Oklahoma State University Extension weed specialist during the Oklahoma Peanut Expo in Lone Wolf, Okla. Reduced or no-till systems limit or reduce mechanical weed control options from many peanut fields. Chemical control has replaced tillage on many farms.

Armstrong said current control recommendations include Valor applied pre-emergence, especially for pigweed. “A new mode of action helps. For post-emergence control, our options are fairly limited. We recommend Valor plus a layby product to get peanuts to canopy stage. Paraquat or Basagran may be good options.”

Armstrong said herbicide resistance has not hit Oklahoma peanut farmers as hard as it has producers in other areas. “But it is creeping in. Weed species we used to control with a specific herbicide may not be controlled as easily any longer.”

He said continuous use of one herbicide, weed mutations, weed escapes and reproduction allow resistance to develop. Time for resistance to develop depends on production practices and the weed population.

“In time, resistant weeds may become the dominant weed in the field.” Seed production, he said, is massive, with one pigweed plant capable of producing up to 1 million seeds. “One Palmer amaranth weed per foot of row could mean 100 million seeds and a 26 percent yield loss,” he said.

Fields that have never been sprayed with an herbicide are much less likely to produce resistant weeds, Armstrong said. “Weeds in those fields have not gone through the selection process, but if resistant weeds are in nearby fields, seed may blow in.”

When farmers identify resistant weeds they should suspend use of an herbicide that is no longer effective. “They may control weeds with a broad spectrum herbicide if they are using something else to control other weed species.”

He recommended rotating crops and herbicides. “Use a different mode of action. Growers also may use other control methods, such as tillage, narrower rows, higher plant populations or hand weeding. Those may not be as efficient or cost effective."

The first step is identification of herbicide resistance. “How do you know? We use a greenhouse test and screen weeds.” Oklahoma State plans a peanut weed resistance screening program his fall.

Armstrong said Extension workers will collect suspected herbicide resistant weeds from growers and test them in the greenhouse. “We also want to know the field location and the herbicide used, an herbicide use history and crop rotation. That information shows us what to test for. We keep information confidential.”

He said pigweed will be the main focus, but also expects growers to bring in Johnsongrass, ragweed and marestail specimen and possibly kochia.

“Early identification is crucial. Control is easier when weeds are small and early control also reduces competition.”

He recommends farmers scout fields for weeds before they spray and then after applications to show how well the treatment worked and to help identify resistance. He said the herbicide screening typically takes two to three months for an evaluation.

“We will get information back to the grower in time to make crop decisions.’

email: rsmith@farmpress.com