Two years. 85 farm fields. 9-inches. Those were numbers Kevin Bradley, University of Missouri Extension weed scientist, shared with farmers during MU Bradford Research Center's annual Pest Management Field day.
In two years of monitoring 85 farm fields under normal management practices, the average height of waterhemp sprayed post emergence was 9-inches. "And we wonder why we didn't kill it?" he asks.
PPO resistant waterhemp?
Bradley has heard concerns over new resistance in waterhemp to PPO-type herbicides, besides glyphosate. Growers in Illinois and Iowa have seen higher rates of PPO-herbicide resistance. But Bradley contends those states practice continuous corn, which increases resistance opportunities. Still there is growing anxiety, so Bradley is researching the situation in the state.
Last fall he collected seed from 170 populations across the state. Those were grown in his greenhouse and tested last winter. PPO resistance was found in only 11% of the populations. "That is low," he says. "We grow rotations of corn and soybeans, so I think that helps us."
While the state does show glyphosate resistant waterhemp, there are still control measures available. Researchers are reaching back into the archives to look at how cultural practices, like row-spacing and tillage practices, can reduce resistant waterhemp density.
In a plot in Moberly, researchers compared 7 1/2 –inch, 15-inch and 30-inch rows. They found that waterhemp density declined as row width decreased. He says more research may come in this area.
Bradley says for now the best way to treat for waterhemp is to catch it early and use multiple modes of action. Not putting down a pre herbicide with resistant waterhemp and only relying on post is an "unwinnable" scenario.