Bill Johnson
WEED CONTROL AND COVERS: Purdue Extension weed control specialist Bill Johnson and his staff are taking a hard look at the value of cover crops in controlling tough weeds.

Confirmation bias can creep into agriculture

Two Purdue University weed scientists suggest caution when analyzing results.

Three organizations issued a report recently that said good things about cover crops. Indeed, that’s not hard to do — farmers are reporting multiple benefits from the practice. However, Bill Johnson, a Purdue University Extension weed control specialist, says when it comes to cover crops and their help controlling specific weeds, it’s best to look at data and make your own interpretation, rather than rely on how someone else interprets that data.

Writing for the Oct. 12 edition of the Purdue Pest & Crop Newsletter, Johnson and Joe Ikley, a weed science program specialist who works with Johnson, introduced a concept called “confirmation bias.”

“Confirmation bias is the tendency to process information by looking for, or interpreting, information that is consistent with one’s existing beliefs,” the pair wrote. “This biased approach to decision-making is largely unintentional and often results in ignoring inconsistent information. Existing beliefs can include one’s expectations in a given situation and predictions about a particular outcome. People are especially likely to process information to support their own beliefs when the issue is highly important or self-relevant.”

The three organizations that issued the report after surveying over 700 farmers are the Conservation Tillage Information Center, the Sustainable Agriculture Research and Education group and the American Seed Trade Association. They’re better known as CTIC, SARE and ASTS.

Interpret results
The survey included a question on whether or not farmers thought cover crops helped control resistant weeds during the next season. According to the survey, 25% said cover crops always help, 31% said they do not help, and 44% said they help “sometimes.” The news release about the survey included this statement: “Sixty-nine percent of respondents said cover crops always or sometimes improve control of herbicide-resistant weeds.”

“We’re very supportive of these groups, but we think it’s important to send the most accurate message possible to growers about this subject,” Johnson says. “It’s all about how you interpret the data. Someone with a negative attitude toward cover crops could see the same data and say, ‘Seventy-five percent of cover crop users don’t see improved weed control after using a cover crop’ if they added up ‘no’ [31%] plus ‘sometimes’ [44%].

“We believe the most unbiased way to read this data is to simply state it as presented: 25% of cover crop users believe they always see improved weed control, 31% don’t see improved weed control, and 44% experience mixed results with cover crops and herbicide-resistant weed control.

“Our concern is that commercial people could be tempted to take the positive interpretation and use it as a selling point,” Johnson continues. “We’ve already found an example where someone is doing so.”

He emphasizes that farmers need another tool to control resistant weeds, and if cover crops will do it, he is all for it. What he isn’t in favor of is offering interpretations of data, even if unintentional, that could sway producers to put more faith in a solution than perhaps they should.

Half of Johnson’s research graduate students are working on projects right now related to learning more about cover crop and weed control interactions. “So far we’ve found about what the survey suggests,” he says. “Cereal rye and marestail has real possibility. With some of the pigweeds, one year we see better control with a cover crop, then another year the control is actually worse compared to the control. We’ve got more work to do here.”

TAGS: Weeds
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