weed-resistance table

Editor’s note: The Iowa Farm and Rural Life Poll is an annual survey of Iowa farmers. The 2017 Farm Poll questionnaires were mailed in February to a statewide panel of 2,080 farmers.


The 2017 Iowa Farm and Rural Life Poll asked farmers who planted corn or soybeans the previous year about weed management and herbicide-resistant weeds.

The first question set asked about the presence of herbicide-resistant weeds and changes in weed management and herbicide programs. Several of the questions had previously been asked in the 2013 Farm Poll survey, allowing comparisons between years.

In 2017, 22 percent of respondents reported they believed they had weeds that were resistant to PPO inhibitor herbicides (e.g., Flexstar, Authority, Kixor). This was a substantial increase from 3 percent in the 2013 survey.

Similarly, in 2017, 12 percent of farmers believed they had weeds that were resistant to HPPD inhibitor herbicides (e.g., Callisto, Impact), compared to 4 percent in 2013.

Several questions focused on weed management behaviors. The first asked farmers if they had changed their weed management program due to concern about herbicide-resistant weeds. In 2017, 77 percent responded affirmatively, compared to 52 percent in 2013.

In 2017, 83 percent indicated they had made more than one herbicide application to a single crop in a single season over the last five years, compared to 81 percent in 2013.

In both 2013 and 2017, 65 percent of farmers indicated they used a custom applicator to spray herbicides. The proportion of farmers who develop their own herbicide programs dropped from 45 percent in 2013 to 36 percent in 2017.

Two final questions on weed management behaviors focused on whether farmers planned to use two relatively new products.

The first asked farmers if they planned to use the dicamba-resistant soybean system with dicamba. Fourteen percent indicated they would, 65 percent would not, and 21 percent were unsure.

The second question asked if they planned to use 2,4-D Enlist with the corn and soybean 2,4-D Enlist resistant systems. Thirteen percent indicated they did plan to use that combination.

Manageability of major weeds

The survey provided a list of weeds that have become more difficult to manage in different areas of the U.S., and asked survey respondents who plant row crops to rate ease or difficulty of control.

Waterhemp was viewed as the most challenging weed: 57 percent reported it has become more difficult to control and 23 percent believed it has become resistant to herbicides. Marestail/horseweed, giant ragweed and lambsquarters were rated as having become more difficult to control by nearly 50 percent of farmers, but fewer than 10 percent believed that these weeds have become resistant to herbicides.

Palmer amaranth is a plant weed scientists have classified as a serious concern for Iowa agriculture because it is highly invasive, fast-growing, produces seed prolifically and has evolved resistance to many herbicides in other states. Resistance is not confirmed in Iowa.

As of August 2017, Palmer amaranth had been identified in 50 Iowa counties, and ISU Extension specialists recommend that farmers be increasingly vigilant in identifying and eradicating it from their fields, conservation plantings, roadsides and ditches.

Given Palmer amaranth’s invasiveness and potential to evolve herbicide resistance, it is critically important that farmers be able to identify and eradicate it. The survey results, however, indicate that a majority of farmers (54 percent) were not familiar with the weed. Twenty-three percent reported that it has become more difficult to control on the land they farm, and 16 percent believed it has become resistant to herbicides.

These results suggest more intensive efforts to increase farmers’ awareness of the weed and eradication methods are necessary.

Concern about resistant weeds

Nearly 80 percent of farmers agreed they were concerned about the spread of herbicide-resistant weeds from other regions of the U.S. (79 percent) and other counties (79 percent). Seventy-seven percent agreed that they are concerned about spread from nearby farms, and 82 percent agreed with the item “Even if I keep my fields clean, I could get herbicide-resistant weeds from neighboring farms.”

In January 2017, after more than a year of collaborative planning among major Iowa agricultural stakeholders, the Iowa Pest Resistance Management Plan was released. The plan outlines a number of activities that will be pursued to help Iowa’s farmers and other agricultural stakeholders increase their awareness of and capacity to address pesticide resistance in the state.

The Farm Poll survey contained a set of items to gauge farmers’ perspectives on the potential effectiveness of several hypothetical approaches to addressing herbicide-resistant weeds. Farmers were provided short descriptions of nine different resistance management approaches and asked to rate their likelihood of success.

The highest-rated options were the “quick fix” approaches entailing new technologies. “Private company discovery and development of new herbicides” and “private company discovery and development of new herbicide tolerant crops” were the two highest-rated approaches.

The survey posed three cooperative approach scenarios, ranging from collaboration just between farmers to a complex collaboration between multiple agricultural stakeholders. The highest-rated option was the one that involved the most stakeholders: “Local farmers, agricultural input supplier representatives, Iowa State University research and extension staff, state agency staff, and commodity group staff working together to improve adoption of weed Best Management Practices.” Sixty-four percent of farmers rated this strategy as likely or very likely to succeed.

These results indicate that although the “quick technological fix” options were viewed as most likely to succeed, cooperative solutions were also seen as promising.

The 2014 Farm Poll survey found that only 14 percent of corn and soybean farmers agreed with the statement, “herbicide-resistant weeds are not a major concern because new technologies will be developed to manage them.” So, although Iowa farmers agreed that a technological fix would be most likely to succeed, many do not believe that such technologies are forthcoming, and support the concept of cooperative pest management as a viable option.

It is important to highlight that the most complex cooperative arrangement, involving stakeholders from across the private and public sectors, was rated as the most likely to succeed. This echoes 2014 Farm Poll findings that multiple private and public stakeholders bear responsibility for resistance management, and indicates that many farmers would support involvement of diverse stakeholders in cooperative resistance management efforts.

The financial incentives and mandate items were the lowest-rated. “Private company financial incentives to farmers to spur adoption of weed Best Management Practices” and “Government financial incentives to farmers to spur adoption of weed Best Management Practices” were rated as likely or very likely to succeed by 36 and 35 percent of farmers, respectively.

“Government-mandated weed Best Management Practices requirements for farmers” was rated least likely to succeed.

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