Diversity key to effective management of herbicide resistant weeds

INDEPENDENCE, Iowa - Simplicity and convenience have run their course when it involves effective weed management, said Mike Owen, Extension weed specialist.

INDEPENDENCE, Iowa - Simplicity and convenience have run their course when it involves effective weed management, said Mike Owen, Extension weed specialist.

An integrated, diverse approach is needed now.

Owen and Extension weed specialist Robert Hartzler talked about strategies for managing herbicide resistant weedsat a recent workshop on herbicide resistant weeds at the Heartland Agribition Center in Independence. Similar meetings were held at other locations in Iowa.

"Weeds have adapted to weed control tactics since the inception of agriculture and the current weed shifts occurring in Iowa corn and soybean fields are not a new problem," Owen said. "However, current agricultural systems are more reliant on a single control tactic - herbicides. We are also now fighting weeds that are adapted to chemical weed control."

Owen said weeds are evolving herbicide resistance faster than the industry can develop new technology.


"It's Darwinian evolution in fast forward," he said.

The evolution of herbicide resistance isn't a herbicide or trait problem, it's behavioral, Owen said.

"It's obvious to me that growers are not widely responding to the message," he said. "Changes in our approach to weed management are needed to sustain the current production system. The greater the diversity, the greater the profitability and sustainability."

Owen, who has testified before Congress on herbicide resistance, sees a disconnect between research and marketing. He thinks that the problem can be managed. Regulation should be avoided.

Owen and Hartzler said mechanical, cultural and herbicidal tactics must be considered.

Annual rotation of herbicide sites of action and using multiple sites of action are useful resistance strategies, Hartzler said. He walked those at the meeting through an exercise to evaluate herbicide programs based on a herbicide group's effectiveness on waterhemp.

"It's not as simple as just adding more products," Hartzler said. "The contribution of each product needs to be carefully evaluated. To eliminate selection pressure on weeds, the different sites of action must provide the same level of weed control. Not only is this difficult to achieve, but many growers see this as economically prohibitive. Managing herbicides alone will not keep us ahead of evolved herbicide resistance in weeds."

Hartzler said it's important to use herbicides efficiently paying attention to application timing, spray additives and spray parameters.


"With glyphosate we got pretty sloppy," Hartzler said. "We're going to have to do a better job."

Planting dates, row spacing and plant population can improve a crop's ability to compete with weeds. Some farmers find that cover crops improve weed control.

Mechanical weed control, which fell out of favor with the introduction of ALS inhibitors in the late-1980s and is now seldom used due to Roundup Ready soybeans, is something to consider.

"I'm not saying that you need to cultivate all your acres, but there is a role for cultivation in some fields or parts of some fields," Owen said.

Primary tillage may be another tool.

"I'm not suggesting that we plow the entire state of Iowa, but in certain fields and in certain areas, it may be an option," Owen said.

Isolate fields with high populations of resistant weeds.

"Plant and cultivate these fields last and clean the equipment when done," Owen said. "We have to look at all these little hammers."

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