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Herbicide resistance continues to be a growing problem

By Jennifer Carrico

WEEDS—Iowa State University agronomy professor Mike Owen discussed weed management during the Integrated Crop Management conference held in Ames, Iowa. (Journal photo by Jennifer Carrico.)

Weather and drought have been the main focus of agronomic production over the past year in Iowa, which has led to major effects in weed management.

Iowa State University agronomy professor Mike Owen said farmers should not make major changes in weed management plans because of the anticipated impact of the weather.

"We have no new herbicide sites of action. Management and stewardship of existing sites remains a major consideration when planning for the coming year," he said.

Glyphosate resistance continues to be a growing problem for farmers in the Midwest and around the country. In the past, a high number of farmers were using only glyphosate, which is one of the reasons for the increase in the problem.

"Glyphosate resistance is not easy to manage and generally costs the producer more money because an alternative must be found," said Owen.

There is potential for herbicide carryover too. The lack of timely rainfall likely impacted herbicide degradation in 2012. Degradation of herbicide will vary considerably from field to field and nothing can be predicted accurately.

"The conditions next spring will be most important in affecting the relative sensitivity of the rotation crop to presumed herbicide residues," he said. "If you are concerned about a certain field, then wait to plant that field until last and make the needed changes at that time."

Owen said there continue to be new products available for weed management, but not new sites of action. He also expects to see new herbicide-resistant crops available eventually, but warns that regardless of what is developed, it will not fix the problems that farmers are currently dealing with.

"Farmers are just going to have to pay a lot more attention to detail in order to prevent problems," he said.

Even with the development of new herbicides, it will be important to follow the strict regulations and handle and use them properly in order to not cause different problems.

Owen said a key to weed management is applying the herbicides at the right time and with the right method. Stewardship in all aspects is essential for effectiveness.

In 2008, approximately 220 fields in Iowa with common waterhemp populations were screened for resistance to glyphosate. In 2011, the Iowa Soybean Association funded a project to evaluate the changes in glyphosate resistant weeds.

More than 200 common waterhemp populations and a number of giant ragweed and horseweed were collected in 2011 and similar collections were made in 2012. Evaluations are being made on the 2012 collections. Approximately 60 percent of the 2011 common waterhemp collections have been evaluated for resistance to five sites of herbicide action. These include ALS inhibitor herbicides, PSII inhibitors, EPSPS, PPO inhibitor herbicides and HPPD inhibitor herbicides.

"As anticipated, most of the common waterhemp populations in Iowa have evolved resistance to the ALS inhibitor herbicides," he said. "Nearly 95 percent of the waterhemp in Iowa is resistant to ALS inhibitor herbicides.

"We have a lot of resistance, but the big issue is that more than 60 percent of the waterhemp is resistant to two or three modes of action," said Owen. "Think about what you will use for weed control if weeds are resistant to all herbicides."

While a farmer can control the operation with which crops he plants, he can't control how those genetics respond to the environment and what the growing conditions are. Being able to control weeds, in order to make a better growing opportunity for the crops is something that is getting more difficult.

"The problem is that we have follow the use of simplicity and now we must apply our knowledge to each situation in order to get a solution," said Owen. "Problem weeds are starting to spread across the country through contaminated seed or other sources. We must be more and more aware of how we are growing our crops in order to be able to control the weeds."

Jennifer Carrico can be reached by phone at 515-833-2120, or by email at

Date: 12/17/2012


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