Weed Resistance To Herbicide Increasing

Weed Resistance To Herbicide Increasing

Herbicide-resistant weed populations are growing at an increasing rate in corn and soybean fields in Iowa. "These weed shifts are the result of management decisions you make," points out ISU weed scientist Mike Owen.

Despite the almost universal adoption of genetically engineered crops, specifically those resistant to glyphosate herbicide, weed problems in Iowa continue to be important and actually, they are getting worse.

That's the observation and latest thinking of Iowa State University Extension weed scientist Mike Owen. He made those comments at the end of the day, as he summed up presentations that were made by weed researchers from the University of Illinois and Iowa State University at a March 31, 2011 conference on weed resistance held at Davenport, Iowa.

Researchers from herbicide companies and seed companies also voiced their opinions and gave their recommendations, regarding the causes of and possible solutions to this increasing problem.

Weeds are continuing to evolve and develop resistance

He says resistant populations of weeds are continuing to evolve and develop resistance to more herbicides. Weed resistance to glyphosate is a growing problem. And other herbicides are being added to the resistance list. For example, resistance to HPPD inhibitor herbicides was documented in seed corn fields. "I suspect resistance to this herbicide group is more widely distributed than most farmers realize," says Owen.

He says it's crucial for farmers, chemical dealers, crop consultants and everyone involved in weed mangement to take steps to help prevent further spread or development of herbicide resistant weeds. "In Iowa we now have resistance in waterhemp to triazine herbicides, ALS inhibitors, PPO inhibitors, glyphosate and HPPD inhibitor herbicides," he adds.

Weeds are most important pest Iowa farmers face each year

Proper weed management will make farmers more money every year than managing any other pest. Diversity of tactics is the key. "Weeds represent the most important and economically damaging pest that Iowa soybean and corn farmers face every year," Owen says.

Evolved resistance to herbicides continues to escalate in Iowa. Glyphosate-resistant populations of waterhemp are widespread and increasing. Similarly, glyphosate-resistance in giant ragweed and marestail are becoming increasingly important. Last year, resistance to HPPD inhibitor herbicides (products such as Laudis, Callisto and Impact) was documented in seed corn production fields.

"I suspect that resistance to this herbicide group is more widely distributed than most growers realize. Thus in Iowa, we have resistance in waterhemp to the triazine herbicides (atrazine), ALS inhibitors (Pursuit), PPO inhibitors (Phoenix), glyphosate and now the HPPD inhibitor herbicides," he notes.

If you aren't getting control of waterhemp, presume it's resistant

Furthermore, many populations of waterhemp have multiple herbicide resistances, he adds. As the number of herbicides to which waterhemp is resistant increases, the difficulty in managing this important weed decreases. "If you are not sure if waterhemp in your fields is herbicide resistant, you are better erring on the side of being conservative," says Owen. "You should presume resistance exists and manage accordingly."

Several representatives from herbicide companies and seed companies also gave presentations and shared their thoughts at the Davenport meeting. Owen offered the following observations and recommendations to farmers and those who advise farmers regarding weed management practices and products.

Steps to help control spread of herbicide resistance in weeds

* DO NOT use only one management tactic or herbicide to control weeds.

* DO use tank-mixes of herbicides with different mechanisms of action (MOAs) that will control the weeds of concern. Tank mixes are better than rotation of MOAs. Refer to the herbicide group number (voluntarily included on many herbicide labels) to determine if the herbicides have different MOAs.

* DO scout early in the spring and continue to scout throughout the season. While you may not think weeds exist in the untilled fields, look closer because they are there and they will cost you money if you do not manage them prior to or IMMEDIATELY after planting.

* DO use a soil-applied residual herbicide on all acres regardless of crop or trait. Whether your plan to till the fields or not, it would be worthwhile to include a residual herbicide that controls the weeds that will germinate first, are most populous, and are of greatest concern.

* DO know what herbicides you are planning to use, what they control (and do not control), what replant restrictions exist and if there is significant potential for crop injury.

Correct management of weeds will make you more money every year

"Correct management of weeds will make you more money every year than managing any other pest complex," Owen emphasizes. "Weeds are universal and exist in economic populations on most, if not all fields in Iowa. Herbicide-resistant weed populations are increasing at an increasing rate in Iowa; these weed shifts are the result of the management decisions you make.

"Anything that is suggested to be simple and convenient (herbicide, crop trait, whatever) will inevitably fail and cost you yield potential," he adds. "No single tactic will protect the potential crop yield nor deter the evolution of herbicide-resistant weed populations. Be proactive and manage herbicide resistance before it becomes a major problem. Diversity of tactics is the key to consistent weed management and high crop yields."

TAGS: USDA Extension
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