Battle Weed Resistance with Herbicides from Different Groups

Weed resistance continues to be a topic of discussion during winter meetings with farmers and others in the ag industry. Many are anxiously awaiting full global approval of herbicide traits like Enlist™, Xtend™ and Balance GT,™ but remember, herbicides alone will not remedy weed resistance.

Diversity of tactics is key to consistent weed management and high crop yields,” said Iowa State Extension Weed Specialist Dr. Micheal Owen while presenting at Latham’s field day in September 2011. He pointed out that rotating crops, rotating traits and rotating pesticides is the best way to slow down the very natural process whereby pests develop resistance to products that mankind uses to protect crops.

Herbicide products kill plants by binding to specific proteins and inhibiting the functions of those proteins. This is commonly referred to as the site (mode) of action. Check the numbering system, usually printed on the first page of the manufacturer’s label, to categorize each herbicide by its site of action.

“Weeds will adapt, so you must know your herbicide action group,” Dr. Owen during a recent Crop Advantage Series jointly presented by Iowa State University and the Iowa Soybean Association.

For example, say a farmer has been using Prowl® and FirstRate® pre-emergence on Roundup-Ready® Soybeans followed by Roundup® and FlexStar® post-emerge.  He wants to rotate chemicals for better weed management. Next year he’s planning to use Treflan® plus Raptor®, followed by Roundup and Resource®.  The problem? Prowl and Treflan are both classified as group 3 herbicides.  FlexStar and Resource are group 14, and Raptor and FirstRate are in group 2.  He might see some improved weed control simply because the herbicides have slightly different active ingredients, but his rotation will not help prevent weed resistance.

A better plan would be to use Dual® (group 15) plus Sencor® (group 5) pre-emergence followed by Roundup plus Basagran® (group 6).  And the BEST plan would be to plant LibertyLink™ Soybeans instead of Roundup-Ready and then switch Liberty® herbicide (group 10) in that plan for the Roundup (group 9) and the Basagran!

Using multiple products with different sites of action, however, is not enough.  Farmers must also make certain the herbicides are effective against the weeds they are trying to manage. Mechanical methods of weed control including cultivation even hand-weeding must be considered, as well as crop rotation and other cultural practices.

More information on which herbicides belong to which site-of-action group is available at  This website also gives information on many of the premix herbicide combinations and the groups to which they belong.