Tools for Herbicide Use

March 2014

There are a lot of tools that have been developed over the last couple of years that can make herbicide application and weed control much easier for corn and soybean farmers.  Here are three of the most important ones:

1. Herbicide Numbering System – Herbicides kill plants by binding to a specific protein and inhibiting that protein’s function. Which protein an individual herbicide attacks is known as the mode of action for that herbicide. While there are hundreds of different herbicide chemicals, these chemicals use less than 20 different modes of action to control weeds. Farmers know that constantly using herbicides with the same mode of action can lead to resistant weed populations but it can be difficult to determine which chemicals have the same mode of action.  For example, you may have traditionally used Harmony (thifensulfuron) to control cocklebur but you now have a population that is ALS-resistant.  So you pick up a jug of Classic (chlorimuron) but it doesn’t work either. That’s because both of these chemicals are ALS-inhibitors although you may not have known it by looking at the brand or chemical name. The Weed Science Society of America has developed a numbering system that takes some of the confusion out of choosing an herbicide. In this new numbering system, all ALS-inhibitors are identified as being in Group 2.  If you have an ALS-resistant weed population, you simply avoid all the dozens of chemicals that are identified as being in Group 2. Charts listing the most common corn and soybean herbicides and their group numbers are widely available.

2. Flag The Technology – Herbicide-resistant weeds have caused producers to move away from planting a near monoculture of glyphosate-resistant soybean varieties. Farmers now utilize different herbicide-tolerant varieties (such as Liberty-Link) and many have gone back to conventional soybean varieties. This has created a situation in which some fields that are not tolerant of glyphosate have accidentally been sprayed with glyphosate. We are now recommending placing colored marker flags near the entrance of fields where spray applicators can see them. Red flags are used to indicate conventional soybean varieties, green is used to indicate Liberty-Link varieties, white is used to indicate Roundup-Ready technology, and yellow is used for STS soybean varieties.

3. Weed ID App – University of Missouri Extension now has a smartphone app that can assist in weed identification.  It is called ID Weeds and is available on I-Tunes and at the Android store. This app asks you a series of questions to identify the weed and then provides pictures and information regarding that weed. 

Finally, use caution in planting corn into fields where Flexstar (Group 14) and Prefix (Group 14, 15) were applied in 2013. These products contain fomesafen and were used extensively last year to control herbicide-resistant pigweeds in soybean fields.  Fomesafen has the potential to cause carryover injury to corn planted in 2014.

Source: Travis Harper, Agronomy Specialist