Breaking Down the Corn Herbicide Numbers

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Early emergence corn

Herbicide group numbers are easily visible on every herbicide label, but how can these numbers help you? 

“While there are several different trade names and new herbicides released every year, you can essentially narrow down herbicide modes of action to about a dozen,” says Kelly Backscheider, field scientist, Corteva Agriscience. “Having a basic understanding of herbicide groups makes it easier to provide effective weed control recommendations.”

You know that rotating herbicides is key to managing herbicide resistance. However, it’s important to keep in mind that while rotating active ingredients is good, rotating modes of action is better.

“In general, corn is our best opportunity to control resistant weeds when compared with soybeans. Everyone wants simplicity these days or products that can fit multiple crops, but if we are using the same actives in both corn and soybeans … then we increase the chance of resistance developing,” Backscheider warns.

“Crop rotation is a great way to keep herbicide resistance at bay, but it’s not going to help if you are relying on the same actives in both crops.”

Here are some key herbicide groups to consider incorporating in customer weed control programs:

  • Group 4: Commonly referred to as the synthetic auxins or growth regulators, Group 4 actives include other herbicides such as 2,4-D and dicamba and typically work by mimicking auxin hormones in the plant, resulting in the disruption of plant growth, and are often visible as twisting, cupping, curling and strapping of plant tissue in affected plants.

    Several actives in this group, including clopyralid, provide none to very little residual control of weeds and will only control emerged weeds. They are also typically only effective on broadleaf species. For example, clopyralid has excellent postemergence activity on troublesome weeds such as giant ragweed and marestail.  

  • Group 5: This group of herbicides includes atrazine, a very important active ingredient for corn. Group 5 herbicides are referred to as photosystem II inhibitors or simply PS II inhibitors. Atrazine provides excellent residual and postemergence control of many key grass and broadleaf weed species. In addition, atrazine — when used in combination with an HPPD herbicide — enhances the control of key weed species such as waterhemp or Palmer amaranth when compared with HPPD inhibitors applied alone.

    It is important to note that, like with many other herbicide groups, there is atrazine herbicide resistance in some areas. Despite this, atrazine continues to be used across many corn acres in the United States, and as always, it is best to use herbicides from this group in addition to other modes of action.

  • Group 15: Commonly referred to as a long chain fatty acid inhibitor, herbicides in this group only provide residual activity of weeds and will not control weeds that have already emerged. However, they are typically very effective on providing residual control of small-seeded broadleaves such as waterhemp and Palmer amaranth and grasses.

  • Group 27: Also known as HPPD inhibitors or the bleacher herbicides, actives in this mode of action obstruct key enzymes essential for pigment synthesis, which results in the bleaching or whitening of leaves. They provide control of weeds that are emerged, but they also can provide residual control of weeds and activity on driver weed species such as waterhemp or Palmer amaranth.

    “I highly recommend using a corn herbicide with an HPPD herbicide,” says Backscheider. Resicore® XL herbicide is a great option, as it contains an HPPD herbicide in addition to a Group 15 and Group 4 and contains a safener so it can be used preemergence or postemergence in corn. Kyro™ herbicide is a postemergence-only option that contains a different HPPD in addition to a Group 15 and Group 4 herbicide.

    “These are just two examples of the newest corn offerings from Corteva that fit really well in a corn herbicide program from a resistance management perspective,” Backscheider says. “The key is to use as many modes of action in corn as possible and to avoid using some of those modes of action in soybeans if you can help it.”

Finally, Backscheider recommends that every retailer keeps an updated copy of the Take Action herbicide classification chart in an easily visible location. This chart is updated every year so you can quickly look at the different modes of action and the actives in common premixes.

Kyro™ and Resicore® XL are not registered for sale or use in all states. Kyro and Resicore XL are not available for sale, distribution or use in Nassau and Suffolk counties in the state of New York. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Always read and follow label directions.