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Why Herbicides Fail

March 26, 2019
A sprayer makes a post-emergence application of corn herbicide.

It’s a problem many growers have experienced at some point. You put down a pre-emergence herbicide marketed as having residual control, only to find unwanted weeds sprouting up a few weeks later. So you follow it up with a post-emergence herbicide, but still find a few pesky weeds. Why?

Let’s consider a few reasons why herbicides can fail.

The herbicide application doesn’t have multiple effective sites of action (SOAs)

It is crucial to use herbicide applications that have multiple effective SOAs against your driver weeds. Using the same herbicide SOA year after year allows weeds resistant to that SOA to go to seed, increasing the population of resistant weeds in subsequent seasons. However – using a pre-emergence application with multiple SOAs, followed by a post-emergence application with multiple different SOAs means the weed has many obstacles to overcome to survive. This strategy offers far more control for far longer and helps prevent the spread of resistant weed species. In fact, applying a herbicide with an average of at least 2.5 effective SOAs per application versus a herbicide with only 1.5 effective SOAs, decreases the likelihood of developing resistance by 83 times.1

Be timely with your applications

Pre-emergence herbicides should be applied before there are any weeds present in the field. Don’t give the weeds a chance to emerge. Hit them hard, and hit them early. If there are any weeds that do emerge, make sure a post-emergence herbicide is sprayed before the weeds reach the recommended label height. For most herbicides this is around 4”. Keep in mind – that recommendation is for the tallest weed in the field, not the average weed height.

Not all herbicides will kill every weed

It’s important to know your fields. Not all herbicides are the same, and just because one works on Palmer amaranth doesn’t necessarily mean it will work on giant ragweed. Know which weeds are in your fields, and work with your retailer to select the right herbicides for the right weeds.

In corn, we recommend using Acuron® corn herbicide. Acuron combines 4 active ingredients (AIs), including the unique AI, bicyclopyrone, and 3 effective sites of action (SOAs). The bicyclopyrone component of Acuron complements the other active ingredients and helps provide more consistent and reliable control of 70+ tough weeds. In fact, our trials show Acuron protects corn from yield-robbing weeds better than any other herbicide on the market, which can lead to higher yield potential. It also offers application flexibility, with the ability to be applied 28 days before planting up to 12” corn.

The atrazine-free counterpart of Acuron, Acuron Flexi, has 3 active ingredients, 2 effective SOAs, and provides the same reliable safety and performance from 28 days preplant up to 30” corn. Acuron Flexi allows for the flexibility to add atrazine or glyphosate based on farming practice.

If you need help designing an effective herbicide program for this season, contact your local Syngenta sales rep.

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1 University of Illinois College of Agricultural, Consumer and Environmental Sciences, 2015.