Plants from a recently collected Palmer amaranth seed sample submitted by Berrien County, Ga., University of Georgia Extension agent Eddie Beasley have been confirmed to be resistant to atrazine. This population was also resistant to glyphosate but not Staple (pyrithiobac). 

The discovery of atrazine-resistance in Berrien County is potentially very important since this seed sample was collected from a row-crop production region and not a dairy production region.  As far as we can tell at this point, the field in question was continuous corn for at least 10 years.  This would suggest that the AR evolved via selection pressure rather than pollen flow or physical seed movement. 

There are a couple of positives here:

  1. 5 lbs/A of atrazine controlled these plants which would indicate that the level of resistance is low (2X) in comparison to what we have observed with glyphosate (6-8X). 
  2. This population was controlled with Evik (ametryn), another member of the triazine herbicide family.  This confirms results from our previous research which suggested that AR in Georgia is metabolism based rather than target-site based.  

How do we plan to address this issue?  In 2013, Dr. William Vencill and several County Extension Agents collected various weed seeds from numerous locations across Georgia. These weeds are now being screened for resistance to multiple herbicides, including atrazine. Also, we are planning to collect additional Palmer amaranth seed this summer in the major corn growing counties.  The plants grown from these seed samples will be screened by Dr. Vencill for resistance to atrazine.    

Although this recent discovery is alarming, we do not think that it necessarily means that AR is widespread.  Results from the above mentioned surveys will answer this question. 

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At this point in time, growers using normal crop rotations likely need not worry about AR.  Fortunately, numerous other herbicides can be used in field corn to effectively control Palmer amaranth (Armezon, Callisto, Capreno, Dual Magnum, Impact, Laudis, Liberty, Realm Q, Status, Warrant, and Zidua).  

Refer to page 155 of the 2014 UGA Pest Control Handbook for specific recommendations regarding the control of GR-ALS-AR-Palmer amaranth in Georgia field corn.