Area producers are concerned about the need for herbicide rotation with good reason. With increasing difficulty of finding and registering new chemicals, it is imperative that we rotate modes of action as well as crops to reduce the amount of selection pressure that we put on weeds. It ultimately falls on us as individual producers and herbicide users to rotate herbicides based on their Mode of Action (MOA). The MOA of a herbicide is how it disrupts a process in the target weed’s growth and development. Many chemicals that we use can have the same mode of action. For example, Milestone (aminopyralid), Garlon (tryclopir), 2,4-D, 2,4-DB , Dicamba, and Grazon (picloram) share the same MOA. Although we see these as different chemicals, we are still using the same MOA and adding selection pressure for the resistant weeds. Regardless of whether we are using herbicides, fungicides, or other chemicals to deal with pests, we should rotate MOA’s whenever possible. This will increase the amount of time that a chemical will remain viable. I have included an exert from Dr. Eric Prostko’s correspondence about atrazine resistant pigweed found in Berrien County, to illustrate how this affects us in row-crops. Note that corn was planted in the same field year after year for at least ten years. If you would like to discuss rotating chemicals on your farm, contact your local county extension agent.
Plants from a recently collected Palmer amaranth seed sample, submitted by Berrien County Extension Agent Eddie Beasley, have been confirmed to be resistant to atrazine (Figure 1). This population was also resistant to glyphosate but not Staple (pyrithiobac). The discovery of atrazine-resistance (AR) in Berrien County is potentially very important since this seed sample was collected from a row-crop production region (i.e. 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. Firstly, 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). Secondly, 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. Vencill and several County Extension Agents collected various weed seeds from numerous locations across Georgia (Figure 2). 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. 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 (http://www.ent.uga.edu/pmh/Comm-Field-Corn.pdf).
Figure 1. 2013 Berrien County Palmer Amaranth Population Treated with 1, 2, and 5 lbs ai/A of Atrazine.