Atrazine-Resistant Palmer Amaranth Confirmed in Berrien County (Prostko and Vencill)

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.

Figure 2.  2013 Herbicide-Resistant Weed Survey (Dr. Bill Vencill).

Liberty-Link Soybean Varieties (Prostko)

Growers searching for Liberty-Link (LL) soybeans varieties that perform well in Georgia may find that this list is rather short at this time.  This is typical for new technologies as varieties tend to get better as the technology becomes more widely adopted over time and space.  Just think back to the 1st generation of RR soybean varieties.  These early RR varieties were not the racehorses of today.  The chart listed below is a summary of the rankings for LL soybean varieties grown in the official 2013 UGA OVT tests (irrigated).

You will note that only 3 LL soybean varieties were ranked in the top 10 (yield) at 3 of 6 locations.  These include HBK LL4950 (indeterminate), HBK LL5350, and Halo 5:45.  Our recently hired UGA soybean breeder, Dr. Zenglu Li, is also incorporating the LL technology into his breeding program.  Thus, it is very likely that Georgia-bred LL varieties will be available in the near future.

Late-Season Options for Morningglory Control in Peanut (Prostko)

Recently, I have been in several fields and have had numerous phone calls about late-season morningglory control in peanut (Figure 1).  Here a few thoughts to consider:

1) Depending upon the anticipated digging date, 2,4-DB can be used to slow down the spread/growth of annual morningglory.  However, this treatment will not provide any real control or result in complete weed death.  For convenience, 2,4-DB could be tank-mixed with a fungicide.  The pre-harvest interval (PHI) for 2,4-DB is 45-60 days depending upon the specific product label.  Only 2 applications/year of 2,4-DB are legal in peanut.

2) Aim 2EC (carfentrazone) can be used as a harvest-aid for annual morningglory in peanut (but not smallflower) .  Please refer to page 448 of the 2013 UGA Pest Control Handbook for more info about this particular treatment.  Applications of Aim will not provide complete control of large morningglories but Aim will make the stems brittle enough to improve harvest efficiency.  ET (pyraflufen-ethyl) could also be used in a similar manner but I have less experience with ET than Aim.  Both of these herbicides will cause minor peanut leaf burn.

3)  Top-mowing can be a very effective method for removing unwanted weed-growth and improving harvest efficiency.  The top few inches of the peanut plants can be mowed just prior to harvest.  However, excessive top-mowing will severely effect the flow of the peanut vines over/through the digger.  I prefer mowing with a flail mower (vertical blades) rather than a rotary mower (horizontal blades) since the rotary mower often leaves a bunch of debris on one side of the row which can cause problems.  Top-mowing is a common practice that I use in some of my weedy research plots.




Figure 1.  Late-season morningglory in peanut.

 

Reflex/Glyphosate Compatibility Issues in Soybeans (Prostko)

Just a quick reminder that there have been a few issues regarding tank-mixtures of glyphosate and Reflex.  Generally, some K-salt formulations of glyphosate have not mixed well with Reflex.  In order to prevent incompatibility problems, please refer to the following Syngenta document:

A Few Thoughts About Peanut Tank-Mixtures (Prostko)

The wet weather has caused significant spray delays for most peanut growers.  Consequently, they are now considering tank-mixtures of any and all chemicals to reduce trips across the field.  Over the last few days, I have had numerous questions about tank-mixing Select (clethodim) with other agri-chemicals.  Here are a few things to consider:

1) Tank-mixtures of Select (or any other grass herbicide) + Cadre should be avoided due to the fact that reduced grass control will occur (Figure 1).  The grass herbicide should be sprayed first and then followed a minimum of 3 days later by the Cadre.  This antagonism is a result of the Cadre reducing the photosynthetic rate of the grass which reduces the sensitivity of the grass plant's ACCase enzyme to Select.

2) Results from an older multi-state (GA, NC, TX) research project suggested that when 2,4-DB is tank-mixed with a postemergence graminicide, grass control can be reduced 8-15% approximately 45% of the time (York et al.  1993.  Peanut Science 20:57-61).

3) For optimum grass control, it not a great idea to mix POST grass herbicides with a fungicide due to the increased potential for antagonism.  Generally, Abound, Bravo WS, Headline, and Folicur have all reduced grass control at some point when tank-mixed with POST grass herbicides.  When a grass/fungicide interaction has occurred, grass control has been reduced by as little as 3% (Poast + Bravo WS - large crabgrass) or as much as 45% (Select + Bravo WS - large crabgrass).  It all depends upon the grass species and fungicide used.  Way too many what if's and maybe's for me to remember.

4) New and seasoned county agents might benefit from reviewing the following publication: Tank-Mixing Chemicals Applied to Peanut Crops:  Are the Chemicals Compatible?
http://www.peanut.ncsu.edu/pdffiles/004993/tank_mixing_chemicals_applied_to_peanut_crops.pdf

Glyphosate and Peanut Yield (Prostko)

In the May/June 2013 issue of the Southeastern Peanut Farmer (page 16), there is an article that suggests that applying low rates of glyphosate to peanut late in the season will improve yields.  This research work is being conducted by scientists at the National Peanut Lab in Dawson.

A few years ago (2006-2007), I conducted similar research with glyphosate (Figure 1).  In my research, there was no positive benefit from applying glyphosate at 105 DAP.  In fact, peanut yields were significantly reduced by several glyphosate treatments.   Because glyphosate is not labeled for such a use in peanut and there are contradictory results regarding the benefits/risks, please encourage your growers to NOT use glyphosate for this purpose.

Figure 1.  Peanut Response to Roundup WeatherMax Applied 105 DAP in Georgia, 2006-2007.

Keep Gramoxone (Paraquat) in the Peanut Mix (Prostko)

Peanut growers need to be reminded (gently, bluntly, harshly, your choice) that Cadre is almost always better following an EPOST (cracking) application of paraquat (Figure 1).  Growers should only consider not using a cracking treatment of paraquat (+ Basagran or Storm) if weather conditions have prevented a timely application (< 28 days after peanut emergence) and/or when there is not enough weed growth to justify the application (i.e. no weeds).