Glyphosate-resistant Palmer amaranth is the greatest pest management threat to Georgia cotton production.  To combat this pest, growers are using more tillage and more herbicides.  Herbicide input costs have more than doubled with growers currently spending $62.50 per acre.  Ninety-two percent of these growers are also hand weeding 52% of the crop at an average cost of $23.70 per hand weeded acre.  Cotton producers desperately need more economically effective tools to manage glyphosate-resistant Palmer amaranth.  Enlist^[TM] cotton would provide cotton tolerant to topical applications of glyphosate, glufosinate, and/or 2,4-D.  Weed management programs using mixtures of 2,4-D and glufosinate can effectively control glyphosate-resistant Palmer amaranth and reduce current herbicide costs.  However, the concern for potential 2,4-D volatility damaging sensitive crops nearby could limit adoption.  Therefore, research was conducted to compare volatility of 2,4-D when formulated as an Ester, Amine, or Choline salt. 

The experiment was conducted at the Sunbelt Agriculture Expo in Moultrie, GA during September 9-11, 2010 and again during August 30-September 2, 2011.  Each formulation of 2,4-D at 2 lb ae/A plus glyphosate at 2 lb ae/A was applied on a 90 foot by 90 foot block with treated blocks being at least 800 feet apart.  Treatments were applied to an 88% sand soil with 10 to 20% of the soil covered with plant debris.  Maximum soil temperatures ranged from 99 to 113 F and the entire study area was irrigated the day prior to experiment initiation.  Immediately after application, cotton plants grown off site in 8-inch diameter pots were placed along transects oriented in 8 directions (S, SW, W, NW, N, NE, E, SE) at distances of 5, 10, 20, 40, 80, and 160 feet from each treated block.  Four cotton plants (5 to 7-lf) were placed at each direction-by-distance location and allowed to remain in-field for 48 hours before being removed.  Four additional plants were placed at each location and were allowed to remain on-site for the first 24 hours after application.  A third set of four plants were placed at each direction-by-distance from 24 hours after application through 48 hours after application.  Additionally, two 40-inch tall by 48-inch wide by 12-feet long tunnels covered with plastic were placed over part of the treated areas for each 2,4-D formulation.  For each tunnel, 10 cotton plants were present for the following: full 48 hours, the first 24 hours, or the second 24 hours.  Once various time intervals expired, plants were removed from the experimental area and transported 35 miles to TyTy, Georgia, where they were placed under irrigation and allowed to grow.  Visual injury, cotton heights, and nodes were measured; however, visual injury 21 to 27 day after treatment is reported. 

Field Results:  When plants remained at the experimental site for the entire 48 hours and data was pooled over years and direction, the Ester formulation injured cotton 63, 57, 48, 29, 13, and 2% at distances of 5, 10, 20, 40, 80 and 160 feet, respectfully.  Less than 2% visual injury was detected with the Amine formulation and only at the distances of 5 and 10 feet from the treated area.  No visual injury was detected with the Choline formulation at any distance.  For plants present during the first 24 hours after application only, the Ester formulation injured cotton 58, 55, 44, 24, 8, and 2% at distances of 5, 10, 20, 40, 80, and 160 feet, respectively.  For plants brought into the experimental area 24 hours after initiation and allowed to remain at the site for the following 24 hours, the Ester formulation injured cotton 23, 18, 14, 7, 2, and 0% at distances of 5, 10, 20, 40, 80, and 160 feet, respectively.  No visual injury was observed with the Amine or Choline formulation when plants remained at the site for only a single 24 hour period.

Direction influenced injury observed by the Ester formulation each year.  As expected, the amount of visual injury observed was greatest along transects in which the majority of winds were blowing for each day (range of 0 to11 mph each day and year).

Low Tunnel Results:  Maximum soil temperatures ranged from 125 to 135 F under tunnels each year.  Averaged over years and tunnels, the Ester, Amine, and Choline formulations injured cotton plants remaining under the tunnels for 48 hours 76, 13, and 5%, respectfully.  Plants present for the first 24 hours were injured 71, 4, and 2% by the Ester, Amine, and Choline formulations, respectively, while 47, 1, and 0% injury was observed for plants present during the second 24 hour period.

^TMEnlist is a trademark of Dow AgroSciences LLC.  Components of the Enlist Weed Control System have not received regulatory approvals; approvals are pending.  The information presented is not an offer for sale.  ©2012 Dow AgroSciences LLC.