Palmer Amaranth (Pigweed) Herbicide Program Suggestions (In Roundup Ready Cotton Only)

The following was sent to us by Scott Brown, Colquitt County, Georgia, Extension Agent

It is not a question of if you will have glyphosate resistant Palmer amaranth in your fields but when. It is going to happen to you sooner or later, so be prepared. Also remember that we have many fields with ALS inhibitor resistant Palmer amaranth in them so precautions should be taken. Use the best yellow and at-plant residual program for your situation.

First Law of Cotton Weed Control: Use a yellow herbicide (Prowl, Treflan or generic equivalent) on every acre.

Second Law of Cotton Weed Control: When in doubt about using a yellow herbicide in cotton refer to First Law of Cotton Weed Control.

RESIDUAL AT-PLANT HERBICIDES: Due to the resistance issue a preemergence at-plant herbicide such as Valor, Reflex, fluometuron (Cotoran), diuron (Direx), or Staple (in some fields) is strongly recommended. Where possible you should irrigate to activate pre-emergence herbicides, if no rain within 5-7 days after planting. Unless you plan to cultivate all residual at-plant herbicides should be broadcast.

Reflex (applied at planting) and Valor (applied preplant) are used interchangeably depending on crop rotation. It is our opinion that only one of these products should be used on the same acreage per crop year. The label does not prohibit the use of both sequentially on the same cropping system but is a UGA recommendation to help prevent PPO resistance development.

Reflex should not be used where sensitive vegetables or other crops will be rotated behind treated cotton (especially brassicas such as: cabbage, collards, turnips, etc). Valor has a short rotational restriction for most crops. Valor can only be used prior to planting: conventionally planted cotton (beds already knocked off) 30 days and a 1 inch rain event; no-till planted cotton 21 days and a 1 inch rain event; strip-till cotton 14 days regardless of rain event). Valent recommends that Valor should always be followed by a preemregence herbicide (Prowl, diuron, cotoran, Staple) at planting. Please check the label. WHEN USING VALOR PLEASE FOLLOW THE VALENT VALOR TANK CLEANOUT PROCEDURE.

Note about STAPLE use: It is our opinion that Staple post plant treatments should not be made if a Staple at plant treatment was applied (Situation 3 below). The label does not prohibit the use of sequential treatments in the same cropping system; this is a UGA suggestion to help prevent ALS inhibitor resistance development.

SITUATION 1.) Fields with major glyphosate resistant Palmer amaranth problems but no ALS inhibitor resistance (if Valor (2 oz/A) is substituted for Reflex then it must be applied at least 14-30 days prior to planting, dependent on cropping system, as outlined above):

Step 1.) Yellow herbicide (use full label rate) PPI or at planting.

Step 2.) Reflex (12-16 oz/A) + diuron (Direx) 4L (16 oz/A) at-plant. Note: Staple at 1.7-2.1 oz/A can be substituted for diuron (Direx) at planting but this eliminates the over-the-top glyphosate + Staple option in Step 3.

Step 3.) Glyphosate (label rate) + Dual Magnum (16-21 oz/A) at 4 leaf stage if no Palmer is emerged. If Palmer is emerged then glyphosate (label rate) + Staple (2.6-3.8 oz/A).

Step 4.) MSMA (32-42 oz/A) + diuron 4L (32 oz/A) + crop oil concentrate. Note: Layby Pro or Suprend may be substituted for diuron. Check label for rates.

NOTE: If Palmer escapes occur earlier than 4 leaf then apply Staple over-the-top beginning at the 2 leaf stage of the crop but before pigweeds are 2 inches tall. This would be between Steps 1 and 2. Do not mix Staple with Dual (or any metolachlor product) or apply within 3+ days either way of a Dual application. Valor is labeled to be used at layby but it is our suggestion to not do so if Reflex was used at planting.

SITUATION 2.) Fields with major glyphosate and ALS inhibitor resistant Palmer amaranth problems:

Same as Situation 1.) but Staple is not an option in Step 2 or Step 3.

SITUATION 3.) Fields with heavy Palmer Amaranth populations but sensitive to glyphosate and ALS inhibitors (if Valor (2 oz/A) is substituted for Reflex then it must be applied at least 14-30 days prior to planting, dependent on cropping system, as outlined above):

Step 1.) Yellow herbicide (use full label rate) PPI or at planting.

Step 2.) Reflex (12-16 oz/A) or diuron (Direx) 4L (16-32 oz/A*) or Cotoran (2-3 pts/A) or Staple at 1.7-2.1 oz/A. If Staple is used at plant then it should not be used in Step 3 or for early escapes.

Step 3.) Glyphosate (label rate) + Dual Magnum (16-21 oz/A) at 4 leaf stage if no Palmer is emerged. If Palmer is emerged then glyphosate (label rate) + Staple (2.6-3.8 oz/A).

Step 4.) MSMA (32-42 oz/A) + diuron 4L (24-32 oz/A*) + crop oil concentrate. Note: Layby Pro or Suprend may be substituted for diuron. Check label for rates.

NOTE: If Palmer escapes occur earlier than 4 leaf then apply Staple over-the-top begining at the 2 leaf stage of the crop but before pigweeds are 2 inches tall. This would be between Steps 1 and 2. Do no mix Staple with Dual (or any metolachlor product) or apply within 3+ days either way of a Dual application.

 *To avoid potential rotational issues do not exceed 3 total pints of diuron (Direx) as a pre and post treatment per year. Further, diuron is potentially more injurious to cotton seedlings at higher rates than Cotoran. On lighter soils lower use rates of diuron should be considered. Diuron should not be used on very sandy soils due to potential injury.

SITUATION 4.) Fields with low-moderate glyphosate and ALS inhibitor sensitive Palmer amaranth problems:

Step 1.) Yellow herbicide (use full label rate) PPI or at planting.

Step 2.) Diuron (Direx) 4L (16-32 oz/A*) or Cotoran (2-3 pts/A) or Staple at 1.7-2.1 oz/A. If Staple is used at plant then it should not be used in Step 3 or for early escapes.

Step 3.) Glyphosate as needed.

Step 4.) MSMA (32-42 oz/A) + diuron 4L (24-32 oz/A*) + crop oil concentrate. Note: Layby Pro or Suprend may be substituted for diuron. Check label for rates. Can layby with Valor, since no Reflex was used.

NOTE: If Palmer escapes occur early then apply Staple over the top beginning at 2 leaf stage of crop but before pigweeds are 2 inches tall. This would be between Steps 1 and 2. Do no mix Staple with Dual (or any metolachlor product) or apply within 3+ days either way of a Dual application.

*To avoid potential rotational issues do not exceed 3 total pints of diuron (Direx) as a pre and post treatment per year. Further, diuron is potentially more injurious to cotton seedlings at higher rates than Cotoran. On lighter soils lower use rates of diuron should be considered. Diuron should not be used on very sandy soils due to potential injury.

Georgia study: why glyphosate stewardship fails to gain ground

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Editor's Note: The following is from a press release received today from Weed Science, a publication of the Weed Science Society of America. The journal publishes articles on the biology, ecology, physiology, or management and control of weeds. It also focuses on herbicides, growth regulators, and related topics. To learn more about the society, please visit: http://www.wssa.net/

Farmers who use herbicide-tolerant crops face a growing challenge from herbicide-tolerant and herbicide-resistant weeds, which are evolving and spreading across cropland.

In the southeastern United States , weeds that can tolerate or resist the popular herbicide glyphosate are threatening cotton crops. In the article “Loss of Glyphosate Efficacy: A Changing Weed Spectrum in Georgia Cotton,” (Weed Science Volume 58, Issue 1, 73-79)

Theodore M. Webster and Lynn M. Sosnoskie examine how the use of glyphosate-tolerant cotton (GTC)—created through genetic modification—has affected weed control issues since its introduction in 1997.

The article is featured in the January–February 2010 issue of Weed Science, published by the Weed Science Society of America. As farmers plant more acres of GTC, they are seeing a change in the most prevalent weeds, which now include varieties that tolerate or resist glyphosate.

The two most troublesome weeds in Georgia cotton are:

• Benghal dayflower, which is tolerant to glyphosate and many of the herbicides used in agronomic crops.
• Palmer amaranth, which has developed resistance to many classes of herbicides including glyphosate.

These weeds typically develop in systems without diverse weed control practices. “The management practices necessary for minimizing the development of herbicide resistance have not been widely implemented,” Webster and Sosnoskie write. They see a need for farmers to work toward greater stewardship, which is the sum of the management decisions and practices used to preserve the utility of a crop trait, and try new or varied weed management strategies that often may be more expensive.

“Agricultural practices aimed at delaying or preventing the development of herbicide resistance are not viewed as being economical in the short term and are not readily used by all growers,” they write. “Because herbicide resistance can spread quickly, indiscriminate use of glyphosate may result in a loss of weed susceptibility for all growers, a tragedy of the commons.” Webster and Sosnoskie identify two reasons why growers are not adopting methods that promote glyphosate stewardship:

• The belief that a new technology will be developed to solve the resistance and tolerance problems, although no new herbicides have been introduced commercially since 1998
• The belief that resistance management strategies will be futile because most farmers think herbicide resistance traits are mobile through pollen-mediated transfer and their fields are affected by other nearby farms

The authors outline several possible incentives for cotton farmers, including industry initiatives to promote better glyphosate stewardship and voluntary government farm programs.

However, they say it likely will take a combination of incentives and a move toward new methods to improve weed control.

“Principles of ecological weed management used in conjunction with herbicide-based weed control systems will likely be an important component of future weed management systems,” Webster and Sosnoskie write.

They also identify several areas for future research: how to limit the development of herbicide-resistant and herbicide-tolerant weeds; whether there is a need to limit the use of glyphosate; factors that may improve control and stewardship of herbicide options; how herbicide resistance traits move across the landscape; what factors could be used to determine a species’ risk of developing resistance; and incentives to help improve herbicide stewardship. The full-text article is available at http://www2.allenpress.com/pdf/WEES_58.1_73-79.pdf.