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Managing Pigweed in Cotton: Midsouth and Southeast

Posted 4/14/2016
Palmer amaranth (Amaranthus palmeri), also known as Palmer pigweed, is an aggressive weed that spreads easily and has evolved resistance to multiple herbicide modes of action, including dinitroanilines (pendimethalin), ALS inhibitors (pyrithiobac), photosystem II inhibitors (atrazine), HPPD inhibitors (mesotrione), and glycine (glyphosate) herbicides.

Palmer Pigweed

Palmer amaranth (Amaranthus palmeri). also known as Palmer pigweed, is an aggressive weed that spreads easily and has evolved resistance to multiple herbicide modes of action, including dinitroanilines (pendimethalin), ALS inhibitors (pyrithiobac), photosystem II inhibitors (atrazine), HPPD inhibitors (mesotrione), and glycine (glyphosate) herbicides. A single female Palmer pigweed plant can produce 500,000 to 1 million seeds. Allowing one plant to produce seeds can lead to a significant weed infestation the following season. However, Palmer pigweed seeds are relatively short-lived in the soil, with about 80% rendered non-viable in 3 years. By following season-long weed management strategies, including residual herbicides, multiple modes of action, and multiple herbicide applications, cotton growers are steadily reducing populations of Palmer pigweed in their fields.

Management Options

“Cotton fields have to be clean and weed-free, when cotton comes up,” says Stanley Culpepper, weeds specialist, University of Georgia. Culpepper urges growers to read herbicide labels and use the appropriate rate of residual herbicides for the soil types and environmental conditions to control weeds while avoiding crop injury. “Sometimes the full recommended rates are not the appropriate rates on some of our soils. We are getting 10:1 injury complaints over complaints about herbicides not controlling weeds. Growers should contact their county Extension agent or attend area cotton meetings to learn the appropriate preemergent (PRE) herbicide rates to use,” he says. (S. Culpepper, personal communication, December 2013).

“The critical thing is to do something to control weeds before they emerge or before they get too large to control,” says Alan York, Professor Emeritus, North Carolina State University. (A. York, personal communication, December 2013).

Effective burndown herbicide applications prior to planting to control all emerged weeds should be followed by timely postemergence (POST) applications of residual herbicides to prevent weeds from emerging. Lay-by applications may be necessary to control weed escapes. Timing herbicide applications so their effectiveness intervals overlap can reduce the chances of weeds emerging.

With an effective burndown and PRE herbicide program, including residual herbicides, growers could be tempted to delay the first POST application. That delay, say southern weed scientists, could be a mistake. Residual herbicide effectiveness begins to decline after about 21 days. POST applications should be timed to overlap their anticipated effectiveness. If Warrant® Herbicide was applied PRE, a second application may be made POST. Only two applications of 3 pints/acre can be made in the same season. Two to three weeks later, growers should follow up with another residual, like Dual Magnum®, Warrant® Herbicide or Pyrimax®.

If Palmer pigweeds escape herbicide controls, growers need to chop or pull the weeds to prevent adding herbicide resistant weed seeds to the seed bank.

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Table 1. Herbicide Recommendations for Genuity® Roundup Ready® Flex Cotton for the Midsouth.

Practice Products
Burndown Roundup PowerMAX® or Roundup PowerMAX® II + dicamba
Preplant Roundup PowerMAX® or Roundup PowerMAX® II + Reflex®
PRE Gramoxone® SL + Roundup PowerMAX® or Roundup PowerMAX® II + Cotoran® 4L or Caporal®
POST Roundup PowerMAX® or Roundup PowerMAX® II + Warrant® Herbicide or Dual Magnum®; or Caporal® + MSMA + Warrant® Herbicide
Lay-by Roundup PowerMAX® or Roundup PowerMAX® II or MSMA + Rowel® Herbicide, Direx® 4L, or Valor®
Hoods Gramoxone® SL + Direx® 4L

Table 2. Herbicide Recommendations for Genuity® Roundup Ready® Flex Cotton for the Southeast.

1 Reduced Tillage
Practice Products
Burndown Roundup PowerMAX® or Roundup PowerMAX® II + dicamba or 2,4-D
Preplant1 Roundup PowerMAX® or Roundup PowerMAX® II + Warrant® Herbicide, Rowel® Herbicide, or Valor®
PRE Gramoxone® SL + Reflex® + Direx® 4L
POST Roundup PowerMAX® or Roundup PowerMAX® II + Warrant® Herbicide or Dual Magnum®
Lay-by Roundup PowerMAX® or Roundup PowerMAX® II + Direx® 4L
Hoods Gramoxone® SL + Direx® 4L, Rowel® Herbicide, or Valor®

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