Glyphosate-Resistance Watch: Palmer Amaranth

April 15, 2010 06:29 AM
Southern farmers call the early stages of a Palmer pigweed infestation "the red tide.” The weed seedlings come on so fast the row middles appear to take on a red hue before your eyes.
They will also tell you the weed is about as stubborn as a pig going through a loading chute backwards. Once thought to be mechanically spread by seed, recent research from Georgia indicates the glyphosate resistant trait can by moved by pollen too.
The first resistant glyphosate populations were discovered in Georgia and North Carolina in 2005. Since that time, Arkansas, Tennessee, New Mexico, Alabama, Mississippi and Missouri have also confirmed glyphosate resistant populations. By 2011, weed specialist Stanley Culpepper expects glyphosate resistant Palmer will likely infest all Georgia cotton-producing counties.
Palmer amaranth is an erect summer annual that may grow to six feet tall or more. Larry Steckel, University of Tennessee weed scientist, says he has seen Palmer amaranth grow a foot in thirteen days after receiving full rates of Roundup PowerMax over the top.
The plant is found in cultivated and fallow fields, gardens and roadsides throughout the southern U.S., from southern California to Virginia. It has vegetative characteristics that make it easy to confuse with other pigweed species. Palmer amaranth leaves have no hair and have prominent white veins on the undersides. It tends to grow faster and is more competitive than any other pigweed species. The weed has male and female plants, allowing for greater genetic variability and adaptability. One plant is estimated to produce a million seeds per plant.
Weed studies at the University of during 2008 and 2009 determined that Palmer amaranth has three potential weaknesses:  
  1. A shallow emergence depth
  2. Short seed life in soil
  3. A significant light requirement needed for germination. 
Each of these weaknesses can be manipulated to improve control by herbicide systems.  
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