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Missouri Cotton Farmers Use Old Tactics to Fight a New Battle against Pigweed

February 10, 2012
old school methods
Ryan Wilson, field manager on the Rone farm in Portageville, Mo., demonstrates how a hooded sprayer is used to target weeds between rows of cotton. More farmers are turning to hooded sprayers again to combat glyphosate-resistant Palmer pigweed.  


The following story was written by a University of Missouri student as part of the 2011 Sonja Hillgren/Farm Journal Ag Journalism Field Reporting Institute. Learn more.

By Katie Alaimo
PORTAGEVILLE, Mo. — As a cool autumn night settled on the Missouri Bootheel region, painting the sky with shades of violet and ushering in the first days of October, two slow-moving giants turned on their lights in the distance as they lumbered back and forth through a sea of white.

This steady procession of machine pickers gathering up their bounty of fluffy white cotton bolls revealed little of the daily challenges farmers contend with in preparation for these early days of harvest. Nor of the time, money and effort spent confronting the relatively new enemy of herbicide-resistant Palmer pigweed. 

Resistant Palmer pigweed first reared its head here in 2008 and has since posed a serious threat to the financial success of Missouri cotton farms, experts say. To keep fields clean, farmers are forced to boost control tactics in a costly battle depending on repeated use of expensive herbicides, field labor, extra tillage and crop rotation.

"This is the number one problem for the Mid-South, whether it’s in cotton, corn or soy," said Jason Weirich, assistant professor of plant sciences for the University of Missouri Delta Research Center, in Portageville.
sprayer cotton


A John Deere Hi-Cycle sprayer rolls through a cotton field near Senath, Mo. Before Palmer pigweed developed a resistance to glyphosate, fields required only two or three applications of Roundup on the Hi-Cyclers. Farmers now find it necessary to make multiple trips over their fields throughout the season using a variety of herbicides. Photo: Katie Alaimo

Nature Adapts. Missouri’s cotton farms are some of the state’s best-kept secrets, but the 325,000 acres sprawling across the region are evidence of the industry’s success, due in large part to the development of Monsanto Co.’s genetically modified Roundup Ready cotton seed. This miracle of modern science enabled farmers to coat entire fields with the herbicide glyphosate, commercially known as Roundup, without risking crop damage. Weeds died as cotton thrived. 

But repeated exposure to a single, powerful herbicide can bring about resistance within previously susceptible weeds. Gradually, Darwin’s theory of natural selection is played out as adapted weeds reproduce to replace their non-resistant predecessors.

"It’s called selection pressure," Weirich said. "The plant evolves, if you will. It can rapidly metabolize the herbicide."

pigweed cotton
Palmer pigweed towers over soybean plants near a cotton field in Portageville, Mo. The weeds can grow taller than corn stalks, with thick bases that present a formidable challenge to farmers in the Missouri Bootheel. Photo: Katie Alaimo
Palmer pigweed has proven itself particularly resilient.

Pigweed has separate male and female plants. Consequently, pollen from the male plant can travel long distances through the air to fertilize female flowers, contaminating many fields at once. A single plant can then produce up to two million seeds, making eradication nearly impossible.

In the past, technology has always been ready to confront the new generation of herbicide-tolerant weeds. For example, when the common cocklebur developed an herbicide resistance, Roundup Ready cotton was available for production.

Currently no new variety of herbicide-resistant cotton has been developed. Instead, extension agents like Weirich believe the solution will come with a change of mindset among farmers toward weed control. 

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