DTN staff reporter Emily Unglesbee has ably covered the dicamba-drift crisis over the past couple of years. Her latest is a detailed, three-part series on Palmer amaranth, “possibly the most aggressive weed American farmers have ever faced” (Unglesbee, 2019a).
Palmer amaranth is so scary because of its “spectacular reproductive abilities” and it’s extreme adaptability. This weed is dioecious, meaning that both male and female plants are required to breed, resulting in high genetic diversity and ability to “evolve quickly to thwart control methods, such as herbicides” (Unglesbee, 2019a).
Once fertilized, a single female plant can produce 500,000 to 1 million seeds that can survive harsh Midwestern winters and linger for years in the soil, waiting for their turn in the sun.
While a common pest in farmers’ fields for decades, particularly in the southeast, Palmer amaranth did not truly become a monster until Roundup Ready crops were introduced, along with the new practice of spraying herbicide over the top of growing fields. Farmers used more and more glyphosate, and at first it seemed like it was tough enough to battle back essentially all the weeds.
But, less than a decade after herbicide-resistant crops were introduced in 2005, the first glyphosate-resistant Palmer amaranth populations were identified in Georgia. It spread rapidly – popping up in four more states (Arkansas, North Carolina, South Carolina, Tennessee) in the next year. By 2012 it had spread across the Southeast from Kentucky to Louisiana.
Unglesbee’s special feature is broken up into three parts, each focused on how a particular region is battling herbicide-resistant Palmer amaranth. Here is a summary of the key issues faced by each group of states:
Part 1, “The Newbie States”
Part 2, “The New Frontier States”
Part 3, “The Veteran States”
To top off these regional concerns, Unglesbee also raises the specter of “metabolic resistance.” Weed scientists in Arkansas are “pretty confident” they have some populations of Palmer amaranth that already have metabolic resistance.
What is metabolic resistance and how does it differ from other resistance mechanisms?
Most resistant weeds are undamaged by herbicides because of a change to their genetics that blocks the action of a specific chemical. Metabolic resistance, on the other hand, means that a weed “learns” how to quickly break down, or metabolize, an herbicide’s so that it does little to no harm. Once a weed figures out how to metabolize one herbicide, it can quickly become resistance to multiple different modes of action herbicides.
In other words, the weeds are getting smarter and tougher, and in many of the same ways insects have to several resistance-prone, persistent.
Unglebee’s three part series paints a vivid picture of how this monster weed is affecting farmers around the country. As the latest national map shows (image below), Palmer amaranth has spread almost throughout the entire continental U.S. in less than 15 years.
Many of these populations are already herbicide-resistant, others are on their way towards developing this adaptation.
While herbicide-resistant Palmer amaranth is obviously a thorn in the side of farmers in many key agricultural regions of the country, it is important to recognize that in the places where it has been the biggest challenge, it has forced farmers to adapt in some positive ways.
Today, farmers in the Southeast are returning to integrated weed management systems, a mainstay of sustainable agriculture that is now getting new attention as herbicide resistance has become more prevalent.
Necessity breeds invention, and resistant weeds like Palmer amaranth have forced farmers to pull out a different mix of tools from their toolbox, rather than just loading up the sprayer and hoping for the best. Maybe, in the long run, the saga of Palmer amaranth will begin the long-overdue transition back to multi-tactic, integrated weed management systems and actually reduce overall reliance on herbicides.
Unglesbee, Emily (2019a), “The State of the Pigweed – 1, Palmer Amaranth’s American Roadtrip: The Newbie States,” The Progressive Farmer, Date published: 01/28/2019, Date accessed: 02/01/2019.
Unglesbee, Emily (2019b), “The State of the Pigweed – 2, Palmer Amaranth’s American Roadtrip: The New Frontier States,” The Progressive Farmer, Date published: 01/29/2019, Date accessed: 02/01/2019.
Unglesbee, Emily (2019c), “The State of the Pigweed – 3, Palmer Amaranth’s American Roadtrip: The Veteran States,” The Progressive Farmer, Date published: 01/30/2019, Date accessed: 02/01/2019.