Palmer amaranth is now resistant to 9 herbicide classes, forcing cotton and soybean farmers back to hand-weeding at $23/acre -- a 475% cost increase

Palmer amaranth (Amaranthus palmeri) has evolved resistance to nine different classes of herbicides, including glyphosate, ALS inhibitors, PPO inhibitors, HPPD inhibitors, and most recently glufosinate. A 2025 study in The Plant Cell revealed the mechanism: Palmer amaranth carries dozens to hundreds of extrachromosomal circular DNA (eccDNA) copies of the EPSPS gene, and recent research shows these eccDNA molecules can rearrange to incorporate multiple resistance genes on a single element. This means a single plant can carry resistance to two or more herbicide classes simultaneously, and it passes this resistance to offspring at near-100% frequency. The practical impact on farmers is devastating. In Georgia, before glyphosate resistance emerged, Palmer amaranth management cost about $23 per acre. By 2010, 50-70% of Georgia's cotton acres required hand-weeding at $22-24 per acre on top of herbicide costs, bringing total weed management to $60-80 per acre -- a 475% increase. The weed grows 2-3 inches per day and can reach 6-8 feet tall. A density of just two plants per foot of row can reduce soybean yield by 79% and cotton yield by similar margins. Some Georgia and Arkansas farmers have abandoned fields entirely rather than fight it. University of Georgia researchers have explicitly stated that resistant Palmer amaranth "can run you out of the cotton business." This problem persists because Palmer amaranth evolves resistance faster than the agrochemical industry can develop new herbicides. The eccDNA mechanism is unique in the plant kingdom and allows resistance genes to amplify to hundreds of copies per cell, creating an essentially insurmountable dose-response barrier. Every new mode of action that Palmer amaranth encounters gets defeated more quickly than the last. Meanwhile, the herbicide development pipeline takes 10-12 years from discovery to market, and no genuinely novel mode of action for broadleaf weed control has been commercialized in decades. The weed is dioecious (separate male and female plants), which forces outcrossing and maintains genetic diversity, accelerating the spread of resistance alleles through populations. Hand-weeding is the last resort, but it is unsustainable at scale because agricultural labor is scarce, expensive, and hand-pulled Palmer amaranth can regrow from stem fragments left in the field.