Palmer amaranth’s aggressive growth habit, extended period of seedling emergence, high water use efficiency, and prolific seed production make it the most problematic weed in agronomic cropping systems in the United States.
The increased adoption of reduced tillage practices, continuous reliance on single mode-of-action post-emergence herbicides such as glyphosate, and decreased use of soil-applied residual herbicides have favored herbicide selection pressure in Palmer amaranth. In Nebraska, Palmer amaranth populations resistant to 5-enol-pyruvylshikimate-3-phosphate synthase (EPSPS)-inhibitor, and multiple resistance to photosystem (PS) II- and hydroxyphenylpyruvate dioxygenase (HPPD)-inhibitors have been confirmed.
Glyphosate is the most widely used agricultural pesticide globally and glyphosate-resistant Palmer amaranth populations have been reported in 26 states due to the continuous and repeated use of glyphosate for weed control. In Nebraska glyphosate-resistant Palmer amaranth had previously been confirmed in the southwest (CropWatch Kruger et al. May 1, 2015).
In 2016, control failure of a Palmer amaranth population following sequential glyphosate applications was observed in a grower’s field under glyphosate-resistant corn-soybean rotation in Thayer County in south-central Nebraska. Palmer amaranth seeds were collected from that field and greenhouse dose response studies were conducted to confirm its suspected resistance to glyphosate and to determine the level of resistance.