Palmer amaranth (Amaranthus palmeri S. Wats.) is a C₄ dioecious species (separate male and female plants) native to the southwestern United States. 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 (Figures 1, 2).

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.

Dose response analysis was performed to estimate the ED₉₀ (effective dose required to control 90% population) values for the glyphosate-resistant Palmer amaranth compared with two glyphosate-susceptible Palmer amaranth populations. Based on the ED₉₀ value, glyphosate-resistant Palmer amaranth exhibited a 37- to 40-fold level of resistance depending on the susceptible Palmer amaranth population being used for comparison (Figure 3). For example, glyphosate─susceptible populations were controlled 90% at a glyphosate rate of 9-10 fl oz/acre, while 365 fl oz/acre was required for glyphosate-resistant Palmer amaranth control. This is 17 times the labeled rate (22 fl oz).

The evolution of glyphosate-resistant Palmer amaranth in south-central Nebraska is of great concern as glyphosate is the most commonly used herbicide in glyphosate-resistant corn and glyphosate-resistant soybean and would not be effective in controlling this weed anymore. Moreover, a Palmer amaranth population with resistance to PS II- and HPPD-inhibitors was reported in a seed corn field within 10 miles proximity (CropWatch Jhala et al. October 4, 2013).

Resource

For more information see Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri) in Nebraska: Confirmation, EPSPS Gene Amplification, and Response to POST Corn and Soybean Herbicides by Parminder S. Chahal, Vijay K. Varanasi, Mithila Jugulam, and Amit J. Jhala, in Weed Technology 2017 31:80-93, Weed Science Society of America. The abstract is publicly available and the full paper is available to journal subscribers.