Basis of Atrazine and Mesotrione Synergirm, Biology, and Management of Palmer Amaranth (Amaranthus palmeri S Wats.) in Nebraska Field Corn

Palmer amaranth, a dioecious summer annual weed species, is the most troublesome weed in agronomic crop production systems in the United States. The confirmation of Palmer amaranth resistant to Photosystem (PS) II- and 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibitor in south central Nebraska justify the need to study the biology and management of Palmer amaranth, and to determine the mechanism of atrazine resistance and basis of atrazine and mesotrione synergism in resistant Palmer amaranth biotype from Nebraska. The objectives of this research were to: (1) determine the mechanism of atrazine resistance and basis of atrazine and mesotrione synergism applied in tank-mixture for control of PS II- and HPPD-inhibitor-resistant Palmer amaranth biotype from Nebraska, (2) determine the effect of degree of water stress on growth, fecundity and seed germination of Palmer amaranth biotypes, (3) develop herbicide programs for management of PS II- and HPPD-inhibitor-resistant Palmer amaranth in conventional corn, and (4) develop herbicide programs for management of PS II- and HPPD-inhibitor-resistant Palmer amaranth in glufosinate-, and glyphosate-resistant corn. Increased absorption of mesotrione applied in a tank-mixture with atrazine could be the basis of atrazine and mesotrione synergism for control of susceptible as well as PS II- and HPPD-inhibitor-resistant Palmer amaranth. Atrazine resistance was conferred by enhanced atrazine metabolism, a non-target site resistance mechanism, via glutathione S-transferase (GST) conjugation. The study conducted to evaluate the effect of degree of water stress on Palmer amaranth growth and fecundity suggested that Palmer amaranth has ability to survive water stress conditions and can produce significant amount of seeds with minimum effect on germination. Palmer amaranth at 100, 75, and 50% field capacity (FC) produced similar number of leaves (588 to 670 plant–1 ), growth index (1.1 to 1.4 × 105 cm3 plant–1) and total leaf area (571 to 693 cm 2 plant–1); however, plants at 100% FC achieved maximum height of 178 cm compared to 124 and 88 cm at 75% and 50% FC, respectively. The field experiments conducted for management of Palmer amaranth in conventional, glyphosate, and glufosinate-resistant field corn demonstrated that most PRE followed by POST herbicide programs provided highest Palmer amaranth control, corn yield and net return