Biology, gene flow, and management of glyphosate-resistant common waterhemp (Amaranthus rudis Sauer) in Nebraska

Common waterhemp is the most troublesome weed in the midwestern United States. Growers from Nebraska reported failure to control common waterhemp following sequential applications of glyphosate in glyphosate-tolerant corn and soybean, which led to moderate to severe yield loss; justifying the need to confirm resistance and study the biology and management of common waterhemp. The objectives of this research were: 1) to confirm the presence of glyphosate-resistant (GR) common waterhemp biotypes in Nebraska and to evaluate their sensitivity to herbicides belonging to alternative sites-of-action; 2) to evaluate the response of common waterhemp to water stress; 3) to quantify pollen-mediated gene flow from GR common waterhemp under field conditions; and 4) to evaluate different herbicide programs for season-long control of glyphosate-resistant common waterhemp in glyphosate-tolerant soybean. Greenhouse dose-response studies conducted to evaluate the response of suspected GR common waterhemp biotypes collected from seven eastern Nebraska counties (Antelope, Dodge, Fillmore, Lancaster, Pawnee, Seward, and Washington) revealed that the biotypes were 3- to 39-fold resistant to glyphosate. The GR biotypes also showed a reduced sensitivity to acetolactate synthase (ALS)-inhibiting herbicides (chlorimuron-ethyl, imazamox, imazaquin, imazethapyr, and thifensulfuron-methyl). Experiments conducted to evaluate the response of common waterhemp to water stress suggested that degree and duration of water stress can adversely affect the growth and seed production of common waterhemp. Highest plant height (≥ 150 cm), growth index (≥ 3.8 x 105 cm3), and seed production (> 34,000 seeds plant-1) were recorded with 100% pot water content applied at 2-d intervals. Pollen-mediated gene flow studies from GR to GS biotypes were conducted under field conditions using a Nelder wheel design. Frequency of gene flow was found to be highest (up to 0.77) at the closer distances (0 to 0.1 m); whereas gene flow frequency declined by 50% at < 2.5 m and 90% at distances < 90 m from the pollen source. Field experiments conducted for management of GR common waterhemp in soybean showed that preemergenece (PRE) followed by postemergence (POST) herbicide programs with multiple sites-of-action provided season-long control of GR common waterhemp and resulted in the highest soybean yield compared to the POST-only herbicide programs.