Evaluating for Palmer Amaranth (Amaranthus palmeri) Resistance in Herbicide Tolerant Cropping Systems in the San Joaquin Valley
Palmer Amaranth (Amaranthus palmeri), is one of the most problematic weeds in herbicide tolerant cotton and other row-crop production systems throughout the United States. Palmer amaranth resistance to glyphosate has been confirmed throughout the southeast United States since 2005.Glyphosate is the world’s most widely used post-herbicide due to its broad spectrum of weed control. The extensive use of glyphosate is a result of a combination of the introduction of glyphosate resistant (GR) crop varieties and the widespread adoption of minimum and no-tillage conservation practices (Culpepper, Steckle). Herbicide resistant crop cultivars now dominate corn (Zea mays L.), soybean (Glycine max L.), and cotton (Gossypium hirsutum L.) production in the United States. Since 2012, growers in California’s San Joaquin Valley (SJV) have made field observations concerning the lack of control of Palmer amaranth in glyphosate tolerant cotton and corn. Palmer amaranth populations from the SJV were screened for glyphosate resistance. Field studies that contained natural populations of Palmer amaranth were also screened at various growth stages. In addition growth chamber dose response efficacy studies that contain glyphosate-susceptible (GS) and known GR biotypes of Palmer amaranth were be evaluated using various rates of glyphosate. Chamber and field studies will obtain a glimpse of potential glyphosate levels that would cause a certain level of reduction of weed control and lead toward alternative management approaches.