Abstract
<p>In Nebraska, 5-enolpyruvyl shikimate-3-phosphate synthase (EPSPS) as well as acetolactate synthase (ALS)-inhibitor-resistant weeds occur in many soybean fields where herbicides from these modes-of-action have been frequently used in the past. Currently, the protoporphyrinogen oxidase (PPO)-inhibitors are the only effective herbicides for POST control of both glyphosate- and ALS-inhibitor-resistant weeds in soybean. Greenhouse experiments were conducted in 2014 to evaluate the efficacy of PPO-inhibitors applied POST for the control of three glyphosate-resistant (GR) weeds and potential for soybean injury, when applied at two growth stages. All herbicide treatments controlled 10- and 20-cm tall GR common waterhemp ≥ 95% at 21 DAT. GR giant ragweed and kochia were controlled 86 to 99% when treated at 10-cm height and 78 to 92% at 20-cm height by 21 DAT. Herbicide treatments reduced shoot biomass in the three GR weeds 88 to 100% when treated at 10-cm height and 73 to 100% when treated at 20-cm height, at 21 DAT. Soybean injury and shoot biomass data revealed that acifluorfen and lactofen were more injurious (≥ 17%), whereas fomesafen, and fomesafen plus glyphosate were relatively safer (&lt; 10% injury). Overall, fomesafen and fomesafen plus glyphosate caused least injury to soybean and were more effective in controlling GR common waterhemp, giant ragweed, and kochia compared with acifluorfen and lactofen.</p>
Highlights
In the United States, glyphosate-resistant (GR) soybean [Glycine max (L.) Merr.], cotton (Gossypium hirsutum L.), and corn (Zea mays L.) were introduced for commercial cultivation in 1996, 1997, and 1998, respectively
Greenhouse experiments were conducted in 2014 at the University of Nebraska-Lincoln, Lincoln, NE to evaluate the control of three GR weed species and potential for GR soybean injury in response to different protoporphyrinogen oxidase (PPO)-inhibitors applied POST
Selected GR weeds were treated with the herbicides when 10- and 20-cm tall while soybean was treated at 3-and 5-trifoliate stages
Summary
In the United States, glyphosate-resistant (GR) soybean [Glycine max (L.) Merr.], cotton (Gossypium hirsutum L.), and corn (Zea mays L.) were introduced for commercial cultivation in 1996, 1997, and 1998, respectively. Weed management in the GR crops became entirely dependent on the POST applications of glyphosate only. As of 2015, 15 weed species have been confirmed resistant to glyphosate in the United States (Heap, 2015). In the midwestern United States, glyphosate has been intensively used on a vast acreage predominantly as a preplant and POST herbicide in GR corn and soybean (Duke, 2005; Owen & Zelaya, 2005). The occurrence of GR weeds is a serious concern, for midwestern soybean growers, because of limited effective POST herbicide options. Most of the GR weeds in the midwest are resistant to ALS-inhibitors such as chlorimuron-ethyl, imazamox, imazaquin, imazethapyr, thifensulfuron-methyl (Legleiter & Bradley, 2008; Sarangi et al, 2015)
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