Weed Technology 2016 30:639–647 The Addition of Dicamba to POST Applications of Quizalofop-p-ethyl or Clethodim Antagonizes Volunteer Glyphosate-Resistant Corn Control in Dicamba-Resistant Soybean Matthew G. Underwood, Nader Soltani, David C. Hooker, Darren E. Robinson, Joseph P. Vink, Clarence J. Swanton, and Peter H. Sikkema* Two studies consisting of six field experiments each were conducted at three locations in southwestern Ontario, Canada, in 2014 and 2015 to evaluate the possible antagonism when dicamba was added to quizalofop-p-ethyl or clethodim for the control of volunteer glyphosate-resistant (GR) corn. At 4 wk after application (WAA), quizalofop-p-ethyl at 24, 30, or 36 g ai ha  1 provided 88, 94, and 95% control of volunteer GR corn, respectively. The addition of dicamba at 300 or 600 g ae ha  1 to quizalofop-p-ethyl (24 g ha 1 ) reduced the activity of quizalofop-p-ethyl on volunteer GR corn by 12 and 20%. At 4 WAA, clethodim at 30, 37.5, and 45 g ai ha  1 provided 85, 91, and 95% control of volunteer GR corn, respectively. The addition of dicamba at 300 or 600 g ha 1 to clethodim (30 g ha  1 ) resulted in antagonism, causing a reduction in volunteer GR corn by 12 and 11%, respectively. In general, there was greater antagonism when the high rate of dicamba was tank- mixed with the lower rate of the graminicide. There was no antagonistic effect on soybean yield by tank-mixing dicamba with either graminicide at all rates evaluated. Based on these results, volunteer GR corn can be controlled effectively by increasing the rate of the graminicide when tankmixed with dicamba. Nomenclature: Clethodim; dicamba; quizalofop-p-ethyl; volunteer corn, Zea mays L.; soybean, Glycine max (L.) Merr. Key words: Antagonistic, density, glyphosate resistance, graminicide, injury, tankmix, weed control, weed emergence, yield. Dos estudios que consistieron de seis experimentos de campo cada uno fueron realizados en tres localidades en el suroeste de Ontario, Canada, en 2014 y 2015, para evaluar el posible antagonismo cuando se agrega dicamba a quizalofop-p-ethyl o clethodim para el control de ma´ ız resistente a glyphosate (GR) voluntario. A 4 semanas despu´ es de la aplicaci´ on (WAA), quizalofop-p-ethyl a 24, 30, ´ o 36 g ai ha 1 brind´ o 88, 94, y 95% de control de ma´ ız GR voluntario, respectivamente. La adici´ on de 300 ´ o 600 g ae ha 1 dicamba a quizalofop-p-ethyl (24 g ha 1 ) redujo la actividad de quizalofop-p-ethyl en ma´ ız GR voluntario en 12 y 20%. A 4 WAA, clethodim a 30, 37.5, y 45 g ai ha 1 brind´ o 85, 91, y 95% de control de ma´ ız GR voluntario, respectivamente. La adici´ on de 300 ´ o 600 g ha 1 dicamba a clethodim (30 g ha 1 ) result´ o en antagonismo, el cual caus´ o una reducci´ on en el control de ma´ ız GR voluntario de 12 y 11%. En general, hubo un mayor antagonismo cuando se mezcl´ o en tanque la dosis alta de dicamba con la dosis baja del graminicida. No hubo ning´ un efecto antag´ onico sobre el rendimiento de la soja al mezclar en tanque dicamba con cualquiera de los dos graminicidas a las dosis evaluadas. Con base en estos resultados, el ma´ ız GR voluntario puede ser controlado efectivamente al incrementar la dosis del graminicida en la mezcla en tanque. Weed management is a critical requirement for profitable soybean production (Vollmann et al. 2010), the most widely produced annual crop in the province of Ontario, Canada. Soybean was seeded on 1.2 million ha in Ontario in 2014, which accounted for 50% of the total grain and oilseed crops grown, accounting for 3.8 billion tonnes produced valued at nearly $2 billion (Kulasekera 2015). Weed management is an ongoing challenge for Ontario farmers who must contend with difficult- to-control weeds, glyphosate-resistant (GR) weeds, and volunteer corn. Although corn is considered a crop when grown in monoculture, it can be viewed as a highly competitive weed when present in soybean. Volunteer corn is one of the most DOI: 10.1614/WT-D-16-00016.1 * Graduate Student, Adjunct Professor, Assistant Professor, Associate Professor, Technical Lead, Professor, and Professor, Department of Plant Agriculture, University of Guelph, 120 Main Street East, Ridgetown, ON, Canada N0P 2C0. Corresponding author’s E-mail: soltanin@uoguelph.ca Underwood et al.: Volunteer GR corn control in DR soybean  639