Weed control and crop tolerance of micro-encapsulated acetochlor applied sequentially in glyphosate-resistant soybean Amit J. Jhala1, Mayank S. Malik2, and John B. Willis2 1Department of Agronomy and Horticulture, University of Nebraska-Lincoln, Lincoln, NE 68583-0915, USA (e-mail: [email protected]); and 2Monsanto Company, 800 North Lindberg Ave., St. Louis, MO 63167, USA. Received 6 December 2014, accepted 21 May 2015. Published on the web 8 June 2015. Jhala, A. J., Malik, M. S. and Willis, J. B. 2015. Weed control and crop tolerance of micro-encapsulated acetochlor applied sequentially in glyphosate-resistant soybean. Can. J. Plant Sci. 95: 973Á981. Acetochlor, an acetamide herbicide, has been used for many years for weed control in several crops, including soybean. Micro-encapsulated acetochlor has been recently registered for preplant (PP), pre-emergence (PRE), and post-emergence (POST) application in soybean in the United States. Information is not available regarding the sequential application of acetochlor for weed control and soybean tolerance. The objectives of this research were to determine the effect of application timing of micro-encapsulated acetochlor applied in tank-mixture with glyphosate in single or sequential applications for weed control in glyphosate- resistant soybean, and to determine its impact on soybean injury and yields. Field experiments were conducted at Clay Center, Nebraska, in 2012 and 2013, and at Waverly, Nebraska, in 2013. Acetochlor tank-mixed with glyphosate applied alone PP, PRE, or tank-mixed with flumioxazin, fomesafen, or sulfentrazone plus chlorimuron provided 99% control of common waterhemp, green foxtail, and velvetleaf at 15 d after planting (DAP); however, control declined to 540% at 100 DAP. Acetochlor tank-mixed with glyphosate applied PRE followed by early POST (V2 to V3 stage of soybean) or late POST (V4 to V5 stage) resulted in ]90% control of common waterhemp and green foxtail, reduced weed density to 52 plants m2 and biomass to 512 g m2, and resulted in soybean yields3775 kg ha1. The sequential applications of glyphosate plus acetochlor applied PP followed by early POST or late POST resulted in equivalent weed control to the best herbicide combinations included in this study and soybean yield equivalent to the weed free control. Injury to soybean wasB10% in each of the treatments evaluated. Micro-encapsulated acetochlor can be a good option for soybean growers for controlling grasses and small-seeded broadleaf weeds if applied in a PRE followed by POST herbicide program in tank- mixture with herbicides of other modes of action. Key words: Acetamide, crop tolerance, post-emergence herbicides, residual herbicides, resistance management, sequential applications, tank-mixture, weed biomass For personal use only. Jhala, A. J., Malik, M. S. et Willis, J. B. 2015. De´sherbage par applications se´quentielles de microcapsules d’ace´tochlor dans le soja re´sistant au glyphosate et tole´rance de la culture au traitement. Can. J. Plant Sci. 95: 973Á981. On utilise l’ace´tochlor depuis de nombreuses anne´es pour lutter contre les mauvaises herbes dans diverses cultures, dont le soja. Les E´tats-Unis ont homologue´depuis peu les microcapsules de cet herbicide a` base d’ace´tamide comme traitement avant les semis, avant la leve´e et apre` s la leve´e pour le soja. Toutefois, on ne posse` de pas d’informations sur le de´sherbage par applications se´quentielles d’ace´tochlor, ni sur la tole´rance du soja a` un tel traitement. La pre´sente recherche devait e´tablir les conse´quences du moment d’application d’un me´lange de microcapsules d’ace´tochlor et de glyphosate en un traitement unique ou en traitement se´quentiel pour lutter contre les mauvaises herbes dans les cultures de soja re´sistant au glyphosate; elle devait aussi pre´ciser l’impact du traitement sur les dommages cause´s au soja et sur le rendement de la culture. Les expe´riences ont e´te´re´alise´es au champ, a` Clay Center (Nebraska), en 2012 et 2013, ainsi qu’a` Waverly (Nebraska), en 2013. L’application du me´lange d’ace´tochlor et de glyphosate avant les semis ou la leve´e, seul ou en combinaison avec de la flumioxazine, du fomesafen ou du sulfentrazone plus du chlorimuron de´truit 99 % de l’amaranthe rugueuse, de la se´taire verte et de l’abutilon quinze jours apre` s les semis. Cependant, le de´sherbage diminue a` 40 % et moins cent jours apre` s les semis. Le me´lange d’ace´tochlor et de glyphosate applique´avant la leve´e, puis au de´but (stade V2 ou V3 du soja) ou a` la fin de la leve´e (stade V4 ou V5) de´truit au moins 90 % de l’amaranthe rugueuse et de la se´taire verte, ce qui re´duit la densite´ Can. J. Plant Sci. Downloaded from pubs.aic.ca by University of Nebraska Lincoln on 09/09/15 des adventices a` un maximum de 2 plants par m2 et leur biomasse a` un maximum de 12 g par m2, avec pour re´sultat un rendement du soja supe´rieur a` 3 775 kg par hectare. Les applications se´quentielles de glyphosate et d’ace´tochlor avant les semis, puis au de´but ou a` la fin de la leve´e assurent une lutte e´quivalente aux meilleures combinaisons d’herbicides examine´es dans le cadre de l’e´tude et de´bouchent sur un rendement en soja e´quivalent a` celui de la parcelle te´moin de´sherbe´e. Les traitements e´value´s causent moins de dix pour cent de dommages au soja. Les microcapsules d’ace´tochlor Abbreviations: AMATA, common waterhemp, Amaranthus rudis Sauer; ABUTH, green foxtail, Setaria viridis (L.) P. Beauv; DAP, days after planting; DBP, days before planting; POST,post- emergence application; PP, preplant application; PRE, pre-emergence application; SETVI, velvetleaf, Abutilon theophrasti Medik Can. J. Plant Sci. (2015) 95: 973Á981 doi:10.4141/CJPS-2014-422 973 974 CANADIAN JOURNAL OF PLANT SCIENCE pourraient constituer une bonne solution de rechange aux cultivateurs de soja pour lutter contre les gramine´es et les dicotyle´dones a` petite graine quand l’application a lieu avant la leve´e et est comple´te´e par un de´sherbage post-leve´e graˆce a` un me´lange compose´de microcapsules et d’herbicides a` mode d’action diffe´rent. Mots cle´s: Ace´tamide, tole´rance des cultures, herbicides post-leve´e, herbicides re´siduels, gestion de la re´sistance, applications se´quentielles, me´lange en citerne, biomasse des mauvaises herbes After corn (Zea mays L.), soybean (Glycine max L. resistant soybean. Weed control can be increased by Merr.) is the most widely grown field crop in the delaying glyphosate applications to allow new weeds United States (United States Department of Agriculture to emerge (Tharp and Kells 1999); however, delayed 2012). Weed management is an important component of application can result in soybean yield reduction as a soybean production to secure higher yields (Norsworthy result of cropÁweed competition during the initial growth 2003). Application of pre-emergence (PRE) and/or post- stage. Therefore, tank-mixing glyphosate and acetochlor emergence (POST) herbicides is the primary method may provide better weed control compared with either for weed control in soybean (Riar et al. 2013). The herbicide applied alone. There is no information in the acetamides are widely used in corn, cotton (Gossypium scientific literature on the effect of sequential applica- hirsutum L.), sorghum [Sorghum bicolor (L.) Moench], tions of glyphosate plus acetochlor in respect to weed and soybean for control of annual grasses and small- control and soybean tolerance. seeded broadleaf weeds. Acetochlor is listed as the fourth Glyphosate has been used extensively for POST weed most commonly used herbicide in the United States control in glyphosate-resistant crops for more than a by the United States Environmental Protection Agency decade; however, herbicide programs depending only (USEPA) (Grube et al. 2011). The solubility of aceto- on glyphosate resulted in the evolution of glyphosate- chlor in water is 223 mg L1 at 258C and generally resistant weeds (Owen and Zelaya 2005). Therefore, provides 3 to 5 wk of residual weed control, depending on soybean growers are tank-mixing herbicides with glypho- application rate, soil type, weed pressure, and weather sate to expand the utility of glyphosate-based herbicide conditions (Parker et al. 2005). Acetochlor is primarily programs (Young 2006). In addition, tank-mixing her- absorbed by emerging plant shoots and seedling roots, bicides with different modes of action is one of the and is xylem-transported by acropetal movement (Boger methods to control herbicide-resistant weeds (Beckie 2006; et al. 2000). Susceptible grass weeds that do emerge Norsworthy et al. 2012; Mellendorf et al. 2013). For appear twisted and malformed with leaves tightly rolled example, acetochlor has been premixed with atrazine in the whorl that cannot unfurl properly, while broadleaf (Anonymous 2012a,b) as well as with clopyralid and seedlings may have slightly cupped or crinkled leaves, flumetsulam for PRE weed control in corn (Anonymous especially under cold conditions (Weisshaar and Boger 2011b). For personal use only. 1987). Weed control programs that include herbicides with Acetamide herbicides are usually applied PRE in corn multiple modes of action are needed due to the rapid and soybean; however, micro-encapsulated acetochlor increases in the number of herbicide-resistant weeds has been recently registered for PP, PRE, and POST (Kaur et al. 2014). Soybean growers can benefit from application in soybean (Anonymous 2011a). Acetochlor micro-encapsulated acetochlor applied PRE or POST in is sometimes applied 14 to 60 d before planting as an early tank-mix with other registered herbicides that provide preplant (EPP) application (Bunting et al.