TECHNICAL REPORT Effect of amitrole and 2,4-D applied preplant and pre-emergence in soybean (Glycine max) NADER SOLTANI, 1 * CLARENCE J. SWANTON, 2 ALLAN S. HAMILL, 3 JOSHUA D. VYN 1 and PETER H. SIKKEMA 1 1 Department of Plant Agriculture, Ridgetown Campus, University of Guelph, Ridgetown, 2 Department of Plant Agriculture, University of Guelph, Guelph and 3 Greenhouse and Processing Crops Research Centre, Agriculture and Agri-Food Canada, Harrow, Ontario, Canada There is limited information on the effect of amitrole and 2,4-D ester applied preplant and pre-emergence in soybean (Glycine max L.) in Ontario, Canada. Six field trials were conducted over a 2 year period (2004 to 2005) at three Ontario locations to evaluate the response of soybean to amitrole or 2,4-D ester applied at 14 days preplant (DPP), 7 DPP, 1 day after planting (DAP), and 7 DAP.The application of amitrole resulted in as much as 5.8, 3.9, 1.7, and 1% visible crop injury in soybean at 7, 14, 28, and 56 days after emergence (DAE), respectively. There was no visible injury in soybean with any amitrole treatment at 56DAE, except for amitrole applied at 7 DAP, which caused 1% visible injury in soybean at 2310 g ha -1 . The application of the 2,4-D ester caused 8.3, 9.7, 4.6, and 1.3% visible injury in soybean at 7, 14, 28, and 56 DAE, respectively.The visible injury decreased over time.There was no visible injury in soybean with any of the 2,4-D ester treatments at 56 DAE,except for the 2,4-D ester treatment applied at 7 DAP, which caused 1% visible injury at 1155 g ha -1 and 1.3% visible injury at 2310 g ha -1 . Soybean generally responded similarly to amitrole and 2,4-D ester when applied at 14 and 7 DPP;however,soybean was more tolerant to amitrole compared to 2,4-D ester when applied at 1 or 7DAP.The application of amitrole and 2,4-D ester resulted in no biomass or yield reduction in soybean compared to the weed-free,untreated control at all doses and application timings evaluated. Soybean is tolerant to the preplant and pre-emergence application of amitrole or 2,4-D ester at the doses evaluated. Keywords: 2,4-D, amitrole, biomass, crop injury, Glycine max L. INTRODUCTION Soybean (Glycine max L.) is an increasingly important crop for growers in south-western Ontario, Canada. Effective weed control is an essential component of profitable soybean production (Miller 1974;Green 1991; Dieleman et al. 1996). Soybean germinates and grows slowly under cooler soil temperatures and, thus, is not a good competitor with weeds in such environments (Owen 1998). Therefore, it is important to control emerged weeds with foliar-applied herbicides before soybean emergence, especially in untilled fields, to provide the crop with the necessary competitive edge against weeds (Hartzler 1998). In untilled soybean pro- duction, most of the emerged perennial, biennial, winter annual, and annual weeds before soybean emergence are broad-leaved weeds. In glyphosate-tolerant, untilled soybean production, the growers might be able to improve the control of many broad-leaved weeds and reduce the selection intensity for the development of glyphosate-resistant weeds by including either amitrole or 2,4-D in their preplant herbicide program. Amitrole is a triazole herbicide that, when used at the manufacturer’s recommended rate, can effectively *Correspondence to: Nader Soltani, University of Guelph Ridgetown Campus, Ridgetown, Ontario, Canada N0P 2C0. Email: nsoltani@ridgetownc.uoguelph.ca Received 28 July 2007; accepted 6 November 2007 Weed Biology and Management 8, 139–144 (2008) Journal compilation © 2008 Weed Science Society of Japan No claim to original Canadian government works doi:10.1111/j.1445-6664.2008.00287.x