Vol.4, No.4, 195-205 (2013) Agricultural Sciences doi:10.4236/as.2013.44028 Copyright © 2013 SciRes. Openly accessible at http://www.scirp.org/journal/as/ Control of glyphosate resistant giant ragweed in soybean with preplant herbicides Joanna Follings 1 , Nader Soltani 1* , Darren E. Robinson 1 , François J. Tardif 2 , Mark B. Lawton 3 , Peter H. Sikkema 1 1 University of Guelph Ridgetown Campus, Ridgetown, Canada; * Corresponding Author: soltanin@uoguelph.ca 2 University of Guelph, Guelph, Canada 3 Monsanto Canada Inc., Guelph, Canada Received 1 February 2013; revised 15 March 2013; accepted 20 April 2013 Copyright © 2013 Joanna Follings et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ABSTRACT Giant ragweed was the first glyphosate resistant weed identified in Canada. It is a very compete- tive weed in row crop production and has been found to drastically reduce yields of soybean; therefore, control of this competitive weed is essential. The objective of this study was to de- termine effective control options for glyphosate resistant giant ragweed in soybean with herbi- cides applied preplant. Eighteen herbicide com- binations were evaluated in field studies con- ducted in 2011 and 2012 at five locations with confirmed glyphosate resistant giant ragweed. Glyphosate plus 2,4-D ester or amitrole provided the best control of glyphosate resistant giant ragweed 4 WAA. Glyphosate plus 2,4-D ester pro- vided 98 to 99% control and was equivalent to the weed free check at all locations. Glyphosate plus amitrole provided 90% to 93% control and was equivalent to the weed free check at 4 of 5 locations. Herbicides providing residual activity provided variable control across all locations. Of the herbicides with residual activity evaluated, glyphosate plus linuron provided the best con- trol of glyphosate resistant giant ragweed; how- ever, control was inconsistent across locations and years. Glyphosate plus linuron provided 23% to 99% control and was equal to the weed free check at one location 8 WAA. Keywords: Chlorimuron-Ethyl; Cloransulam-Methyl; Flumioxazin; Herbicide-Resistant Weeds; Imazethapyr; Linuron; Metribuzin; Saflufenacil/Dimethenamid-P; Soybean 1. INTRODUCTION Glyphosate is a nonselective herbicide that inhibits the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) [1,2]. The inhibition of EPSPS stops plants from synthesizing certain aromatic acids that are essen- tial for plant growth [2]. Glyphosate has low environ- mental and mammalian impact. There are various prop- erties of glyphosate that make it a safe herbicide. The EPSPS enzyme can only be found in plants, bacteria and fungi [2]. Therefore, glyphosate has a low toxicity for non-target organisms such as mammals, birds and fish. Glyphosate has limited mobility and is rapidly de- graded in the soil. Glyphosate is rapidly inactivated when applied to the soil due to adsorption to clay and organic matter through the phosphonic acid moiety [3]. In addi- tion to having minimal activity in the soil glyphosate is also unlikely to evaporate from the soil surface due to its low volatility [4]. It is because of these properties that glyphosate does not provide residual control of weeds. There has been rapid adoption of glyphosate resistant crops since the introduction of glyphosate resistant soy- bean in 1996 [5]. The use of glyphosate resistant crops has resulted in changes in crop production systems. Since the introduction of glyphosate resistant crops there has been an increase in conservation tillage and a concomi- tant decrease in the use of herbicides with a different mode of action [6]. These practices impact selection pressure and have facilitated weed population shifts and the selection of glyphosate resistant weeds [6]. It was previously thought that glyphosate resistance was very unlikely to occur in weeds. Some of the reasons for the remote possibility of glyphosate resistant weeds included glyphosate’s unique mode of action, limited uptake from the soil and rapid degradation in the soil [7]. However, there is wide spread glyphosate resistant mo- nocot and dicot weeds in many agricultural producing areas in the world. In 1996 the first glyphosate resistant weed was reported [8]. A population of rigid ryegrass (Loliumrigidum) was found to have a 7- to 11-fold resis-