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Mueller et al 2005 - Weed Technology 2005 Volume 19:924933...

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924 Weed Technology. 2005. Volume 19:924–933 Proactive Versus Reactive Management of Glyphosate-Resistant or -Tolerant Weeds 1 THOMAS C. MUELLER, PAUL D. MITCHELL, BRYAN G. YOUNG, and A. STANLEY CULPEPPER 2 Abstract: The value of glyphosate has been compromised in some fields where weed populations have developed resistance or tolerant species increased. Three case studies related to reduced control from glyphosate are: (1) a weed population that has become resistant to glyphosate, with horseweed in Tennessee as an example; (2) a weed population increases due to lack of control in ‘‘glyphosate only’’ systems, with tropical spiderwort in Georgia cotton used as an example; and (3) the hypo- thetical resistance of common waterhemp to glyphosate in Illinois. For each of these case studies, an economic analysis was performed using a partial budget approach. This economic analysis pro- vides the cost of control to the farmer when glyphosate fails to control these weeds and gives a critical time in years to compare different glyphosate resistance management philosophies (applicable only before resistance has evolved). The cost of glyphosate-resistant horseweed in cotton-soybean- corn rotation in Western Tennessee was calculated to be $30.46/ha per year. The cost of tropical spiderwort in cotton in southern Georgia was calculated to be $35.07/ha per year. The projected cost if common waterhemp were to develop glyphosate resistance in a corn-soybean rotation in southern Illinois was projected to be $44.25/ha per year, and the critical time was determined to be greater than 20 yr, indicating that a resistance management strategy would extend the value of glyphosate- resistant crops. Nomenclature: glyphosate; common waterhemp, Amaranthus rudis Sauer. # 3 AMATA; horseweed, Conyza canadensis L. Cronq # ERICA; tropical spiderwort, Commelina benghalensis L. # COMBE. Additional index words: economic analysis, herbicide cost, herbicide resistance. Abbreviations: ALS, acetolactate synthase; C managing , economic cost of managing resistance; C resistance , economic cost of resistance; GR, glyphosate resistant; NPV, net present value; NPV proactive , net present value of proactive resistance management; NPV reactive , net present value of reactive management; PPO, protoporphyrinogenoxidase; R resistance , net economic return once resistance has occurred; R without , net economic return without resistance; T critical , time at which net present value for reactive and proactive resistance management are equal; T resistance , time at which resistance occurs. INTRODUCTION From several perspectives, glyphosate is a unique molecule of tremendous commercial importance. After more than 25 yr of sales, glyphosate is the world’s larg- est selling and fastest-growing agrochemical (Baylis 2000). The original discovery was based on the desire of researchers at Monsanto to control perennial weeds (Magin 2003), but this molecule has become a preferred 1 Received for publication October 29, 2004, and in revised form April 1, 2005.
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