Proactive Versus Reactive Management of Glyphosate-Resistant or -Tolerant Weeds1

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 hypothetical 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 provides 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; Cmanaging, economic cost of managing resistance; Cresistance, economic cost of resistance; GR, glyphosate resistant; NPV, net present value; NPVproactive, net present value of proactive resistance management; NPVreactive, net present value of reactive management; PPO, protoporphyrinogenoxidase; Rresistance, net economic return once resistance has occurred; Rwithout, net economic return without resistance; Tcritical, time at which net present value for reactive and proactive resistance management are equal; Tresistance, time at which resistance occurs.