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.
