Selection Pressure, Shifting Populations, and Herbicide Resistance and Tolerance

BRAD HANSON, UC Cooperative Extension Weed Specialist, Department of Plant Sciences, University of California, Davis; ALBERT FISCHER, Professor, Department of Plant Sciences, University of California, Davis; ANIL SHRESTHA, Associate Professor, Department of Plant Sciences, California State University, Fresno; MARIE JASIENIUK, Associate Professor, Department of Plant Sciences, University of California, Davis; ED PEACHEY, Department of Horticulture, Oregon State University, Corvallis; RICK BOYDSTON, USDA Agricultural Research Service, Prosser, Washington; TIM MILLER, Mount Vernon Research and Extension Center, Mount Vernon, Washington; and KASSIM AL-KHATIB, UC Cooperative Extension Weed Specialist and Director of Statewide IPM Program, Department of Plant Sciences, University of California, Davis Selection Pressure, Shifting Populations, and Herbicide Resistance and Tolerance Herbicides can provide impressive levels of weed control in many crop and noncrop situations; however, not all weedy species are equally controlled, due to varying levels of natural tolerance or evolution of herbicide-resistant weed biotypes. Herbicides impose a great degree of selection pressure on weed populations— and if the same herbicide or herbicides with the same mode of action are used repeatedly, herbicide-resistant or herbicide-tolerant species can build up in the population after several generations (fig. 1).