ABSTRACT
Herbicides have been used to control weeds selectively in crops that were naturally tolerant to specific herbicides since the mid-1940s. Herbicide-resistant crops (HRCs) may be produced either by breeding for resistance through selection or by using biotechnology to generate HRCs. This chapter discusses the potential weediness and possible spread of volunteer and escaped HRC plants in agricultural and natural ecosystems. HRC cultivars are less or equally fit compared to existing crop cultivars, and in the absence of the herbicide selection pressure likely will be equally or less invasive as compared to existing cultivars. Gene transfer can occur between HRC and herbicide-susceptible crop cultivars, weeds growing in areas adjacent to fields containing HRCs, and between volunteer HRC plants and weedy relatives growing outside field boundaries. These herbicide-resistant weeds may spread to new areas, including fields where HRCs are or will be grown. Biological factors affect the evolution of resistance and include fitness, gene flow, and inheritance.
