ABSTRACT
The main factors responsible for major herbicide-resistant (HR) weed problems worldwide are (1) multiple applications of highly efficacious herbicides with the same mode of action; (2) annual weed species that occur at high densities, are widely distributed, are genetically variable, are prolific seed producers, and have efficient gene (seed or pollen) distribution systems; and (3) simple cropping systems that favor a few dominant weed species (Owen, 2001a; Thill and Lemerle, 2001). The global occurrence of HR weed biotypes is continually updated (Heap, 2005). Resistance-prone species include annual ryegrass (Lolium rigidum Gaudin), blackgrass (Alopecurus myosuroides Huds.), wild oat (Avena spp.), common lambsquarters (Chenopodium album L.), kochia [Kochia scoparia (L.) Schrad.], and Amaranthus spp. The majority of cases of field-selected herbicide resistance are conferred by a single gene with a high degree of dominance (Powles et al., 1997), although there are numerous exceptions.
