ABSTRACT
This chapter addresses cross resistance and multiple resistance, best documented in the weed species Lolium rigidum Gaud, and Alopecurus myosuroides Huds. The type of sexual reproduction in a weed species is crucial to the resistance profile a population may develop. In such cases the degree of resistance at the whole plant level, while sufficient to provide resistance at the recommended rates, is much less than conferred by the target site cross resistance mechanisms. The biochemical bases of this so-called "membrane recovery response" are unclear, as is the causal connection between response at the membrane level and resistance at the whole plant level. Resistance, multiple resistance, and cross resistance, endowed by a variety of physiological mechanisms, have appeared rapidly and repeatedly in L. rigidum and in A. myosuroides in response to local agronomic practices. Biotypes of L. rigidum with resistant acetolactate synthase are present in Australia.
