The inheritance and linkage of enhanced metabolism-based herbicide cross-resistance was examined in a multiple resistant population of rigid ryegrass. F₁ hybrids between resistant and susceptible populations showed an intermediate response to all the four herbicides tested, with no indication of maternal inheritance. Segregation of F₂ families fitted a single-gene model for resistance to simazine, chlorotoluron, and chlorsulfuron. But there was more than the expected mortality from the low dose of tralkoxydim. Segregating F₂ populations were selected by high rates of each of the four herbicides to create selected F₂ families. Analysis of the response of these families demonstrated that simazine resistance is linked to chlorotoluron resistance. No other herbicide resistances were linked. Therefore, in this population of rigid ryegrass, at least three separate genes are responsible for metabolism-based cross-resistance. This study shows that multiple herbicide resistance in rigid ryegrass is the result of accumulation of a number of different resistance genes.

Nomenclature: Simazine; chlorotoluron; chlorsulfuron; tralkoxydim; rigid ryegrass, Lolium rigidum Gaud. LOLRI.