SELECTION PRESSURES FOR DICLOFOP-METHYL RESISTANCE AND GERMINATION TIME OF ITALIAN RYEGRASS

To formulate strategies for controlling herbicide resistance in Italia n ryegrass (Lolium multiflorum Lam.), the selective pressures acting o n wild Italian ryegrass populations to encourage evolution of resistan ce must be identified. Field and greenhouse experiments were conducted in 1990 to 1992 to compare the seedling emergence patterns of diclofo pmethyl-resistant and susceptible Italian ryegrass. Timing of seedling emergence in full sunlight differed for the two biotypes. In general, the herbicide-susceptible biotype emerged earlier than the resistant biotype. Greatest emergence for the susceptible biotype was in August and September, the period prior to typical winter cereal crop sowing. Soil shading enhanced resistant seedling emergence during August and S eptember, suggesting that soil temperature plays an important role in regulating seed germination of this biotype. Before current-year seed shed, the seed bank in the experimental plots was found to contain thr ee times as many viable resistant seeds as susceptible seeds. In a cor roborating experiment, we found that Italian ryegrass seeds with a gre ater degree of dormancy were more likely to be herbicide resistant tha n those with a lesser degree of dormancy. Our results support the hypo thesis that the timing of field tillages and herbicide use is a select ive force for increasing seed dormancy and thus for changing seedling emergence patterns as herbicide-resistant Italian ryegrass weed popula tions evolve. The linkage between changes in seed dormancy and herbici de resistance traits has ecological and agronomical implications.