Selection Pressures for Diclofop‐Methyl Resistance and Germination Time of Italian Ryegrass

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Abstract
To formulate strategies for controlling herbicide resistance in Italian ryegrass (Lolium multiflorum Lam.), the selective pressures acting on wild Italian ryegrass populations to encourage evolution of resistance must be identified. Field and greenhouse experiments were conducted in 1990 to 1992 to compare the seedling emergence patterns of diclofopmethyl‐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 September, 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 three times as many viable resistant seeds as susceptible seeds. In a corroborating experiment, we found that Italian ryegrass seeds with a greater degree of dormancy were more likely to be herbicide resistant than those with a lesser degree of dormancy. Our results support the hypothesis that the timing of field tillages and herbicide use is a selective force for increasing seed dormancy and thus for changing seedling emergence patterns as herbicide‐resistant Italian ryegrass weed populations evolve. The linkage between changes in seed dormancy and herbicide resistance traits has ecological and agronomical implications.

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