USE OF GENE FLOW TO CONTROL DICLOFOP-METHYL RESISTANCE IN ITALIAN RYEGRASS (LOLIUM-MULTIFLORUM)

Flower production and pollen dispersal patterns of diclofop-methyl-sus ceptible and -resistant Italian ryegrass were examined to determine wh ether evolution of herbicide resistance could be controlled by cross-f ertilization between biotypes. The susceptible and resistant ryegrass biotypes differed in timing and abundance of both ovule production and pollen release such that pollen from the susceptible plants had a muc h greater chance of fertilizing the resistant plant population than vi ce versa. Susceptible ryegrass, growing with or without wheat competit ion, produced more than 60% of its seeds before any pollen from resist ant plants was released. In contrast, throughout the course of resista nt plant seed generation, pollen from susceptible plants composed at l east 30% of the total pollen load. These phenological differences, in conjunction with herbicide treatment results for third-generation ryeg rass seedlings, suggest that gene flow can be used to reduce developme nt of diclofop-methyl resistance in ryegrass populations within wheat cropping systems. In a wheat field infested with resistant ryegrass, c essation of herbicide application and sowing of a susceptible ryegrass biotype can be expected to reduce the evolution of diclofop-methyl re sistance by at least 6% per year.