Glyphosate-resistant Italian ryegrass ( Lolium multiflorum) on rice paddy levees in Japan.

The rapid range expansion of naturalized Italian ryegrass ( Lolium multiflorum Lam.) in farmland is a serious problem in Fukuroi city in Shizuoka Prefecture, Japan. Glyphosate has been used to control Italian ryegrass in the levees of rice paddy fields and wheat fields for ∼20 years, but this weed in Fukuroi city is poorly controlled by glyphosate. In order to elucidate the level of resistance to glyphosate in Italian ryegrass populations, seed bioassays and a foliar application experiment, using seeds collected from 16 wild populations in and around Fukuroi city and from three susceptible cultivars, were conducted. For the susceptible cultivars and one population from a site where glyphosate had not been applied for >10 years, the shoot length in the seed bioassays was greatly suppressed at a glyphosate concentration of 10 mg ai L⁻¹ and no seedling survived after the foliar application of glyphosate at a rate of 2.3 kg ai ha⁻¹. Nine wild populations from levees in the southern part of Fukuroi city showed vigorous shoot growth at a glyphosate concentration of 10 mg ai L⁻¹ and had at least a 78% survival rate after the application of glyphosate at 2.3 kg ai ha⁻¹. Four wild populations from levees in the northern part of Fukuroi city showed a slight suppression of the shoot growth as a result of the glyphosate treatment and their survival rates ranged from 20 to 64%. The results suggested that resistance to glyphosate has evolved in the wild populations of Italian ryegrass that are growing on the levees. This is the first report of a glyphosate-resistant weed in Japan. [ABSTRACT FROM AUTHOR]