In Europe, 18 weedy grass species had been confirmed to have biotypes with resistance to herbicides. The most frequent is that of atrazine resistance, with nine resistant biotypes found. These biotypes are mainly resistant because of changes in the D1 protein of photosystem II. All atrazine-resistant biotypes, except that of bristly foxtail, show cross-resistance to s-triazine and as-triazines. From an agriculture point of view, the most important cases of resistance are those found in blackgrass, wild oat, Italian ryegrass, rigid ryegrass, and barnyardgrass. In these species, cross- and multiple resistances were observed due to metabolism or changes in the target protein by genetic mutations or both. These biotypes are extremely difficult to control with alternative herbicides.

Nomenclature: EPSF synthase, 5-enolpyruvylshikimate-3-phosphate synthase; barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG; blackgrass, Alopecurus myosuroides Huds. ALOMY; bristly foxtail, Setaria verticillata (L.) Beauv. SETVE; Italian ryegrass, Lolium multiflorum Lam. LOLMU; rigid ryegrass, Lolium rigidum Gaudin LOLRI; wild oat, Avena fatua L. AVEFA.