Poa annua control with norflurazon was different between two field trials in 1999. At the field site with suspected diuron-resistant P. annua, control was less than at the site with known diuron-susceptible P. annua. Diuron-resistant and -susceptible biotypes of P. annua were subjected to a range of norflurazon rates in the greenhouse. The diuron-resistant P. annua biotype (R/S : of 2.5) was collected in 1995 from a Lolium perenne L. seed production field near Tangent, OR. The R/S : for norflurazon in these experiments was 4.8; thus, confirming that diuron-resistant P. annua is resistant to norflurazon.

Nomenclature: Diuron; norflurazon; Poa annua L. POAAN, annual bluegrass; Lolium perenne L. LOLPE, perennial ryegrass.