Annual Grass Control in Established Mohave Valley Alfalfa Using Thiazopyr (Visor), Trifluralin (Treflan) and Norflurazon (Zorial)

Annual grasses such as wild barley and Mexican sprangletop are of economic concern to alfalfa growers in western Arizona. Thus, granular formulations of thiazopyr (Visor 2.5G), trifluralin (Treflan TR-10), and norflurazon (Zorial 5G) herbicides were evaluated for annual grass control in an established alfalfa field in Mohave County when applied after the first hay cutting in early spring and prior to irrigation. Visor 2.5G was applied in spring 1998 and 1999 at application rates of 0.25, 0.38, and 0.5 lb a.i./acre. Treflan TR-10 was applied at 2.0 lb a.i./acre in early spring or in both early spring and late summer. Visor 2EC was applied at 0.5 lb a.i./acre in spring 1998 but was replaced with Zorial 5G at 2.0 lbs a.i./acre in spring 1999. In 1998, good wild barley control (81%) resulted from a spring application of Visor 2.5G at 0.50 lb a.i./acre. The Visor 2EC formulation at 0.5 lb a.i./acre provided poor weed control (40%) due to the retention of herbicide on the alfalfa foliage and lack of incorporation into the soil by irrigation. In 1999, fair wild barley control (73 to 79%) resulted from spring applications of Visor 2.5G at 0.50 lb a.i./acre and Zorial 5G at 2.0 lb a.i./acre. During summer 1998, good Mexican sprangletop control (83-93%) resulted from Visor 2.5G applied at 0.38 and 0.50 lb a.i./acre and Treflan TR-10 applied in both spring and summer at 2.0 lb a.i./acre. No alfalfa phytotoxicity was observed at the rates applied in this study. 1