Giant smutgrass is a perennial, clump-type, grassy weed that commonly infests Florida pastures. Experiments were conducted in 1998 and 1999 in Immokalee, FL, to evaluate multiple mowing treatments in combination with hexazinone applications at 0.56 to 1.7 kg ai/ha, to control giant smutgrass and bahiagrass density. Mowing did not influence giant smutgrass control in 1998 or 1999. Hexazinone application increased giant smutgrass control at all application rates. In 1998, regression analysis determined that hexazinone applied at 0.56 kg/ha provided >90% control of giant smutgrass 182 d after treatment (DAT) and >80% control 321 DAT. Both 1.1 and 1.7 kg/ha of hexazinone provided >90% control for 365 DAT in 1998. In 1999, due in part to excessive rainfall, 0.56 kg/ha provided >80% control for only 48 DAT. It was also concluded that application rates of 0.83 to 0.98 kg/ha hexazinone were the lowest rates that consistently provided 80% or better control over both years. From 0 to 30 DAT, bahiagrass density increased by 17% for the 0.56 kg/ha rate and 2% at the 1.7 kg/ha rate. From 30 to 365 DAT, bahiagrass density increased at 0.04% per day compared with 0.1% per day for 0.56 and 1.7 kg/ha, respectively. Increased bahiagrass injury by the higher application rates of hexazinone was responsible for low levels of bahiagrass growth from 0 to 30 DAT. However, bahiagrass soon recovered from injury, and the higher application rates resulted in a more rapid rate of bahiagrass spread, likely due to less competition of giant smutgrass in plots treated with 1.1 and 1.7 kg/ha rates. These data prove that mowing prior to hexazinone application is an unwarranted expense, and that the 1.1 kg/ha rate provided the most effective and consistent giant smutgrass control with acceptable levels of bahiagrass injury.
Nomenclature: Hexazinone; giant smutgrass, Sporobolus indicus (L.) R. Br. var. pyramidalis (P. Beauv.) Veldkamp; bahiagrass, Paspalum notatum Fluegee, #3PASNO.
Additional index words: Bahiagrass density, hexazinone, pasture weeds, tropical forage.
