West Indian Dropseed/Giant Smutgrass (Sporobolus indicus var. pyramidalis) Control in Bahiagrass (Paspalum notatum) Pastures1

Abstract

Field experiments were conducted in 1997 and 1998 to evaluate the effect of mowing followed by hexazinone for West Indian dropseed/giant smutgrass (Sporobolus indicus var. pyramidalis) (hereafter referred to as dropseed) control. The experimental design was a split plot, with mowing (nonmowed [mature] and 35-cm regrowth) as the whole plot and hexazinone rate (0.0 [control], 0.56, 0.84, 1.12, 1.40, and 1.68 kg ai/ha) as the subplot treatments. The application of 0.84 kg/ha hexazinone provided 94 and 81% dropseed control, 365 d after treatment (DAT) during 1997 and 1998, respectively. Increasing application rate to 1.12 kg/ha hexazinone provided 87 and 88% dropseed control, 365 DAT during 1997 and 1998, respectively. Both the 0.84 and 1.12 kg/ha rates provided the same average control (87.5%); however, the 1.12 kg/ha rate provided consistent control over years. Mowing dropseed, followed by hexazinone application at 35-cm regrowth, provided no additional control when compared with no mowing treatments. Rates of hexazinone at 1.40 and 1.68 kg/ha caused phytotoxicity to bahiagrass and increased bare soil surface area, especially 90 and 120 DAT. Phytotoxic effect on bahiagrass and on bare soil decreased 365 DAT, resulting in 75 to 80% total forage cover. Concentration and yield of total nonstructural carbohydrates were significantly lower for the mowed 35-cm regrowth treatment than for the nonmowed plants; however, even in its weakened condition this reduction had no effect on dropseed control.Nomenclature: Hexazinone; West Indian dropseed/giant smutgrass, Sporobolus indicus (L.) R. Br. var. pyramidalis (P. Beauv.) Veldkamp; S. indicus (L.) R. Br. var. indicus #3 SPZIN; S. jacquemontii Kunth.; S. poiretii (Roem. & Schult.) A. S. Hitchc. # SPZIN; bahiagrass, Paspalum notatum Fluegge # PASNO.Additional index words: Bermudagrass, CYNDA, Cynodon dactylon (L) Pers., hexazinone, pasture weeds, tropical forage.Abbreviations: DAT, days after treatment; GC, percent ground cover; TNC, total nonstructural carbohydrates.