Evaluating Alternative Herbicide Strategies for Effective Control of Navua Sedge (Cyperus aromaticus) in Queensland’s Wet Tropics, Australia

Abstract

Cyperus aromaticus (Ridley) Mattf. & Kük. (Navua sedge) is an aggressive perennial weed threatening pasture systems, sugarcane production, and roadsides in Queensland's wet tropical regions. This study assessed the efficacy of seven herbicide programs: (i) halosulfuron-methyl (single and multiple applications), (ii) Halosulfuron-methyl (multiple applications—four times), (iii) Halosulfuron-methyl (single application) followed by glyphosate spray (6 weeks after Halosulfuron application), (iv) Paraquat (single application) followed by three applications of Halosulfuron-methyl, (v) Imazapyr (single application), (vi) Imazapyr (multiple applications—four times), (vii) Florpyrauxifen-benzyl (multiple applications—four times), and (viii) an untreated control. Treatments were applied at two plant growth stages (post-mowing and flowering) across three land-use types: pasture (Malanda), sugarcane (Gordonvale), and roadside (Ingham). Weed kill percentage and tiller density were evaluated at 6, 12, and 18 months after the first herbicide application. When treatments were assessed 6 months after the first herbicide application, Imazapyr, performance averaged over the three locations in mowed plots either as a single or multiple application, resulted in poor weed control (3%–8% control); however, it showed improved performance (32%–42% control) when applied in non-mowed plots. At 6 and 12 months after the first herbicide application, multiple applications of florpyrauxifen-benzyl applied four times delivered the highest and most consistent control (77%–91%) and significantly reduced tiller production. Sequential applications of halosulfuron-methyl with glyphosate or paraquat provided effective early-stage control. Single applications of imazapyr or halosulfuron-methyl were ineffective (3%–20%) and linked with high tiller density. Six months after the first herbicide application, sequential applications of halosulfuron-methyl followed by glyphosate, and paraquat followed by halosulfuron-methyl, also provided effective weed control. In contrast, single applications of imazapyr or halosulfuron-methyl provided poor control (3%–20%) at this time and were associated with higher tiller counts. By 18 months after the first herbicide application, averaged over locations and herbicides, mowed plots achieved higher weed control (29%) and resulted in less tiller production than those at the flowering stage (20%). Further, when averaged across location and application stage, no herbicide achieved more than 34% control. Additionally, averaged over herbicides and application stage, the roadside site had better weed control (56%) than the sugarcane (11%) and pasture (7%) sites. This study highlights the importance of multiple herbicide applications, particularly halosulfuron-methyl or florpyrauxifen-benzyl applied in mowed plots for long-term suppression of C. aromaticus.