Efficacy of pre-emergent herbicides on sicklepod (Senna obtusifolia) in the Dry Tropics of Cape York

Abstract

This study determined the efficacy of multiple herbicide treatments applied at a pre-emergent stage on field populations of sicklepod (Senna obtusifolia (L.) Irwin and Barneby) in the dry tropics of far north Queensland. The aim was to identify herbicide treatments that can be applied towards the end of the dry season and activated with the first substantial rains of the Wet Season, to produce more consistent control outcomes, particularly in situations where infestations are inaccessible due to the annual northern wet season. Field sites were established in dense sicklepod at three locations on Cape York: Morehead River, West Normanby River and Oaky Creek. The trial was established just prior to the Wet season in 2019 (October) and monitored at approximately three-monthly intervals for three years. After the initial treatment, several of the herbicides showed high efficacy, with the most efficacious herbicides reapplied after 12 and 24 months to evaluate annual re-treatment effects on seedling regeneration numbers in the following wet seasons. High efficacy was achieved with Conqueror®, Grazon Extra®, Stuka flexi® and Tordon75-D® all of which contained the active ingredient picloram. The complete absence of sicklepod after three years and replacement with grasses or other herbaceous species was observed in some plots. Pre-emergent herbicides have significant potential for future use, particularly for the strategic management of access to infrastructure such as rest stops, property access, and stock watering points.