Optimising spot spraying for controlling Guinea grass

Established Guinea grass stools are commonly found in our sugarcane-farming system. No registered herbicides that are both effective on perennial Guinea grass stools and selective to sugarcane are available. Directed-spray strategies using registered herbicides and current spray configurations are not effective in controlling Guinea grass stools, so growers with Guinea grass escapes rely on spot spraying. To optimise spot spraying, the mixture applied needs to be effective in a single application with minimal impact on the adjacent sugarcane stools. Two pot trials screened herbicides registered in sugarcane for their efficacy to control established Guinea grass stools. The first pot trial showed that good spray coverage was crucial for successful control of Guinea grass spot sprayed with 1.35 L per 100 L of Weedmaster® Argo® (glyphosate 540 g/L) + 0.3 L of LI700* per 100 L. The second pot trial showed that isoxaflutole (75 g/ha) + MSMA (2.16 kg/ha) was the most effective herbicide combination and the application rate could potentially be reduced. Bobcat® i-MAXX (imazapic 25 g/L + hexazinone 125 g/L) was also effective when used at 2 L per 100 L, but Barrage® (diuron 468 g/kg + hexazinone 132 g/kg) was only effective when used at twice the recommended rate (2 L per 100 L) and only when the entire foliage was soaked to the point of runoff. The most effective spot-spraying options for Guinea grass control with the lowest impact on adjacent sugarcane were: isoxaflutole + MSMA, but this mixture is not registered for spot spraying application; and Bobcat® i-MAXX. Bobcat® i-MAXX SG (granular formulation), registered since 2018, includes a spot-spraying rate at 350 g per 100 L that is equivalent to the effective rate tested in the second pot trial. Key words Guinea grass, Megathyrsus maximus var. maximus, directed spray, spot spray, isoxaflutole, MSMA