Low-volume high-concentration applications of glyphosate to control gamba grass (Andropogon gayanus)

Abstract

Gamba grass (Andropogon gayanus Kunth.) is a tussock-forming perennial species capable of out-competing other pasture grasses to form dense stands up to 4 m tall. Infestations occur in the Northern Territory, Queensland, and Western Australia, but its current distribution is only a small proportion of its potential range. Once established, gamba grass impacts on the biodiversity and ecosystem function of an area, whilst also imposing a significant fire hazard due to the large biomass that it produces. Controlling invasive grasses amongst other desirable grass species is a challenge, particularly in difficult to access areas where movement of vehicles and equipment is impaired.

To overcome some of these challenges, we undertook a rate response trial on gamba grass to test the efficacy of low-volume high-concentration applications of glyphosate. At a field site near Mt Garnet in North Queensland, a dense stand of gamba grass was slashed in December 2017 and allowed to regrow until April 2018. A randomised complete block experiment comprising seven treatments, three replicates and clusters of 15 gamba grass plants as experimental units was then established. Using a gas operated splatter gun attached to backpack style equipment, six rates of glyphosate (0, 9, 18, 27, 36, 45 and 54 g a.i. L-1) were applied, with each plant directly receiving 4 mL of herbicide mixture per half metre of plant height. An untreated control was also included for comparison. After 3 months, gamba grass showed a strong dose-dependent response ranging from 30% mortality at 9 g a.i. L-1 to 100% mortality at rates of 36 g a.i. L-1 or higher. Regrowth of any surviving plants was also adversely affected at rates above 9 g a.i. L-1 with plants taking longer to reshoot. While promising, a follow up trial has been undertaken on more mature gamba grass plants to determine if similar results can be achieved.