Integrated shrub management in semi-arid woodlands of eastern Australia: ground and aerial application of defoliant to shrubs regenerating after disturbance

Abstract

This paper describes experiments undertaken at several sites in semi-arid woodlands of eastern Australia to determine if chemicals applied either on the ground or from the air reduce the density of shrubs regenerating after disturbance. Ground-spraying of Roundup® in the autumn was more effective than spring application in defoliating shrubs, especially 2-year-old coppice growth. Spraying of Roundup with a hand-held boom at 0.5 up to 2.5 kg glyphosate/ha identified rates to be used for boom spraying. Aerial spraying experiments were then undertaken across several sites and involved several target species. The location of sufficiently large areas where shrub regeneration was of an optimum age (i.e. about 2–3 years) proved to be extremely difficult due to prevailing drought conditions precluding the use of prescribed fire as a preliminary treatment. Nonetheless in one experiment, young (1-year-old) regrowth of firebush (Senna pleurocarpa) exhibited increased sensitivity to Roundup with significant shoot mortality recorded after it had been applied at 0.5 kg glyphosate/ha. Aerial spraying based on an ultra-low volume application of 10 L/ha further enhanced cost-effectiveness on this occasion. Economic analyses structured around 20-year partial budgeting and determination of net present value (NPV) suggested a profitable return could be expected where treatment was based on Roundup applied at this threshold rate 2 years after a prescribed fire, especially when the rehabilitation costs were spread over an entire paddock that had been only partially sprayed. Finally, operational aspects involving aerial spraying in these semi-arid woodlands are also discussed.