Effects of Cotton Defoliants on Native Trees in Cotton Production Areas of the Namoi Catchment at Boggabri, NSW, Australia: Field and Glasshouse Experiments

Abstract

Native tree decline and dieback are issues of general concern in many rural landscapes including cotton production regions of the Namoi Catchment in NSW, Australia. Landholders and cotton producers are interested in the potential effects of cotton defoliants on surrounding native trees via the mechanism of spray drift from cotton defoliation activities; whilst researchers, resource managers and sections of the wider community are keen to define all potential causal agents of the increasing frequency of tree loss in regions of north-western NSW. The aim of this research project was to determine the potential effects of cotton defoliants on localised Australian native tree species from north-western NSW and to assess potential of observed effects to contribute to native tree decline and dieback or act as causal agents of these conditions. Five experiments (three field-based and two glasshouse-based) were designed to test the relative effects of four commercially-applied cotton defoliants (utilised in the Australian cotton industry), including: Dropp Liquid® (thidiazuron, TDZ), Dropp Ultra® (thidiazuron + diuron, TDZ+DN), Prep 720® (ethephon, ETP) and Atlacide® (sodium chlorate, NaCl3). The two major field-based experiments examined the relative effects of annual and periodic exposure of young native trees to defoliants over three years, whilst the third measured the impact of an intentional over-spray during routine, commercial defoliation activities on mature trees (growing in a retained stand).