The current Australian sugarcane industry transition towards adoption of an ‘alternative’ herbicide strategy as part of improved environmental stewardship is increasingly complicated by recent farming system, regulatory and herbicidal product changes. This study quantified and compared the efficacy, economic costs and environmental risk profile s of a range of established, emerging, and recently registered pre -emergent herbicides across field trials in the Wet Tropics region of North Queensland. Several herbicides were effective on certain weed species, but lacked broad spectrum control. Better efficacy results from products with multiple active ingredients (i.e., imazapic -hexazinone) demonstrated the benefits of using mixtures of active ingredients to widen the spectrum of weed control efficacy. All tested pre – emergent herbicides behaved quite similarly in terms of their propensity for off-site movement in water (surface runoff losses generally > 10 % of active applied), with their losses largely driven by their application rate . Herbicides with lower application rates consistently contributed less to the total herbicide loads measured in surface runoff. Results demonstrated alternative choices from the more environmentally problematic herbicides (such as diuron) are available with effective alternative formulations providing between 4 and 29 less risk than the traditional diuron -hexazinone ‘full rate’. However, considerable challenges still face canegrowers in making cost -effective decisions on sustainable herbicide selection. Additional research and effective grower extension are required to address information gaps in issues such as specific weed control efficacy of alternative herbicides and potential blending of some herbicides for more effective broad spectrum weed control, while also minimizing environmental risks.

Introduction

The decline of fresh and marine water quality associated with land -based runoff from adjacent agricultural catchments is a major cause of the poor state of many of the coastal ecosystems of Australia’s Great Barrier Reef (GBR) World Heritage Area (Waterhouse et al., 2017). Pesticides have been specifically identified as among the most important diffus e source pollutants from catchment areas of the GBR (Brodie et al., 2012). High value freshwater, estuarine and inshore waters of the GBR lagoon are regularly exposed to pesticide runoff from agricultural lands, particularly during wet season riverine flood events (December to April) (e.g. Lewis et al., 2009 ; Davis et al., 2012). The pesticides that have been most commonly detected in the GBR lagoon are herbicides that inhibit electron transport at photosystem II (PSII) in plants and include diuron, atrazine, hexazinone, ametryn, simazine and tebuthiuron (Haynes et al., 2000a ; Shaw and Müller, 2005 ; Shaw et al., 2010). Concentrations during elevated stream flow events can at times exceed ecological water quality protection guideline trigger values for up to several weeks for some of these PSII herbicides (Lewis et al., 2009 ; 2012 ; Brodie et al., 2012 ; Davis et al., 201 3 ; O’Brien et al., 2016; Novic et al., 2018).