4.2.2 Reducing PSII herbicide losses
The main factor that influences PSII herbicide losses in both wet and dry seasons is also the rate of application which is also linked to the application method ( e . g . banded spraying ) and timing of application .
Reducing the amount applied , for example banded spraying and adopting integrated pest and / or weed management , will reduce pesticide losses . Expanding on this :
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Reducing overall usage and use of precision application . Given the direct relationship between the amount of pesticide on an area and the run-off losses from that area ( discussed below ), reducing applications of pesticides to fields will also reduce losses ( Rohde et al ., 2013a , 2013b ; Masters et al ., 2013 ; Silburn et al ., 2013b ; Davis , 2013 ; DNRM , 2016 ). Pesticide applications can be reduced through practices such as band spraying of residual herbicides ( Donaldson et al . 2015 ; Davis and Pradolin , 2016 ; Masters et al ., 2013 ; Nachimuthu et al ., 2016 ; Oliver and Kookana , 2006 ; Oliver et al ., 2014 ; Rohde et al ., 2013b ; Silburn et al ., 2013a ), employing shielded sprayers , substituting less toxic knockdown herbicides for residual products ( e . g . Melland et al ., 2016 ) and adopting integrated weed management , which aims to strategically manage the weed seed bank rather than rely on prophylactic or precautionary spraying ( Armour et al ., 2013a , 2013b ; Davis , 2013 ). Therefore , management practices that reduce the amount of product applied — by applying at a lower rate , banded or precision application ( e . g . spotspray / WeedSeeker ®)— have repeatedly been demonstrated in catchment , irrigation and rainfall simulation studies to provide a nearly proportional reduction in run-off losses ( see Eberhard et al ., 2017 ). For example , field experiments in the Lower Burdekin and Mackay catchments using banded spraying have shown 90 per cent reductions in losses of residual herbicides on irrigated sugarcane paddocks ( Davis and Pradolin , 2016 ; Oliver et al ., 2014 ) and at least 50 – 60 per cent reductions in rainfed systems ( Devlin et al ., 2015 ; Masters et al ., 2013 ; Nachimuthu et al ., 2016 ).
Integrated weed management . A critical step in reducing the amount of pesticide applied is effective strategic weed control over the longer term . This approach is termed ‘ integrated weed management ’ and it requires planned and timely management of many operations over many crop cycles to reduce the weed seed burden over time . Fundamental principles of integrated weed management include suppression of weeds through high levels of crop residues and crop competition and diligent weed control during fallow phases to avoid regeneration of the weed seed bank ( SRA , 2017a ). Case studies have illustrated the effectiveness of integrated weed management in sugarcane crops . For example , In the Burdekin region ( Davis , 2013 ), a farmer was able to change entirely to shorter lived knockdown herbicides in ratoon stages of the crop cycle and thus reduce the risk of herbicide loss from the field . Green-cane trash blanketing in sugarcane is widely considered an efficient practice to manage weeds in sugarcane production . However , little information exists on the optimal thickness of a green-cane trash blanket for weed control or the optimal timing of the herbicide applications in this situation . Fillols ( 2012 ) showed that , in comparison to bare soil , trash at all levels reduced weed coverage and contributed to additional yield and profitability . In particular , increasing the level of trash led to improved management of broadleaf weeds and grasses , and strategies involving early pre-emergent herbicides were more efficient . Adoption of green-cane trash blanketing in inefficient irrigation systems is unlikely to be successful due to issues associated with inflow rates , field length and field slope .
Maximising the time between application and likely runoff events . There is a logical relationship between the amount of pesticide on a field at the time of a run-off event and the amount transported in run-off ( Leonard et al ., 1979 ; Silburn and Kennedy , 2007 ; Thorburn et al ., 2013 ; Melland et al ., 2016 ). It has been known for a long time that loads and concentrations of pesticides in run-off are greatest soon after application and decline with time ( Wauchope , 1978 ; Leonard et al ., 1979 ). This behaviour is characteristic of constituents that are decaying at the site of application ( e . g . soil , crop residues ). If no run-off occurs within the first three weeks after application , large losses are typically avoided . Field studies in the Lower Burdekin have supported this result ( e . g . Davis et al ., 2013 ).
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