Evaluating farming practices for water quality outcomes

Sugar Research Australia (SRA) recently completed a four-year project aimed at reducing nutrient and pesticide losses from cane farms in the Pioneer and Plane Basins. In this project we have investigated different farming practices and their impact on water quality. The water quality knowledge gained by the project can be used by growers to select farming practices that are beneficial for the environment and assist in reducing losses of nutrients, herbicides and imidacloprid (cane grub control chemical) to the Great Barrier Reef lagoon.

Key findings:

1. Even small amounts of nutrients, herbicides, and imidacloprid wash out during rain, and can be detected in waterways. 

   
   

2. Best practice matters – but must be tested. Some practices reduce losses, but others assumed to be safe still exceeded water quality guidelines. We need to monitor, including complex interactions between recommended practices. 

   
   

3. Armed with local trial data, growers can make better decisions that support environmental sustainability.

Snapshot of the trials

Trial sites were established and run-off waters were monitored throughout the respective growing seasons, the trials included:  

• 11 paddock sites were set up across the region, Each site had six strips (six cane rows wide)

• The trials ran for four years and monitored multiple growing seasons

• A range of farming practices, including BMPs, were trialled and compared

• Water monitoring equipment collected run-off during rainfall events

• Flow-paced, composite samples were taken from each strip

• Sub-samples were sent to labs to test for nutrients, herbicides, and Imidacloprid (cane grub control chemical)

  
  

What have we learnt from the trials

All chemicals leave the paddock

From our measurements we determined a percentage of the applied nutrients, herbicides and imidacloprid leave the paddock in run-off events.  Whilst in general only limited percentages of the applied chemicals are washed out of the paddock, the amount washed out will be detected in waterways. Therefore, growers have a direct role to play with regards to environmental sustainability. For all the chemicals used by growers there are guidelines for amount of the chemicals which are acceptable in waterways. The guidelines commonly used are for the protection of 95% of species in freshwater streams and it is those guidelines by which the industry is judged. 

Industry knowledge

Many of the project trials involved farming practices where the water quality outcomes were known, and as such we gathered data to support established industry knowledge, for example:

• Importance of correct depth placement (100–125 mm) of liquid imidacloprid in ratoons to minimize run-off losses.

• Most surface herbicide losses occur in the first flush run-off event.

  • Aim to apply herbicides away from high rainfall events.

  • In periods of storms/heavy rains, consider herbicides with less mobile actives.

  • Herbicides are highly vulnerable to run-off in the first 48 hrs following application.

• Similar nutrient run-off results for surface applied liquid dunder and sub-surface applied granular fertiliser.

• Lower nutrient run-off achieved by accounting for the N in banded mill mud when calculating amount of N to apply.

For growers this industry knowledge is available in Best Management Practice (BMP). For example, BMP requires chemical application training which involves following the chemical labels, using correct rates, and ensuring application equipment is in good order and calibrated, which are practices that help minimise chemical losses to waterways. 

Unexpected outcomes to consider

An unintended outcome of the project was that we found some farming practices were not as environmentally beneficial as had been assumed by the industry, for example:

• Banded mill mud in ratoons – we observed higher residual herbicide run-off losses compared to where no mill mud had been applied.  This result had not been previously measured or reported to industry.

• 6 EASY STEPS (6ES) Toolbox recommendation to reduce applied N where there is banded mill mud – nutrient run-off was lower where N rate was reduced but nutrient losses were still higher than where no mill mud was applied.

• Enhanced Efficiency Fertiliser (EEF) – the specific timing of rainfall and loss events may result in some situations where a nitrification inhibitor fertiliser does not result in reduced nutrient losses to the environment, as compared to a non-nitrification inhibitor product.

• Liquid imidacloprid applied at best practice in ratoons – in 60% of our trial sites high concentration levels of imidacloprid were recorded in run-off water; our data suggest such high levels would result in exceedances of the 95% species protection freshwater guideline for imidacloprid in local waterways. 

• Spot spraying using diuron can lead to very high diuron concentrations in run-off water.

  
  

Our trial data did not question the productivity benefits of the above farming practices, rather highlighted limitations with regard to water quality. The project provided evidence that farming practices need to be checked for water quality outcomes rather than assumed, else we as an industry can be supporting and promoting farming practices which have limited impact on reducing nutrient and pesticide loads to the Great Barrier Reef lagoon.

SRA Central project Cane to Creek Mackay Whitsunday was fully funded by the Great Barrier Reef Foundation in partnership with the Australian Government’s Reef Trust.

For more information, see: Cane Matters Spring 2024 - Sugar Research Australia