What is the first flush?

Sugarcane crops are harvested annually during the dry season, followed by the application of fertilisers and pesticides to support new growth and pest protection. After pesticides are applied to the paddock, a small rainfall or irrigation event is needed for some products, like residual herbicides, to be absorbed into the soil profile.

The 'first flush' happens when these recent fertiliser and pesticide applications coincide with the initial heavy rainfall or excessive irrigation after the dry season. Typically, fertiliser and pesticides are most susceptible to runoff and leaching during the first three weeks following product application.

Water quality monitoring data shows that this often leads to the highest concentrations of nitrogen, phosphorus, and pesticides being washed from paddocks into streams – and late fertilised ratoon crops pose a higher risk.

Growers can make timely locally relevant adjustments to reduce fertiliser runoff during this high-risk period, minimise environmental impacts, and improve farm productivity.

How can this high-risk period be managed?

Despite best efforts by growers, this high-risk runoff window can be difficult to manage due to the unpredictable nature of the Queensland wet season.

The best opportunity to reduce the risk of fertiliser and pesticide losses is to manage application methods and product choices carefully. For fertilisers, this includes managing timing and rates of application, placement and product selection. For pesticides, this includes timing of application, product choice and strategic application.

Growers are also trialling drainage interventions to divert, capture and retain water from the first flush events, which contains elevated nutrient and pesticide concentrations. These drains promote denitrification processes to remove reactive nitrogen before it is exported to downstream aquatic ecosystems. However, they have limited capacity to capture runoff from large rainfall events.

Drainage interventions are being tested, measuring nitrogen levels in inflow and outflow waters at various trial sites. If these interventions prove successful, they will provide growers and managers with an additional tool to mitigate nitrogen losses to the Great Barrier Reef.

Sensor technology detects peak losses

Water quality monitoring programs are increasingly using high-frequency sensors to provide accurate measurements to growers, including nitrate concentrations at a very high frequency. These new sensor-based technologies deliver data to growers every 15 minutes – detecting their peak losses, particularly during the first flush.

Scientists and extension staff use this technology to identify nutrient 'hotspot areas’ by placing sensors throughout a stream network in different locations and scales within a particular catchment area. They are highly suitable for smaller stream networks where water quality can vary considerably over short time periods.

Extension staff are working with growers to interpret the data and tailor management options for local water quality issues, answering key questions for growers, including:

• Is it my nitrogen?

• Am I losing nitrogen from my farm?

• Can I trust the monitoring results?

• Is my approach effective?

• What more can I do?

• How much nitrogen comes from the rainforest?

  
  

The data provides local validation of fertiliser losses to build confidence in growers to adopt improved agricultural practices.