Paddock Scale Water Quality Monitoring of Vegetable-Sugarcane and Legume-Sugarcane Farming Systems - Summary report 2010-2013 Burnett Mary Region

Abstract

The project has delivered a number of key findings from what were years in which summer rainfall was 50‐100% greater than the long term average. These were as follows –
- Sediment and nutrient losses during grain legume or vegetable rotations with sugarcane were dominated by losses occurring during the sugarcane crop.
- The most sensitive period for soil and nutrient loss occurred during the transition period between crops in the rotation, and during the early stages of crop establishment.
- Soil disturbance, the presence of groundcover (crop residues/trash/living mulch) and soil compaction were the major factors affecting runoff volumes and loads of
sediment and total nitrogen (N) and phosphorus (P). The most effective management systems that ameliorated soil compaction, minimised soil disturbance and maintained ground cover reduced sediment and nutrient loads by 50‐60%.
- Legume residues or legume companion crops were effective at providing groundcover and at reducing soil loss, but also tended to increase losses of the biologically active fractions of N (Dissolved Inorganic N) and P (Filterable Reactive P).
- Runoff losses of DIN were relatively small in all systems tested (0.7‐ 2.7 kg DIN/ha), but leaching losses of nitrate‐N were estimated in excess of 140 kg N/ha from the current commercial practice intensive vegetable systems. This leached N was lost before being able to be recovered by the subsequent sugarcane crop and represents a risk to groundwater quality.
- The risk of offsite losses from herbicides with long half‐lives in the field was illustrated by high concentrations of Diuron recorded in runoff that occurred more than 2.5 months after herbicide application. There was also concern about increased losses of Metribuzin when applied in systems with reduced tillage and surface residues/trash.
- Similarly effective weed control during the plant cane crop could be achieved by reduced application rates of residual herbicides and/or the replacement of residual
herbicides with less persistent knockdown products. However, excluding Diuron in the ratoon crop resulted in poor weed control and the need for additional herbicide
applications.
- The most substantial improvements in runoff (if not drainage) water quality were achieved at the expense of cropping system productivity – especially in the systems
with intensive vegetables. The management strategies showing most promise involve strategic/zonal tillage, reduced nutrient inputs and reduced rates of residual
herbicide use. These promising systems will need research attention to fine tune management so as to limit constraints to productivity and profitability.