The water quality and economic benefits of agricultural management practices
Improved land and agricultural management practices are proven to reduce the runoff of suspended sediment, nutrients and pesticides at the paddock scale.
Our knowledge of the effectiveness of specific management practices in terms of water quality benefits and economic outcomes has improved significantly since 2008 and improved the ability to prioritise management action. However, the costs and risks for landholders associated with changing management practices can prove significant barriers to adoption and are not well understood.
Summary of evidence
- In grazing lands, sediment loads are reduced by: setting stocking rates that maintain ground vegetation cover and biomass (particularly during droughts and at the end of the dry season) and vegetation diversity (including maintaining some tree cover particularly in riparian areas); and managing stock access to, and increasing ground cover in, riparian or frontage country and wetlands, and rilled, scalded and gullied areas. Techniques for managing gully and streambank erosion, which are known to be a significant source of sediments in grazing lands, are important and require further investigation as to their economic viability and effectiveness.
- Soil management practices that reduce runoff and sediment movement reduce loads of particulate and total nutrients in runoff.
- In most cropping systems of the Great Barrier Reef, management systems that reduce or eliminate tillage and maximise soil cover (via crop residue retention) and the use of grassed headlands, and where appropriate grassed inter-rows, reduce soil loss. Controlled traffic and contour embankments also reduce runoff and soil loss. Targeting practice improvement to areas contributing most to soil loss, considering erosion rates, soil texture and location of sediment traps including reservoirs, can increase the effectiveness at the Great Barrier Reef scale.
- Losses of nitrogen are related to nitrogen fertiliser applications and the nitrogen surplus (i.e. the difference between nitrogen inputs and nitrogen in crop offtake) at both the field and whole-Great Barrier Reef scales. Where surpluses are high, nutrient loads are most effectively reduced by reducing nutrient inputs and surpluses. The same principles should apply to phosphorus. When nitrogen applications closely match crop requirements (i.e. nitrogen surpluses are low), management ‘tactics’ such as splitting or altering the timing of fertiliser applications, altering fertiliser types and burying fertiliser, can help manage the risk of nitrogen supply limiting yield.
- Nutrients from sources such as nitrogen from legumes and nitrogen and phosphorus from mill mud in sugarcane areas may substantially increase nutrient surpluses and thus have water quality impacts.
- In furrow irrigated sugarcane, increasing irrigation efficiency (i.e. reducing over-application of irrigation) reduces nutrient losses. Efficiencies can be increased either by better managing irrigation within a given system, or moving from systems with lower (e.g. furrow) to higher (e.g. trickle) efficiency.
- Soil management practices that reduce runoff and sediment movement (e.g. retention of crop residues, controlled traffic) reduce pesticide runoff. Managing pesticide application timing (i.e. increasing the time between application and runoff) as well as the amount, placement and application method (e.g. banded spraying) will reduce pesticide runoff greatly, especially for the highly soluble photosystem II inhibiting herbicides. Applying products with rapid degradation rates (e.g. ‘knockdown’ herbicides) will reduce concentrations and loads in runoff.
- Constructed wetlands need to be maintained, with accumulated sediments, particulate nutrients and associated weed biomass removed off-site. If not maintained, these accumulated materials can be ‘flushed out’ of the wetlands and flow to the Great Barrier Reef during flood events, reducing the water quality improving capacity of wetlands.
- Wetlands have much broader values in the landscape than just water quality improvement, so they should not be used as a substitute for poor land management practices. Poor quality water flowing into freshwater wetlands can impair wetland functions and be detrimental for wider landscape health and productivity.
- The costs of changing management practices to improve water quality vary greatly (e.g. two orders of magnitude) between different agricultural enterprises on a per-hectare basis. Therefore, it is difficult to assume a single economic outcome of changed management.
- Improving farm management to meet an industry-based Best Management Program generally gives positive economic benefits in the long-term, and so little or no external support may be needed to encourage this change. However, the economic benefits of Best Management Programs alone may not be large enough to drive adoption. The benefits from changing management practices also varies between farms, so the cost-effectiveness of changes can differ significantly across practices, farms and industries. This means that there is a challenge to find the most efficient solutions that will deliver improvements where they achieve the largest benefits.
- The transaction costs associated with changing management and the largely risk-averse nature of agribusinesses are factors that can prove significant barriers to adopting improved practices. Increasing adoption rapidly and in cost-effective ways remain key challenges.