RRRD055: Validating the cost/benefit of improved fertiliser practices and quantifying nutrient loads and pathways from irrigated dairy pastures in the Wet Tropics and the Burnett-Mary regions

David Rowlings1, Jack Koci2, Paul Nelson2 and Rick Kowitz3

1 Institute for Future Environments, Queensland University of Technology
2 School of Earth and Environmental Sciences, James Cook University
3 Queensland Dairyfarmers' Organisation Limited

pdfDownload the RRRD055 Research Outcomes Report4.64 MB

Executive Summary


In developing this project, the dairy industry identified a number of key priorities where further research and development would assist the industry to improve water quality outcomes for the Great Barrier Reef Lagoon. These included the following:

  • Improved quantification of nutrient inputs and outputs at a whole-of-farm and paddock scale to improve nutrient use efficiency and reduce risks of nutrient loss from dairy farms.
  • Regional evaluation of nitrogen stabilisation, modified fertiliser products, and delivery systems (e.g. slow release fertilisers, nitrogen (N) stabilisation and liquid nitrogen).
  • Quantifying the costs and benefits of specific practices in relation to water quality outcomes and improved nutrient use efficiency.
  • Development of a practical tool to assist on-farm soil nutrient management decisions, and to identify nutrient balancing issues and opportunities for improving nutrient use efficiency and identifying potential risks.
  • Determining the level to which Farm Management System (FMS) practices would deliver on NRM Water Quality targets.

Through a strategic partnership between the Australian Government’s Caring for Our Country Reef Rescue program, Queensland Dairyfarmers’ Organistation, Queensland University of Technology, James Cook University, and Incitec Pivot Fertilisers, it was possible to combine a number of these research priorities into one project.

The aim of these investigations was to determine the cost/benefit of using urea treated with a nitrification inhibitor compared to using standard urea in terms of reduced nitrogen losses through leaching and runoff, and the potential production efficiency gains. The contribution of Incitec Pivot Fertilisers made it possible to value add to the water quality research with an investigation of gaseous losses of nitrogen, therefore completing the nitrogen loss pathways from dairy pastures.

Until now there has been little quantification of N loss from fertiliser application on dairy farms in tropical and sub-tropical Queensland, and the management practices that might reduce these losses. Such research is vital, given the proximity of farming systems to sensitive environmental areas, such as the Great Barrier Reef World Heritage Area, and the increasing economic constraints on dairy production in the tropics.

This research was conducted on two commercial irrigated dairy farms, situated at Gympie and Ravenshoe. Management practices (e.g. grazing, fertiliser application and irrigation) were performed as per normal farming operations and are common to many dairy production systems in the Great Barrier Reef catchments.

Key findings

This study investigated the effect of fertiliser application rate and a nitrification inhibitor (DMPP) on N losses and pasture growth, on two dairy farms within the Australian tropics. The experiments were conducted on commercially operating dairy farms near Ravenshoe in North Queensland, and Gympie in southern Queensland, where all management practices (e.g. grazing, fertiliser application and irrigation), were performed as per normal practice. While undertaking such a field trial presented several challenges, the results show promising avenues for improving N use efficiency and decreasing losses of N to the environment.

While it was not possible to accurately allocate losses to particular mechanisms, N losses at the Ravenshoe Red Ferrosol appeared to be mainly as a result of deep drainage, whereas runoff and denitrification dominated losses in the heavier textured Gympie soil. At the Gympie site, the annual nitrogen use budget was dominated by the unusual climatic conditions rather than any potential management decisions. Optimal pasture growth was limited in what is traditionally the prime growing season by the extended and unseasonal dry period from August to mid-January, which accentuated losses from the extreme rain event at the end of January and skewed nutrient losses substantially towards runoff. In such extreme climatic events there is little that good management can achieve that will alleviate such losses. However improved nitrogen use efficiency (NUE) can still be achieved during the drier, cooler fertilisation period by the closer matching of N fertiliser application to plant N demand.

A clear response to fertiliser rate, and the effectiveness of the DMPP inhibitor were demonstrated during the fertilisation period across a range of parameters (yield, N removed, NUE), though N2O losses were unaffected at Ravenshoe. Pasture yield was almost as high in the low N + DMPP treatment as it was in the high N treatment. NUE and the effect of DMPP was substantially improved at the lower rate of N application suggesting that N was not the limiting factor for production, and indeed soil water was in deficit for long periods of the experiment at both sites. As such reducing N application during periods of high water deficit is an effective strategy for improving NUE.

The results indicate that fertiliser N inputs can be reduced from the standard commercial rates with no economic or yield penalty when urea is used in conjunction with DMPP. Halving the fertiliser rate when an inhibitor was used, reduced the amount of N potentially lost to the environment by more than half. DMPP did not significantly reduce N2O emissions at either site, though a trend towards lower emissions was observed at Gympie. In the low N treatments most of the N was utilised by the pasture. In the high N rate treatments, however, there were large losses of N that were unaccounted for possibly through denitrification to N2.

The following key recommendations can be drawn from this research:

  1. The increase in pasture crude protein (CP) content associated with DMPP application suggests that farmers can apply urea at lower rates to achieve up to 19% CP. In this way, less N is applied to maintain the same level of production by dairy cows. The cost of DMPP is approximately 20% more than conventional Urea so this additional cost needs to be accounted.
  2. Management of N fertiliser application rates according to soil moisture deficits, irrigation availability and predicted precipitation could substantially reduce N losses. This allows for a closer matching of plant N demand to fertiliser supply. In periods of adequate to excess soil moisture, the use of the DMPP inhibitor increases NUE efficiency.
  3. Before the observed improvements in agronomic NUE lead to higher overall farm NUE, cows grazing pasture with higher CP may require rumen fermentable carbohydrate (RFC) supplements. Feeding RFC supplements, in combination with increased stocking rates, provides an opportunity for harnessing agronomic gains in NUE and increasing milk production.
  4. The higher pasture N concentration can be balanced by supplementing cow feed with fCHO in combination with an increased stocking rate. This approach to harnessing improvements in agronomic NUE leaves the rate of N application unchanged but increases farm production.

Benefits and application of the work

At both sites there was a significant increase in biomass with DMPP applied at half the industry standard rate compare to urea at the same rate. At Gympie the yield was 20% higher than in the urea only treatment and similar to that observed from the higher N rate treatments. At Ravenshoe the biomass from the lower rate when combined with DMPP was greater than the higher N rate treatments.

These findings will be of most interest for farmers looking to maximize their productivity at the lowest possible cost. The use of lower rates of urea nitrogen fertiliser should also reduce the potential risk of losses via other pathways, including leaching and runoff.

Dairy farmers and service providers are already benefiting from the research through the extension of these findings through field days, fact sheets and dairy industry publications. Industry recommended practices and priorities for future investment will be reviewed when the research results are conclusive.

Future directions

This research has contributed to our understanding of nutrient pathways in irrigated tropical and subtropical dairy pastures, and will help to quantify the amounts of nitrogen and phosphorus fertiliser lost through the various pathways. The research has also measured the value of the DMPP inhibitor in reducing losses. However, it is recognised that these findings are highly dependent on seasonal conditions. Differences in some results for the two sites will provide additional insights into the climate, soil and management factors affecting N loss from dairy systems and in developing recommendations for good practice to improve NUE in tropical and subtropical dairy farms.

Now that the methodology is well established, there would be considerable value to the dairy industry in continuing this research over a longer period of time to understand the impacts of seasonal variability on these investigations, and to measure the effect of using additional improved practices and technology to further reduce losses and improve fertiliser efficiency.

There is an additional need to examine animal management practices that allow farmers to harness improvements in agronomic NUE derived from optimised fertiliser management. Factors such as pasture utilisation rates and ruminant N nutrition also need to be considered in a holistic approach to dairy N management.