Reducing Sediment and Nutrient Loss from the Pastoral Lands of Northern Australia: Sustainable Grazing Strategies for the Seasonably Variable
Tropical Savannas

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Declining water quality is seen as one of the major threats to the Great Barrier Reef lagoon with elevated inputs of sediments and nutrients having the potential to profoundly effect the ecology of a number of marine communities (ENCORE, in press; Schaffelke and Klumpp 1998; Dennison and Kirkman 1996; Van Woesik et al. 1999).

Estimates of total river discharges of sediment and nutrients (nitrogen and phosphorus) into the lagoon have been derived from models relating erosion to regional land-use patterns in catchments adjacent to the Reef (Moss et al. 1993; Neil and Yu 1996). Current estimates are of an annual input of 22 million tons of sediment, 90 000 tons of nitrogen and 12 000 tons of phosphorus. This is estimated to be approximately four times the load before European settlement of the catchments. Overall, 66% of the estimated nutrient and sediment flux is estimated to come from grazing lands (Moss et al. 1992).

The Fitzroy and Burdekin Rivers are the biggest source of nutrients and sediments discharged into the Great Barrier Reef lagoon. The amount of sediments and nutrients emanating from these catchments is not static, but strongly dependent upon land use, condition and management. Grazing has the potential to have a major impact on water quality through its effects on plant cover and soil condition (figure 1). In general soil loss and run-off increase sharply when cover declines on grazing lands, e.g. McIvor et al. (1995). The importance to the Great Barrier Reef lagoon of these large ‘dry’ catchments where cattle grazing is the dominant land-use is therefore self-evident.

Figure 1a. Drought, coupled with overstocking, can lead to severely reduced cover over significant areas of the interior grazing lands.

Figure 1b. Sustainable grazing systems result in healthy, productive landscapes. Widespread adoption of such systems is essential for the long-term viability of the beef industry.

More than 98% of the 129 000 km² Burdekin catchment is under some form of extensive cattle grazing. It may therefore be assumed that this form of land use contributes a major portion of the nutrient and sediment inputs (Moss et al. 1993) into local rivers and ultimately to the Great Barrier Reef (figure 2).

Figure 2. Extent of beef grazing on the Great Barrier Reef catchments

Unfortunately, many areas of the upper Burdekin catchment show some form of landscape degradation (De Corte et al. 1991). Most of this degradation appears to be initiated in dry years when overstocking in combination with low rainfall results in the over-utilisation and eventual death of perennial grasses species. This results in their replacement by annual grasses, reduced ground cover and increased soil erosion and nutrient loss (figure 3). Possible solutions to the problem of rainfall variability are that graziers stock conservatively, spell parts of their property on a regular basis and/or adopt some variable stocking strategy that matches animal numbers to available feed.

Figure 3. Processes leading to land degradation in the semi-arid grazing lands

Unfortunately however, these strategies have not been widely adopted by the beef industry. As with many other resource management problems the reasons for this non-adoption are complex and include a host of cultural, social, economic and legislative issues. However, possibly the biggest single reason is the absence of quantitative data to show that sustainable strategies are economic and therefore equal to, or superior to, existing management strategies.

To address the above issues, the Department of Primary Industries established a grazing trial in 1997 to produce objective data on the impact of different grazing systems on sustainable production and to demonstrate the economic benefits of sustainable management.

This large scale (1000 ha), long term (> 10 years) trial is being conducted on the property ‘Wambiana’, 70 kilometres south of Charters Towers (figure 4). The research involves investigation of a range of grazing strategies and their effects on animal production, pasture condition, biodiversity and nutrient and sediment loss. The data will also allow the economics of the different grazing strategies to be calculated (table 1). Management strategies being investigated are those already used by local graziers, together with new ones that utilise the latest climate forecasting techniques or strategic pasture spelling.

Table 1. Description of sampling design

Figure 4. Outline of property and experimental strategies of the ‘Wambiana’ grazing trial

Concerns about sediment and nutrient run-off and potential downstream impacts have prompted the Great Barrier Reef Marine Park Authority to be one of the major funding bodies of the grazing trial. The investigation of particular relevance to the Authority, is how different stocking strategies effect soil and nutrient loss. Although this question has received attention previously (e.g. McIvor et al. 1995) there is little data on how grazing affects nutrient run-off, while extrapolation of results to larger areas has been problematic due to the small plots used in these studies. Earlier work also failed to link soil loss to cattle production and hence the economics of the trade-offs between these two variables were not addressed.

A series of five bounded run-off catchments have been installed on the Wambiana site to quantify soil and nutrient loss under the different grazing strategies (figure 5).

Figure 5. One of the bounded catchments at the ‘Wambiana’ site. The wing walls funnel run-off water through the San-Dimas flume to allow sampling and measurement of the flood profile.

Each experimental catchment consists of a 1 ha bounded area with wing walls to funnel run-off water through a San-Dimas flume, each of which is fitted with an electronic device to record the height of the run-off water. Calculations based on flow height and duration will provide accurate estimates of the total amount of run-off. To collect data on erosion rates flumes are also fitted with a sediment box to trap heavier soil particles before they move through the flume.

Automatic pumping samplers are used to sample the run-off water for dissolved nutrients and sediments. These samplers are programmed to sample at specific intervals over the flow event to obtain an accurate estimate of nutrient concentration in the run-off water. Rainfall amount and intensity is also measured at each site using a pluviometer. Although the equipment was installed in December 1998 in the middle of the wet season, as yet, no measurable run-off has been recorded. This is due to the excellent ground cover that has resulted from the above average rainfall and the fact that most of the rain that fell was of unusually low intensity and therefore failed to run off.

Detailed measurements of aerial, basal and ground cover, as well as species composition and standing biomass in each catchment, are also made on a regular basis to allow the quantification of the relationship between run-off and these variables.

Run-off data will ultimately be integrated with economic information, pasture production and biodiversity data to allow an objective assessment of the cost-benefits associated with different grazing strategies. Hopefully this data will provide an impetus for graziers to adopt sustainable management practices. The development and adoption of these strategies by the grazing industry will be facilitated by a Grazier Advisory Committee, which has played a key role in the development and management of the project. The committee was formed before the trial started and has been very important in ensuring that the project remains as relevant as possible to the grazing industry. Extension programs detailing the benefits of sustainable management will also be used to encourage uptake of sustainable grazing strategies by the industry. Appreciation of catchment water quality issues and their potential impact on marine systems will also assist in this process.

In the longer term, the adoption of sustainable grazing strategies will not only ensure a sustainable and viable beef industry but will also have flow on effects to the wider community. These include reduced drought subsidies, reduced rural debt, better woody weed control, improved biodiversity and better water quality. The last will have direct benefits for two major Queensland industries, i.e. tourism and fisheries which currently generate $850 million for the State’s economy (Driml 1999).

This cooperative research is one of the first to demonstrate a recognition of catchment-to-coast connectivity. Improved grazing strategies will benefit the local grazier as well as reduce sediment and nutrient run-off to the Great Barrier Reef. This is vital for the continued preservation of the unique and beautiful features of the Great Barrier Reef.

We gratefully acknowledge the cooperation of the Lyons family, ‘Wambiana’, and the financial assistance provided by the Drought Regional Initiative, the National Heritage Trust, the Great Barrier Reef Marine Park Authority and the CRC for the Sustainable Development of Tropical Savannas.

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De Corte, M., Cannon, M., Barry, E., Bright, J. and Scanlan, J. 1991, Land degradation in the Dalrymple Shire: a preliminary assessment, CSIRO, Davies Laboratory, Townsville.

Dennison, W. C. and Kirkman. H. 1996, Seagrass survival model, in Seagrass Biology: Proceedings of an International Workshop, eds J. Kuo, R. C. Phillips, D. I. Walker and H. Kirkman, Faculty of Sciences, University of Western Australia, Nedlands, pp. 341–344.

ENCORE (in press), Encore: The effect of nutrient enrichment on coral reefs, 2, Synthesis of results and conclusions.

McIvor, J. G., Williams, J. and Gardner, C. J. 1995, Pasture management influences run off and soil involvement in the semi-arid tropics, Aust. J. Exp. Agric. 35: 55–65.

Moss, A. J., Rayment, G. E., Reilly, N. and Best, E. K. 1992, A Preliminary Assessment of Sediment and Nutrient Exports from Queensland Coastal Catchments, Queensland Department of Environment and Heritage Technical Report No. 4, Brisbane.

Neil, D. T. and Yu, B. 1996, Fluvial sediment yield to the Great Barrier Reef lagoon: Spatial patterns and the effect of land use, in Downstream Effects of Land Use, eds H. M. Hunter, A. G. Eyles and G. E. Rayment, Department of Natural Resources, Queensland, Australia, pp. 281–286.

Schaffelke, B. and Klumpp, D. W. 1998, Short-term nutrient pulses enhance growth and photosynthesis of the coral reef macroalga Sargassum baccularia, Mar. Ecol. Prog. Ser. 170: 95–105.

Van Woesik, R., Tomascik, T. and Blake, S. 1999, Coral assemblages and physico-chemical characteristics of the Whitsunday Islands: evidence of recent community changes, Marine and Freshwater Research, 50: 427–440.