School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemSuitability of subsurface drip irrigation for sustainable pasture production in the Riverine Plain( 2012)The dairy industry of the Riverine Plain in the southern Murray-Darling Basin has traditionally relied on border-check (B-C) irrigation for year round productivity of perennial pasture. Recent prolonged drought conditions coupled with unprecedented low water allocations severely constrained B-C pasture irrigation. Also, concern about the ecological impacts of water extraction and its use for irrigation in the Murray-Darling Basin is increasing. Consequently, farmers and resource managers are considering the role of subsurface drip (SSD) irrigation in improving pasture irrigation sustainability. SSD irrigation is a water efficient technology that delivers water and nutrients through emitters directly to points within a plant’s rootzone, often leading to improved crop yield. Past SSD research has focussed on row crops including vegetables, fruit, maize and lucerne. SSD irrigation of pasture on Riverine Plain soils represents a significant departure from this past research in three main respects: (1) the pasture will be grazed by dairy cattle, (2) pasture production occurs across the full soil surface rather than as individual plants separated by bare soil, and (3) Riverine Plain soils are duplex, with irrigation performance influenced by contrasting horizon hydraulic conductivity, soil shrinkage tendencies and the role of macropore flow in infiltration. Given SSD irrigation’s substantial cost, a greater understanding of likely water, pasture productivity and environmental benefits is necessary before its use is recommended for pasture production in the Riverine Plain. The overall objective of this study was to identify the combinations of SSD irrigation design (tape spacing) and operation (irrigation frequency) that deliver optimal environmental performance for perennial pasture irrigation across the range of soils found in the Riverine Plain. A three step approach was used. Firstly, replicated field experiments were conducted at two case study sites, allowing rigorous quantification of pasture accumulation, water and nutrient movement on duplex soils with differing B horizon permeability. Combinations of tape spacing (0.6, 1.0 and 1.4 m) and irrigation frequency (daily or 4 days irrigation) were imposed. The second step assessed the ability of the Hydrus-2D model to predict the hydrological processes observed at the case study sites, with the intention of then applying the validated model in the third step, investigating SSD design and operation impacts on water and nutrient movement for the major soil classes of the Riverine Plain. The field experiments showed that seasonal pasture accumulation exhibited sensitivity to tape spacing and irrigation frequency whereas pasture botanical composition was insensitive.The degree of sensitivity in pasture accumulation differed markedly between the two sites, reflecting soil characteristics. Where the soil B horizon was moderately permeable (East Shepparton fine sandy loam, with B horizon saturated hydraulic conductivity of ~10 mm/h), pasture accumulation was reduced by 17% as tape spacing increased from 0.6 m to 1.4 m. Where the soil B horizon had low permeability (Lemnos loam, with B horizon saturated hydraulic conductivity of ~5 mm/h), pasture accumulation was insensitive to tape spacing or irrigation frequency. Pasture yields on the soil with the moderately permeable B horizon were 30% lower than on the soil with the low permeability B horizon. The extent to which tape spacing and irrigation frequency affected water and nutrient movement was controlled by the subsoil’s hydraulic behaviour. At the seasonal scale, tape spacing and irrigation frequency had no consistent impact on water balance components at both sites. Universally, minimal runoff occurred and deep percolation was the main pathway for water loss. Subsoil hydraulic behaviour controlled rootzone soil moisture redistribution and the fate of applied irrigation water, the mechanisms of deep percolation and its consequence. Soil moisture and deep percolation were more sensitive to irrigation frequency than tape spacing. On the soil with the moderately permeable B horizon, soil moisture 0.1 m above the tape responded to 4 days irrigation but not to daily irrigation. Nitrate-N travelled with irrigation water, consequently tape spacing had little impact on nutrient movement while irrigation frequency was important for the soil with the moderately permeable B horizon. Use of Hydrus-2D as a tool to generalise the assessment of SSD performance was attempted. However, in validating Hydrus-2D’s performance in simulating the case studies it was clear that Hydrus-2D did not capture key hydrological processes. Hydrus-2D was unable to predict water and solute movement with sufficient accuracy due to its inability to account for macropore flow, which is a recognised characteristic of Riverine Plain soils. Subsequent identification of optimal SSD design and operation combinations for environmental performance thus drew solely on the experiment findings. For texture-contrast duplex Riverine Plain soils, SSD operating regime (irrigation frequency) has greater impact on water and nutrient movement than SSD design (tape spacing), but the degree of impact depends on the subsoil’s hydraulic properties. For duplex soils with moderately to highly permeable subsoils (for example East Shepparton fine sandy loam), irrigation duration will dictate the balance between gravity-dominant downward movement or lateral and upward redistribution of irrigation water. On duplex soils with low permeability subsoils (for example Lemnos loam), the subsoil restricts downward movement, allowing water to spread laterally and upwards regardless of SSD operating regime. Where the subsoil exhibits very low permeability (for example Congupna clay, with B horizon saturated hydraulic conductivity of ~1 mm/h), surface runoff may become more important as a loss pathway. Provision of incentives would improve economic feasibility of SSD in pasture production, if the environmental benefits to the wider community warranted such intervention. This study has highlighted potential environmental advantages and disadvantages of SSD irrigation, clarifying where its use is appropriate. Field studies demonstrated that SSD was not ideal on soils with moderately to highly permeable subsoils, leaving sprinkler irrigation as the most appropriate option for irrigators. For other Riverine Plain soils, SSD irrigation has clear advantages over B-C irrigation in the minimisation of evaporative water loss and surface runoff. The absence of surface water and the reduction in surface runoff in turn will assist nutrient management. Use of fertigation with SSD will reduce volatilisation loss of fertiliser, but this may be offset by loss of nitrate-N in drainage water. Similarly, managing deep percolation from SSD may prove more challenging than managing runoff from B-C irrigation. SSD depends on a pressurised water supply, has high embodied energy and thus higher energy requirements and associated greenhouse gas generation than B-C irrigation. Ultimately, the question of whether adoption of SSD irrigation substantially improves overall pasture irrigation sustainability or not will have to be assessed on a case by case basis, given its dependence on current farm practices.
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ItemTactical choices of medium and high input dairy systems( 2013)In the last decade dairy farms in northern Victoria were exposed to increased volatility of input and output prices as well as variable climate conditions that include a big dry period. Two representative case study pasture based dairy farms of ‘medium’ and ‘high’ input were selected to examine the production and financial outcomes that arise from a multi-year sequence of tactical farm management decisions. The approach of the research had several key aspects; case studies were selected as the method of investigation, on-farm interviews of the case study farmers were carried out and their financial and physical history was collected. A stochastic multiyear whole-farm biophysical and economic spreadsheet model was developed to analyse the physical and economical performance of the case study farms. The study found that both farming systems had different optimum choices available year to year to increase profitability. In many of the scenarios tested, the decision option with the highest growth in equity compared to other options tested did not always result in the highest net cash flow. The decision maker would need to evaluate the net cash flow implications of their decisions to determine if they are worthwhile choices. For both farms, in years with greater upside, there was a greater range of outcomes between decisions compared to years with poor financial outcomes. This suggests farm managers cannot get too relaxed and complacent in the good years and need to ensure they are gaining the benefits of the good year as well as minimizing losses in the poor years.
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ItemA systems approach evaluating alternative dairy feeding strategies in south-east Australia( 2012)The Australian dairy industry relies primarily on pasture for its feed supply. However, the variability in rainfall negatively effects plant growth, leading to uncertainty in dryland feed supply, especially during periods of high milk price. New (complementary) systems combining perennial ryegrass with another crop and/or pasture species have potential to mitigate this seasonal risk and improve productivity and profitability by providing off-season feed. To date, the majority of research studying the integration of alternative crops into pasture-based systems has focused on substitution and utilisation of alternative feed sources. There has been little emphasis on the impacts of integration of forage crops on the production of surpluses or deficits in feed supply (pasture and pasture products such as hay and silage) for subsequent lactations. This study, unlike others where pasture consumption is estimated, uses actual annual pasture consumption rates measured over four years from an on-farm trial. In addition, while the ultimate aim of adopting different feeding systems is to increase productivity and profitability of dairy farmers in Victoria, the benefits and costs associated with transitions of systems have to be addressed. These include whether alternative feeding systems produce different amounts of greenhouse gas (GHG) emissions, and whether the proportions of carbon-based and nitrogen-based emissions produced in each system are comparable. Further, if a direct cost is placed on carbon emissions, whether Australian dairy farmers will seek mitigation strategies to reduce their GHG emissions. The impact of a direct price of carbon emissions on dairy farm operating profits has not been widely studied in Australia. This thesis examines and analyses actual systems data from two pasture-based feeding systems trials conducted in southern Australia. The systems examined are a ryegrass-based (RM) system comprising pasture and pasture silage and a complementary forage-based (CF) system comprising double-cropping (winter cereal crop followed by a summer grazing crop) as well as pasture and pasture silage. A comparison of pasture production and pasture consumption profiles of the two pasture systems revealed that both are under supplied during the months of July, December, January, February and March (with at least a 50% probability). Feeding strategies were developed for the rest of the production year when the pasture production exceeded the pasture consumption. The integration of double-cropping systems into pasture-based dairy production systems in south-east Australia was investigated for providing feed at critical times of the production period to further increase milk production. The partial life cycle assessment (LCA) used to evaluate the GHG emissions produced in each feeding system indicated that the RM and the CF systems produced 9.6 t and 12.3 t of carbon dioxide equivalent (CO2-eq) emissions per ha respectively. This equalled to 7.4 kg and 7.1 kg CO2-eq emissions per kg milk solids (MS) produced respectively. A nitrogen (N) surplus balance technique showed that the ratio of N in milk to N in all the inputs used was similar, 25.2% and 26.6%, for the RM and the CF systems respectively. A novel risk analysis comparing the variability in income in response to different prices on carbon emissions was also conducted in this study. Imposing a carbon price ($20–$60) and not changing the systems reduced the operating farm operating profits by 28.4% in the RM systems and 25.6% in the CF system compared to a scenario where no carbon price was imposed. In conclusion, production and utilisation of home-grown feeds will remain paramount among the south-east Australian dairy farmers to maintain and increase profitability in this competitive dairy sector. However, the risk associated with the higher operating profits in the CF system requires further evaluation. There is opportunity for future studies to focus on the impacts of different mitigation and adaptation strategies and policy impact on farm operating profit. Further evaluation and improvement of the assumptions and model predictions as well as the monitoring technologies are suggested to enable further policy research.
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ItemProfitable irrigated dairy farming systems: exploring efficiencies in feeding systems with decreasing water availability( 2010)Changes in climate and water policy, and increasing competition between urban, environmental, amenity and consumptive uses are expected to decrease the amount and increase the variability of water allocations to irrigated dairy farmers in northern Victoria. This thesis examines whether changing to more efficient feeding systems and increasing milk production per cow and/or per farm may enable some dairy businesses overcome the impacts of these changes.