School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemGreenhouse gas emissions from Australian beef feedlots( 2011)Emissions of the greenhouse gases, methane (CH4) and nitrous oxide (N2O) and the indirect greenhouse gas ammonia (NH3) play an increasing role in public concern about the environmental impact of concentrated animal feeding operations, including feedlots. However, there is a lack of emissions measurements under typical commercial conditions and there is high uncertainty in the estimation. The lack of accurate measurements and baseline emissions also makes it difficult to evaluate efficiency of current mangemange practices and identify the potential reductions under mitigation options. The objective of this study was to achieve increased understanding of greenhouse gas emissions from Australian beef feedlots, elucidating the biophysical factors controlling emissions from feedlot systems. Specifically, the study utilises measurements of greenhouse gas emissions undertaken at commercial feedlots in Australia using micrometeorological methods and integrates data collected from the feedlot operators into empirical models with the aim to identify and quantify the sources of variation in measured emissions between sites and seasons; test the validity the modelling approach used specifically for feedlots and quantify the link between animal behaviour and diurnal emissions patterns. This study comprised two detailed modelling exercises. The first utilising the results of published studies to validate a range of equations for predicting enteric methane emissions and for predicting emissions of methane, nitrous oxide and ammonia from manure. The second modelling exercise utilised the results of measurements undertaken in two commercial Australian feedlots to evaluate a range of models under commercial conditions. Finally, the diurnal variation in micrometeorological measurements of CH4 and NH3 were examined in the context of animal feeding behaviour in order to examine implications for measurement accuracy and examine correlations between fluxes and behaviour. This thesis indicates that the current Australian Inventory methodology for estimating greenhouse gas emissions from feedlots (enteric CH4, manure CH4, N2O and NH3) suffers from considerable inaccuracies. Although more accurate estimates of CH4 emissions appear to be associated with utilising an equation based on ration composition, particularly carbohydrate fractions the current approach over estimates emissions considerably. Inaccuracies in prediction of emissions of N2O and NH3 are related primarily to the use of single “emissions factors” which do not adequately reflect the changes in potential emissions associated with changing environmental conditions. This thesis also explored the contribution of CH4, N2O and NH3 using IPCC default factor of 1.25% deposited NH3 is lost as N2O to total feedlot emissions, represented as CO2-e. Initial estimates suggest that feedlot emissions were dominated by CH4, with minor contributions of direct and indirect N2O. However, based on the measurements nitrogenous greenhouse gases are predicted to contribute up to 52% of total CO2-e. These results indicate that mitigation options to reduce feedlot emissions need to be applied to both enteric CH4 and nitrogenous gas emission, particularly NH3.