Greenhouse gas emissions and potential mitigation options for the Australian dairy industry

One of the biggest challenges facing the world today is how we feed an everincreasing population while reducing greenhouse gas (GHG) emissions that are contributing to global warming. Unquestionably, the livestock sector represents a significant source of emissions, generating carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O).

In 1988, the Intergovernmental Panel on Climate Change (IPCC) was established to prepare a comprehensive review and recommendations concerning to the state of the science of climate change, the social and economic impact of climate change, and possible response strategies and elements for inclusion in a possible future international convention on climate. The first assessment report of the IPCC served as the basis for negotiating the United Nations Framework Convention on Climate Change (UNFCCC). Via the IPCC, a series of algorithms and emission factors (EFs) were developed to calculate GHG emissions that conform to the UNFCCC, thus allowing individual countries to calculate their GHG emissions.

The Australian Federal Government began accounting and reporting the nation’s GHG emissions in 1990 according to the UNFCCC rules. Currently, agriculture is responsible for 13% of Australia’s GHG emissions and is the primary source of CH4 and N2O emissions. The national accounting of GHG emissions adopts a large-scale approach. As such, it does not estimate individual farm GHG emission profiles, nor identify potential mitigation strategies to reduce total farm emissions.

The purpose of this thesis was to determine the GHG emissions of Australian dairy farms using the Australian GHG methodology and examine potential mitigation options to reduce on-farm GHG emissions attributed to milk production. To ascertain GHG emissions, a localised focus within one region was explored, where the milking herd grazed pastures year-round with supplementary feeding occurring either in the dairy parlour or grazed paddocks. Sixty dairy farms in Tasmania were examined, with their total GHG emissions varying between 704 and 5,839 t CO2 equivalents (CO2e)/annum. A metric of emissions intensity (EI) of milk production, defined as kg CO2e/kg fat and protein-corrected milk (FPCM), was calculated to allow comparison between farms. The mean EI was 1.04 kg CO2e/kg FPCM, with individual farms varying between 0.83 and 1.39 kg CO2e/kg FPCM. Linear regression analysis showed that 93% of the difference in total farm GHG emissions could be explained by annual milk production. The study also found that 60% of the difference in the EI of milk production between farms was explained by differences in feed conversion efficiency (FCE; kg FPCM/kg dry matter intake (DMI)) and nitrogen (N) fertiliser application rates (kg N/ha.annum).

This on-farm evaluation at a local level (Tasmania) was expanded nationally to 41 Australian dairy farms. Farms varied between grazing pastures with supplementary feed delivered in the dairy parlour and paddocks through to farms where, in addition to grazing and supplements delivered in the dairy, cows spent a proportion of their time off paddock consuming partial mixed rations. Individual farm total GHG emissions varied between 411 and 9,416 t CO2e/annum. The Australia-wide mean was estimated as 1.04 kg CO2e/kg FPCM, with individual farms varying between 0.76 and 1.68 kg CO2e/kg FPCM. Linear regression analysis showed that 95% of the difference in total farm GHG emissions was explained by annual milk production. Milk production per cow (kg FPCM/cow.lactation) explained 70% of the difference in EI between farms. Farms were grouped according to their farming system (FS), indicative of the level of grain feeding and supplementary feeding management. The mean EI of milk production for FS1 farms (refers to grain feeding of < 1 t dry matter (DM)/cow.lactation) was 1.23 kg CO2e/kg FPCM. This was significantly (P < 0.05) greater than the mean EI for FS2 and FS3 farms, at 0.98 and 0.97 kg CO2e/kg FPCM, respectively. FS2 farms refers to grain feeding of > 1 t DM/cow.lactation with supplementary forage fed in the paddock while FS3 farms refers to grain feeding of > 1 t DM/cow.lactation and the incorporation of supplementary feeding into a partial mixed ration delivered on a feedpad.

The Australian inventory methodology for calculating GHG emissions is an estimation method based on current science. As new science pertaining to GHG emissions emerges, the inventory is updated, with the most recent occurring in 2015. An assessment was undertaken to ascertain the consequence of the updated methodology on the EI of milk production utilising the same 41 Australian dairy farm case studies. Mean EI increased by 3% to 1.07 kg CO2e/kg FPCM (ranged between 0.84 and 1.54 kg CO2e/kg FPCM). Annual milk production remained a strong determinant, with 96% of total farm GHG emissions explained by this. A Concordance Correlation Coefficient analysis was undertaken to estimate the extent of agreement between the two methodologies. There was moderate agreement between methodologies for estimating individual farm EI of milk production. However, primarily due to a regional variation in an emission factor (EF) for manure management, there was poor agreement between methodologies for estimating regional EIs. This study reaffirmed that while enteric CH4 emissions remains the largest component of on-farm GHG emissions, waste CH4 emissions has emerged as a more substantial source of on-farm GHG emissions.

The need to identify mitigation options that are considered ‘win:win’ options in reducing the on-farm GHG emissions while maintaining or improving productivity and/or profitability are critical to meeting the need to reduce ruminant livestock GHG emissions. In addition, win:win strategies may be more readily implemented by farmers, as opposed to mitigation strategies that erode productivity or profitability, in the pursuit of reducing GHG emissions. This thesis explored the GHG emissions reduction potential of two mitigation options applicable for Australian dairy farms;

(i) evaluating dietary and breeding approaches for improving animal N use efficiency (NUE);

(ii) improving feed quality to increase liveweight gain (LWG) promoting earlier mating of dairy heifers.

Reducing the overall diet N concentration was found to be a more effective means to improving NUE and reduce N2O losses than increasing the concentration of N in milk of lactating cows when modelled across three climatic regions. Nitrous oxide emissions were reduced by 50 to 57% when the supplementary feed was reduced from 4% to 1% N (total diet N concentration of 4.1 and 2.5%, respectively). In contrast, when the N concentration of milk was increased from 0.50 to 0.65%, reflecting 3.1% and 4.1% milk protein, N2O emissions were only reduced by 7 to 11%. This was an important finding, highlighting that reducing the source of N intake in the diet resulted in a more significant reduction in emissions compared to increasing the sink of N into milk. In addition, manipulation of dietary N would be a much easier mitigation option to implement than manipulation of N concentration in milk through breeding. This is a currently available mitigation option for all Australian dairy farmers to consider implementing, especially for those farms currently feeding a high protein diet.