The purpose of this MScCCAFS thesis is to assess the sensitivity of comparative milk footprints from dairy farm systems implementing conventional and legume-based animal diets to choice of allocation method. To do this an understanding of and life cycle assessments (LCA), carbon footprinting (CF), allocation methods and system expansion, the Scottish dairy industry and legume-based animal diets is required.

LCA and CF are methods that allow for the environmental impact of products and supply chains to be investigated. CF is a form of LCA and focuses on the environmental impact of Greenhouse gas (GHG) while LCA looks at more impact categories, for example water use, land use, acidification, global warming, eutrophication, acidification, ozone depletion (Broekema & Kramer, 2014; Dalgaard et al., 2008). Nitrous oxide (N2O) and methane (CH4) are potent GHG that account for a large portion of the agricultural emissions generated (Moumen et al., 2016). They have high global warming potential (GWP), GWP is an index for the amount of warming a gas causes over a given period of time. 1kg of methane causes 25 times more warming over a 100 year period compared to 1kg of CO2 (Raga Mexico et al., 2007).

Allocation methods play an important part in LCA. Allocation is the “partitioning the input or output flows of a process or a product system between the product system under study and one or more other product systems” (International Organization for Standardization, 2006). Allocation methods play a key role when LCA studies are carried out on agricultural systems, such as on dairy farms, as there are many co-products being generated along with the main product milk – these co-products include meat from culled cows or calves that enter the beef industry, or manure/slurry. There are different allocation methods available, such as mass, economic, energy and protein based allocation. The different methods of allocation can have a significant influence on footprint results, so choosing the most suitable method is critical (Rice et al,. 2017). Mass and economic allocation were selected for this MScCCAFS thesis project. Mass allocation is based on physical properties (mass or volume), dairy farms look at the relations between total farm GHG emissions and the total milk and meat production in kilograms (Kristensen, Mogensen, Knudsen, & Hermansen, 2011). Economic allocation also uses physical properties, but the different values of the products are taken into account, so for dairy farms it looks at the total amount of milk and meat produced at a standard unit price (Kristensen et al., 2011).

System expansion is a method used to avoid co-product allocation. In system expansion co-products are considered alternatives to other products on the global market. As previously stated, a dairy farm’s main product is milk while the slurry is a co-product. This co-product may also be regarded as a “waste” depending on whether there is a nearby demand (value) for it. The slurry from the dairy system can replace fertilizer on the market, which in turn means there is an avoided production of fertilizer and thereby a negative contribution to the environmental impact from the lifecycle of dairy (Cederberg & Stadig, 2003; Q-PorkChains, 2017). But it must be noted depending on method used to apply the slurry that emissions associated with application of slurry/manure can be higher than those associated with application of fertilisers, countering the avoided manufacturing benefit, for example application using a trail shoe instead of a splash plate can see a 20% reduction in emissions released (Dowling et al., 2008; Laboski et al., 2013). The timing of the application can also have an effect on the emissions, very hot and dry conditions lead to higher levels of emissions being released (Laboski et al., 2013).

The Scottish dairy industry is changing, all-year-round indoor feeding and continuous housing are becoming more prevalent in the UK, the number of farms practising traditional all-summer grazing is decreasing (March et al., 2014). Legume-based animal diets have the potential to fit well into the traditional summer grazing systems and all-year-round indoor feeding. The amount of greenhouse gases (GHG) being generated in these systems will vary. Reducing emissions and relevant energy flows will be critical for farming in the future (Aragón-Durand et al., 2018; Food and Agriculture Organization, 2015). Climate change is becoming an ever-growing concern and it is causing a push to decarbonise areas such as transport, industry and agriculture. Transport and industry are already moving in this direction, with transport using electric vehicles and industry starting to use more renewable energy sources such as wind turbines to help reduce their carbon footprint. The agricultural sector must also find ways to reduce their footprint (Hoegh-Guldberg et al., 2018; Rogelj et al., 2018).

Improving the genetic merit of dairy cows is a way that the footprint of dairy production can be reduced, as a more productive cow will give rise to lower emissions per litre of milk or kg of meat produced. Dairy cows with higher genetic merit can have higher milk yields, increased fertility, lower calving intervals, increased longevity of the cow within the herd, lower feed requirement, less emission production during rumination (Byskov et al., 2017; Crowley et al., 2017; Hurley et al., 2018). Improving these traits through genetics can reduce the amount of emissions produced by dairy cows in Scotland (Ross et al., 2014; Vellinga et al., 2018). The progression of genetics within the Scottish dairy sector has been steadily improving since the 1980’s (Wall, 2018). This means the chance and system to reduce dairy footprints through genetics is already in place, and the there is large potential to continue to reduce the emissions through genetics.