| Strategies for greener milk
Midair (Greenhouse gas mitigation for organic and conventional dairy production) is a research project funded by the European Commission’s Fifth Framework Programme for RTD. Launched in 2001, the project unites 19 agricultural and environmental research organisations from across the EU, coordinated by the Institute for Energy and Environment in Leipzig, Germany. Earlier EU-funded research had revealed a lack of knowledge about biogenic GHG emissions from arable and animal farming. In response, the Midair project formed two objectives: first, to give better estimates of these emissions and their sources, and second, to investigate options to reduce (mitigate) them.
|Farms are an important source of methane and nitrous oxide|
Dr Søren O. Petersen, a Midair partner from the Danish Institute of Agricultural Sciences, explains the need for the project. “Dairy herds produce methane through fermentation in the cow’s stomach, which is then exhaled to the atmosphere, but also methane is produced in cow manure during storage. Nitrous oxide is produced from the nitrogen in manure and mineral fertilisers. And the amounts are significant. The Kyoto Protocol has committed the EU-15 to an 8% reduction in GHGs, which is the equivalent of around 340 million tonnes of carbon dioxide. In Midair, we estimate that mitigation measures on the types of dairy farms that we have investigated could make an appreciable contribution to the Kyoto commitment.” Midair also investigated regional differences in dairy farm emissions, as well as comparing conventional and organic farming methods.
What goes in must come out – the FarmGHG model
To arrive at better estimates of carbon and nitrogen flows; Midair developed a model called FarmGHG that describes inputs, outputs and internal flows within a model dairy farm (see figure 1).
FarmGHG allows quantification of GHG emissions, based on internationally agreed models and, importantly, it also accounts for energy use and emissions occurring in the whole supply chain – critical information for mitigation strategies. The analysis has shown that GHG emissions per unit of milk produced rise with the livestock density. In other words – producing a bottle of milk by intensive dairy farming produces more GHGs than a bottle from less intensive farming. Regional differences were found, for example between Mediterranean, Atlantic and Continental regions. For a given livestock density, GHG emissions from organic dairy cows will be similar to those from conventionally farmed cows. While organic farms usually have fewer cows, and thus farm less intensively with lower total emissions, they also have lower production.
Precise but mucky measurements
As well as modelling, the consortium performed long-term monitoring of methane and nitrous oxide emissions on farms and in crop rotations of five dairy producing regions in Europe. These regions have different climates and soils, varying crop rotation strategies for animal feed and employ different manure management practices. Søren O. Petersen again: “Methane emissions come from the cows themselves as well as animal houses and manure storage facilities. Nitrous oxide arises from nitrogen inputs to fields, when manure or mineral nitrogen is used for fertiliser and interacts with crops, the climate and the soil. We monitored all these environments to establish GHG emissions using a variety of techniques, such as repeated point measurements in fields, a tracer method for manure storages, and a ‘fast box’ device to track other point sources on the farm, such as ditches or silage stores.” To verify the FarmGHG model, overall emission plume measurements were made with a tunable diode laser downwind from 20 farms in the Netherlands.
Such emission plumes can be seen in figure 2. The levels of methane measured from the farms compared well with the emissions calculated using the FarmGHG model.
Mitigating factors investigated
The consortium investigated several aspects of dairy production where GHG production might be reduced, for example:
- Dairy cattle spend much time on pastures where their dung and urine add nitrogen to the soil, which is then disturbed by treading. Varying impact by animals and urine properties were investigated to understand if management practices would reduce emissions, such as rotational grazing proposed in earlier studies. The Midair results suggest not and identified soil moisture levels as important in regulating emissions.
- Manure can be stored and handled in several ways, such as mixing with straw and using covers to reduce aeration. These options were compared to establish the most promising manure-management practices. A significant finding was that surface crusts on stored slurry reduce methane emissions appreciably, this was documented for the first time in Midair and it appears to be a cost-effective GHG mitigation option."
Using the FarmGHG model, mitigation scenarios were considered including manure application techniques, biogas production, reducing feed imports, and others. Direct injection of slurry into fields was found to be preferable to broadcasting, and the production of biogas from anaerobic digestion was shown to be a very cost-effective GHG mitigation option, as this treatment produces energy thus reducing inputs, while also cutting down the emission of methane during manure storage.
Optimising management practices
“A significant finding of our modelling is that GHG emissions in dairy farming increase with the intensity of the production system for the same amount of milk produced per cow, and this is independent of whether organic or conventional farming methods are employed,” says Søren O. Petersen. This means that ‘extensification’, or lowering the intensity of farming by lowering livestock densities, would reduce GHG emissions for a given herd size. However, low intensity farming would reduce production volumes of the farm with some far-reaching societal consequences, so in the short term the Midair partners advocate better management practices to reduce GHG emissions. As Peterson explains, “From the work accomplished in Midair we see much scope to reduce methane emissions in the handling and storage of manure. In addition, using methane captured from manure and slurry to generate energy increases the environmental benefits as this substitutes for fossil fuels.”