High Quality Forage to Dairy cows and Low Quality Forage Biogas

High quality roughage feed is an important part of feeding strategies for milk production striving to reduce the dependence of imported soymeal. One way of obtaining this high quality silage is to use only the first cut ley. The regrowth ley would then need to find another application, where a possible solution is to use the regrowth for biogas production, also adding the possibility to digest the manure to biogas. The purpose of this study was to analyze the environmental and economic performance of a dairy and cereal farm system before and after introducing the dual-purpose ley production scheme.

Materials and method

Two cases were analyzed, one case that represents the farms before the introduction of the dual-purpose ley production scheme (the base case), and one that represents the farms with a quality assorted ley system and biogas production (the dual-purpose ley case). The base case consisted of a separate milk farm and cereal farm, whereas in the dual-purpose ley case the two farms cooperated via a biogas reactor, a combined heat and power plant (CHP), and by introducing ley in the crop rotation of the cereal farm. The economic analysis encompassed the costs and incomes associated with the production on the farms. Costs not differing between the cases, such as for the milking stable, were not included in the calculations.

The environmental analysis was performed using life cycle assessment (LCA) methodology, including primary fossil energy use, global warming potential (GWP), acidification potential (AP) and eutrophication potential (EP). Both cases produced milk and meat from 240 dairy cows as well as 1900 tonnes of cereals, composing the functional unit (FU) of the study. The base case also produced extra wheat and rapeseed, whereas the dual-purpose case instead produced extra heat and electricity. Each case was compensated for the extra production by system expansion. System expansion for the extra heat and electricity was analyzed in two scenarios, one where the biogas energy replaced fossil energy sources and one where renewable sources were replaced.

Results

Economically the largest difference between the base case and the dual-purpose ley case were the decreased income from cereal, the new income from electricity, increased costs for the ley production and investment and production costs for the biogas plant including the CHP. Compared to the base case the income increased for the dual-purpose ley case with approx. 81,500 per year, but the costs also increased with 203,500 per year. Consequently the profit decreased with 122,000 per year. A number of sensitivity analyses were performed to explore conditions when the dual-purpose ley concept would be profitable. It was found that the opportunity to sell excess heat was of critical importance. Approx. 30% of the produced heat could be utilized on the farms. The rest had to find other uses.

The base case used 10 TJ primary fossil fuels per FU. The dual-purpose case produced around 12 TJ (3,300 MWh) surplus secondary energy from biogas. If all heat from this biogas could be sold and thereby replace other energy sources, the dual-purpose ley case a) used 5TJ primary fossil fuels when the surplus energy replaced renewable primary energy, or b) replaced 15 TJ primary fossil fuels when all the surplus energy replaced fossil energy sources. For global warming potential, the base case emitted 2600 tonnes CO2-eq per FU. The dual-purpose ley case had a GWP of 2,600 tonnes CO2-eq when the biogas replaced renewable energy sources, but 1,200 tonnes CO2-eq when fossil sources were replaced. If only electricity could be sold, the GWP would be approx. 1,700 tonnes for the fossil alternative. Both EP and AP were dominated by on farm emission and were similar between the cases.

Discussion and conclusions

The need for heating on the dairy farm and farm households only covers for a fraction of the heat from the CHP, and there is a need to develop better solutions to economically exploit the heat generated. One possibility could be to use the heat for sorption-cooling of milk, to dry wood chips for energy production or to cooperate with horticultural or poultry production, where heating of the buildings are required. Changing to thermophilic anaerobic digestion could also be an alternative, since this increases the demand for process heat. However the potential income from utilizing the heat is reduced.

For the environmental results it was important to find usages for the biogas that reduces fossil fuel use. System expansion was used to account for the surplus energy produced from the biogas. When expanding the system to account for replaced processes, marginal production data is often used. In this study we used two alternatives for marginal energy data. Noticeably, the assumption regarding what kind of energy that is replaced is of critical importance for the results. The same is true for the assumption regarding production method and origin of avoided cereal production.

The assumption on ratio of plant available N in the digestate was important both for the economic and environmental analysis, as this determined the total amount of applied N. The N-fertilization rates were calculated based on plant available N in the digestate. A higher ratio of plant available N would lower the mineral N requirements, N2O emissions from soil and nitrate leaching.

Introducing ley in a cereal based crop rotation increases soil carbon. The impact from carbon sequestration from the atmosphere was only accounted for in a sensitivity analysis. Amount of carbon sequestrated depend on many factors such as agricultural management, climate, soil type etc. This in combination with the unknown indirect land use effects of a lower cereal production in the dual-purpose ley case are the main reasons for omitting soil carbon in the main study. In the sensitivity analysis, a yearly carbon sequestration of 125 kg C per ha was tested , which decreased on-farm GWP with ca 10% due to increased carbon sequestration at the cereal farm. This is small compared to the effects of biogas production.

The main conclusions were that the proposed dual-purpose ley system could turn the former energy consuming farms to energy producers, if the produced electricity and heat would replace fossil energy sources. The scenario could also substantially reduce GWP (-60%), on the same condition. If the produced heat and power would only replace other renewable energy sources, no difference compared to the base case occurred regarding GWP, since the reduction due to avoided renewable energy was in the same range as the reduction due to the avoided cereal production in the base case.

The observed decreases in GWP from reducing the need for concentrate feed, mineral fertilizers and changed manure management were not decisive. The largest potential for decreasing GWP was instead connected to the biogas produced, when this substituted fossil energy use.

The studied concept, to use high quality ley to dairy cows and to use low quality ley to biogas, is one way to obtain high quality fodder, while stimulating biogas production. In order for theconcept to be profitable it is essential that some conditions are fulfilled, for example that the produced heat is utilized. In order for the environmental potential to be fully utilized the biogas need to be used so that fossil energy sources are replaced.

Publications

• Project report (in Swedish): Mer och bättre vall till mjölkproduktion och återväxtvall till biogas. Download from: http://www.slu.se/hqvall.
• Poster presentation at World Bioenergy 2012, Jönköping: A fossil based offset for produced biogas can turn a dairy & cereal farm from energy consumer to energy producer.
• Scientific article (manuscript) Ingrid Strid, Carina Gunnarsson, Hanna Karlsson, Mats Edström, Jan Bertilsson, Systems analysis of a dual-purpose ley production system for Swedish milk and biogas. Planned submission to the journal Agricultural Systems.