Arbejdsrapport fra Miljøstyrelsen Nr. 11/2005 – Evaluering af mulige tiltag til reduktion af landbrugets metanemissioner

Evaluering af mulige tiltag til reduktion af landbrugets metanemissioner

Summary and conclusions

The measures for reducing greenhouse gas emissions from agriculture in Denmark have so far concentrated on reducing emissions of nitrous oxide, primarily as a consequence of the Aquatic Action Plans. There are, however, several options to further reduce agricultural greenhouse gas emissions, and so far only few measures have been implemented to reduce methane emissions from agriculture. These emissions primarily originate from enteric fermentation in animals (in particular cows, but also pigs) and from manure storages.

Effects in the national emissions inventory

It has been estimated that a change in inventory method for methane from enteric fermentation of dairy cows will contribute about 0.1 mill. ton CO₂-eq to the Danish national reduction commitment. Achieving this requires, however, a structured research effort, which could probably be completed by the end of 2006 if initiated now.

There is great uncertainty related to methane emissions from stored animal manure. It is estimated that the database required for changing the national emissions inventory for methane from manure storages will require a considerable research effort.

The current emissions inventory for nitrous oxide from field applied fertilisers and animal manure uses the same emission factor (1.25%). In the literature there is considerable support for using a differentiated emission factor, i.e. a higher value for manure N and a lower value for mineral N. The emissions from manure may possibly be reduced by removing the easily degradable organic matter in the manure. A specific Danish research in this area is recommended, because of the generally lighter soils in Denmark compared with other European regions.

The inclusion of LULUCF (Land Use Land Use Change and Forestry) in the emission inventory could contribute a reduction of 0.7 mill. ton CO₂-eq. by 2008-12 from the agricultural sector (excluding forestry). To include this amount in the emission inventory under the Kyoto-protocol will require a considerable effort to document land use and changes in land use. It is currently not possible to estimate the costs involved in this, nor has it been finally decided how carbon sequestration in mineral soils can be included in the emission inventory for the Kyoto Protocol.

If national CO₂ reducing projects within the LULUCF area is introduced, it will soon have to be decided how this is to be integrated in the national emissions inventory. It will require establishment of guidelines and routines, including collaboration with other authorities, in order for these projects to be included in an easy and flexible way.

Methane emissions from dairy cows

A model-based analysis has been conducted to assess the development in methane emissions from dairy cows during the period 1991 to 2002. The winter-feeding of the cows has changed during this period so that the feeding level and the starch content has increased, whereas the sugar content has decreased. A review of relevant literature shows that these changes have probably led to a reduction in methane production per kg dry matter in feed and as percentage of the gross energy content of the feed.

The simulation model Karoline was used for quantitative prediction of the methane emissions from dairy cows under varying conditions with respect to level feed feeding, proportion of concentrates in the ration, digestibility of roughages as well as fat, sugar and starch content in the feed. The results of these simulations are in accordance with the corresponding experimental data from the literature, and Karoline is thR_refore considered a reliable model for predicting methane production by dairy cows.

The Karoline model was used in comparison with three other models (regression equations) to estimate the decrease in methane emissions from 1991 to 2002 based on the changes in winter-feeding practice that had taken place over this period. These simulations showed that the enteric methane emissions (as percent of gross energy intake) from dairy cows in Denmark was reduced by 5-6% from 1991 to 2002.

Methane emissions from pigs

A review of existing Danish data on methane emissions from pigs shows that the production of methane from piglets is insignificant, about 0.1% of gross energy intake. Slaughter pigs fed with a low-fibre diet or standard feed mixtures have methane emissions of 0.2 to 0.5% of gross energy intake. When feeding with a higher fibre content the production of methane may, depending on fibre type, increase to about 1% of the gross energy. There appears to be both individual and race-related differences in the methane production from pigs. Dry and pregnant sows fed at maintenance level have methane productions of 0.6 to 2.7% of gross energy depending on both feed level and fibre type. The methane production from lactating sows has been measured at about 0.6% of gross energy intake.

The review has only included typical examples from Danish experiments, where measurements of methane production have been included. Quantification of methane emissions was not a major objective in any of the experiments. There has thus not been conducted any systematic experiments to explore the variation in methane production or the possibilities to reduce the emissions.

Methane oxidation in surface crusts of slurry tanks

A fixed cover of slurry tanks appears to reduce methane emissions from slurry stores. In light of new knowledge on methane oxidation in the surface crust of slurry tanks it was hypothesized that control of the air exchange above the slurry surface crust may be able to reduce methane emissions to the atmosphere. Within this project a laboratory scale experimental facility was constructed, which controls the concentration of methane or oxygen on the basis of on-line gas analyses. The system consists of two storage containers (70 litres) with tightly sealed lids. Sampling of the venting air stream before, inside and after each container enables quantification of methane emissions from stored slurry with and without surface crusts. The air exchange is controlled via a PC by two peristaltic pumps. Air sub-samples for gas analysis are pumped to a portable gas chromatograph (GC) via a 6-port valve. The software written for this application analyses the GC output and calculates the deviation from a set-point, from which a modified pump-rate for the following time interval is calculated and communicated to the relevant pump. The pumps have been calibrated individually. The functionality of the system was tested with a known methane standard, corresponding to a constant production, and with pig slurry. The emission interval that the pumps can compensate is up to about 30 g CH₄ m^-2 d1, which covers the range normally expected from stored slurry.

Methane emissions from manure handling

Models for methane and nitrous oxide emissions from stored slurry and solid manure have been described. The available literature is more extensive for CH₄ than for N₂O, and it is also more extensive for slurry than for solid manures. This means that for some greenhouse gas sources it is difficult to derive emission models, and that the models can not have the same level of detail, i.e. information is missing on the influence of some of the controlling factors.

The model for methane emissions from stored slurry includes a temperature response with a considerable uncertainty, as quantified by different enthalpies for methane formation. This gives an uncertainty of 15-90% in mean annual CH₄ emission. A temperature increase of 4C as projected for 2100 will increase the annual emissions by 35-220%. The observed change in storage practice over the period 1990 to 2003 has increased the mean annual CH₄ emission by 30-90%, mostly due to changes in the amount of manure being stored.