Most proposals for GHG management fail to consider all of the factors. This one does a better job.

Greenhouse gas (GHG) emissions from New Zealand dairy farms are significant, representing nearly 35% of New Zealand’s total agricultural emissions. . .

Dairy farms have a high demand for electricity, with a 300-cow farm consuming nearly 40 000 kWh per year. However, because only ~10% of the manure produced by the cows can be collected (e.g. primarily at milking times), a maximum of only ~16 000 kWh of electricity per year can be produced from the effluent alone. . .

A solution for smaller farms is to co-digest the effluent with unutilised pasture sourced on the farm, thereby increasing biogas production and making the system economically viable. A possible source of unutilised grass is the residual pasture left by the cows immediately after grazing. This residual can be substantial in the spring–early summer, when cow numbers (demand) can be less than the pasture growth rates (supply). The cutting of ungrazed grass (topping) is also a useful management tool that has been shown to increase pasture quality and milk production, especially over the late spring–summer. . .

For the average dairy farm, the net reduction in emissions of 0.68 CO2e/ha would equate to nearly 14% of the direct and indirect emissions from farming activities and if implemented on a national scale, could decrease GHG emissions nearly 1.4 million t CO2e or ~10% of New Zealand’s Kyoto Protocol obligations while at the same time better manage dairy farm effluent, enhance on-farm and national energy security and increase milk production through better quality pastures.

If the compost remaining afterwards is spread on the pastures another benefit is gained. When a system like this is continued for several years there is continuous improvement. Many of the objectives of the enterprise are realized, and externalities are reduced.

It would be better if the material was gasified in a farm scale biochar system since the char would have even greater benefits for the pastures and would sequester carbon in a durable form. I suspect that the enterprise could end up being carbon negative when the amount stored in the soil is subtracted from the amount emitted from operations.

I'd like to see much more effort invested in developing affordable farm scale systems. Depending on configuration they can produce liquid fuels and fertilizer as well as energy (heat/electricity etc.) and biochar. I'd like to see them utilize recent advances in thermoelectrics as well. Farms and ranches could perhaps compensate for the harms caused by cities and industries. They should be paid for this of course.