To reach net zero by 2050, the UK must reduce greenhouse gas emissions from agriculture and methane emissions from farm livestock pose a thorny problem. Ruminants such as cattle and sheep emit methane from their digestive systems and their manure. Scientists are trying to find ways to reduce these methane emissions without wiping out large parts of the agricultural industry.
But my recent field research on dairy farms suggests emissions from slurry, cattle dung mixed with water, could be four to five times greater than the official government estimates.
While most attention is focused on enteric emissions, largely the burps from the animals themselves, my new study with colleagues means that much more attention needs to be paid to how to reduce methane emissions from storing slurry.
Fortunately, new technologies can capture, process and use methane from slurry and turn it into a valuable resource – biomethane. Where there’s muck there’s money, the Victorians used to say, and this is no truer than in the reduction of livestock methane emissions to address the net zero challenge.
The UK government’s national inventory report on greenhouse gas emissions says that methane emissions from dairy cattle comprise 75% enteric emissions and 25% emissions from manure management – that’s the storage and spreading of livestock poo. These calculations are based on a methodology developed by the Intergovernmental Panel on Climate Change, involving a complex formula based on the number of animals, what they are fed, environmental conditions and so on.
Scientists seem comfortable with these existing enteric emissions calculations, but the formula for calculating emissions from manure management is now coming into question as field studies around the world find these emissions are often higher than assumed.
My colleagues and I monitored emissions from the slurry lagoons on two dairy farms in Cornwall for a year. The lagoons, which hold slurry in concrete-lined pits, were installed with airtight covers and the gas released from the slurry was collected and weighed.
On one farm, we found methane emissions were almost four times greater than that were reported by the UK government to the UN (145kg per cow per year, rather than 38kg).
On the second farm, methane emissions were more than five times greater (198kg per cow per year). If these figures are representative – and more research is needed to confirm this – then much more methane is coming from slurry storage than previously thought. The ratio of enteric to manure management could be closer to 50:50, and the total methane emissions from the dairy sector would be around 40% greater than the UK government officially reports.
There has been a lot of attention paid to addressing enteric emissions, but the slurry question has been treated as much less of a priority in comparison.
Approximately two-thirds of dairy cows across a total of 12,500 farms in the UK are on slurry systems.
Technological advances do offer solutions. The biogas industry is developing techniques to capture methane by covering slurry storage facilities. The methane gas is then cleaned and compressed and available as a fuel source.
Tractor manufacturers have developed tractors that run on methane, and businesses and local councils are exploring converting vehicle fleets to run on the gas. Methane captured from slurry storage can heat and power farm buildings.
Some of this technology is already up and running in Cornwall and beyond. Our estimates show that this energy saving could be worth tens of thousands of pounds to dairy farmers with an average-sized herd of milking cows. Biogas from slurry could be of huge value to the UK’s agriculture sector, too – potentially in the order of £400 to £500 million a year, according to our calculations.
So reducing greenhouse gas emissions in agriculture can bring economic opportunities for farmers. The next challenge is to explore how these new supply chains for biomethane can be rapidly developed.
