Revised methane method pegs dairy emissions 20% lower

Net carbon emissions from a sample of livestock farms fell significantly when analysed by a carbon methodology that accounts for biogenic methane.

Distinguishing between biogenic (methane from ruminants) and fossil fuel methane (methane from below the earth) reduced two dairy farms’ net carbon emissions by 19% and 24%, according to one trial.

See also: Project shows how farming can hit – or better – net zero

Using this approach, emissions from a sheep and beef farm fell 66%, making it a net carbon sink (see “Net carbon emissions positions of seven ARCZero farms falls 74% with GWP*”). 

This shows the vital importance of “dual methodology reporting”, farmer and policy informer Professor John Gilliland told the British Cattle Breeders Conference.

The findings come from work done by ARCZero, a Northern Ireland (NI) initiative. It used Agrecalc to calculate the carbon position of seven participating farms across NI.

Agrecalc used two different methodologies, as reported by the University of California Davis:

• GWP 100: Looks at the global warming potential (GWP) 0f gases over 100 years. Because methane has a half-life of about 12 years, the life of methane is overestimated by a factor of three to four.
• GWP*: Takes short-lived methane into account, treating biogenic methane from ruminants as a “temporary pulse” rather than a “constant agent of warming”.

John Gilliland, a farmer based near Derry, an adviser to AHDB and of the Institute of Global Food Security at Queens University Belfast, is chair of ARCZero.

He stressed he was not a scientist, but was responding to scientists calling for GWP* to be used to assess livestock system emissions.

“Critically, farmers need to do dual reporting of their farms, GWP100 and GWP*,” John told the conference.

“It will be how we stage a reputational recovery in the eyes of doubters. Please include GWP* when you have your carbon assessment done.”

He said it was critical that ruminant livestock production was recognised for the methane it produces, which takes carbon already in the atmosphere in the biogenic carbon cycle.

Massive change in carbon position

John said the results importantly show three things:

• Net emissions on farms were lower than gross emissions.
• GWP100 lowered the net emissions.
• GWP* lowered emissions further (when factoring in 20-year livestock trends for Northern Ireland).

Take home messages

John advised farmers do the following: 

• Know your own data. Understand your own net carbon position before worrying about the world around you.
• Work out your net carbon position. Factor in what you are sequestering and storing, as well as your gross emissions.
• Carbon first. Quantify and manage your carbon figures before looking at nitrous oxide emissions in detail. Then move onto methane. Methane inhibitors come last, as they are an additional cost.
• Choose your carbon tool wisely. Is it independent and accredited? Does it account for woodland and grassland or just the product? Can it work with GWP*? Is is affiliated with a science-based group? Is it supported by a globally recognised science-based institution?

Spreading the message

John said ARCZero had acted quickly since two publications last year stated the case for GWP* and a reclassification of biogenic methane.

Praising the work of Professors Michelle Cain, University of Cranfield, and Myles Allen, University of Oxford, he said 2023 had been a turning point for carbon accounting on farm because of the following publications:

• July 2023: A joint letter signed by nine academics from eight independent academic agricultural institutions was published in the Environmental Research journal calling for dual reporting of carbon, treating methane as short-lived in the environment. They peer-reviewed the work of Myles Allen, who had been calling for a distinction to be made between biogenic and fossil fuel methane.
• September 2023: A United Nations Food and Agriculture Organisation report stated that methane is more shorter lived than carbon and nitrous oxide , that biogenic methane is different to fossil fuel methane, having a GWP of 27 rather than 29.8. It also recognised GWP* as a useful metric available to use.

“Big thanks go to Agrecalc for working the figures and taking the project on, and Myles Allen, who said the assumption that biogenic methane stays in the atmosphere for 100 years is flawed and studied it.

“It’s still important to be efficient though,” John told Farmers Weekly. “Farms can reduce emissions on farm through efficiency gains like lower days to slaughter and less nitrogen fertiliser by using legumes for the same output of milk or meat.

“Deep down we always knew our grandparents were wise farming the way they did. But now we have the science to understand their wisdom.”

He said farms should focus on the following: Getting age to slaughter down through genetic gain, weaning systems off synthetic nitrogen and ryegrass monocultures. and transitioning to legume-based farming, after hitting a pH of 6.5 in mineral soils.