GRAZING CATTLE’S greenhouse gas emissions have been over-estimated, according to groundbreaking new research by UK scientists.
Previous work on livestock emissions cut corners by including generalised data on the chemical composition of cattle waste – but more detailed work by Rothamsted Research has found that cattle on pasture mixes including white clover produce almost half the amount of nitrous oxide that is currently used in the official figures driving UK climate change policy.
Nitrous oxide is a potent greenhouse gas that is 265 times more harmful than CO2, and can account for 40% of beef supply chain emissions. As such, the Rothamsted results may offer a route by which UK farming can achieve its ‘net zero’ ambition by 2040.
Atmospheric chemist, nitrous oxide expert and co-author of the study, Dr Laura Cardenas said: “Due to technical and logistical challenges, field experiments which measure losses of nitrous oxide from soils usually add livestock faeces and urine they have sourced from other farms or other parts of the farm, meaning that the emissions captured do not necessarily represent the true emissions generated by the animals consuming the pasture.”
Writing in the journal Agriculture, Ecosystems and Environment, the Rothamsted team described how they created a near ‘closed’ system whereby the circular flow of nitrogen from soil to forage to cattle and, ultimately, back to soil again, could be monitored.
On its Devon ‘farm lab’ at North Wyke, herds of 30 cattle were grazed on either land that had long been pasture; a high-sugar grass commonly sown by farmers; or a high sugar grass and white clover mix, and every input and output was measured on-site, with no recourse to averaged outside data.
Lead author of the study, Dr Graham McAuliffe and his colleagues had previously discovered system-wide reductions of greenhouse gas emissions associated with the inclusion of white clover in pasture, primarily driven by the lack of need for ammonium nitrate fertiliser, whose production and application create greenhouse gases. But in the absence of evidence at that time, the team had relied on figures which assumed all cattle urine or faeces deposited to soils cause the same volume of nitrogen-based emissions irrespective of pasture type.
The most recent Intergovernmental Panel on Climate Change figures provided to scientists estimate this ‘emission factor’ as 0.77% – but Rothamsted’s latest experiment found it was 0.44% on the white clover-high sugar grass mix, once the additional nitrogen captured from the air by clover was accounted for.
Dr McAuliffe said: “These differences might not sound like much, but when used in calculations of the climate impact of beef, they have a considerable effect as nitrous oxide emissions can account for over 40% of entire supply-chain greenhouse gas losses.”
According to Dr Cardenas, further research is required: “Although white clover is unlikely to be a ‘silver bullet’ for agriculture’s net-zero ambitions on its own, adopting combinations of multiple emissions-abatement interventions, such as increasing legume-inclusion in pasture compositions and utilisation of ‘low-carbon’ fertilisers, will be essential to maximise farming’s national and international contribution to a cooler planet.”
Professor Bob Rees from Scotland’s Rural College commented: “Rothamsted’s research provides valuable new insights into the role of grassland management on emissions of the powerful greenhouse gas nitrous oxide.
“SRUC has been working with Rothamsted Research at North Wyke over the past 10 years to improve our estimates of nitrous oxide emissions from grazed pastures. This work has demonstrated that emissions derived from livestock excreta are very much lower than previously thought, and these new findings have now been incorporated into our national reporting of greenhouse gases,” said Prof Rees.
“Rothamsted’s work has highlighted the complexity of understanding emissions in a farming system, since changing one component such as diet can have effects elsewhere for example on soil emissions from deposits of animal excreta. They also identify the importance of clover based swards in reducing greenhouse gas emissions, which is supported by work undertaken at SRUC.”
He added that SRUC was currently leading a review that includes Rothamsted Research on the role of livestock in contributing greenhouse gas emissions, funded by the Centre for Innovation and Excellence in Livestock.