The climate impact of grass-fed cattle herds may be overestimated, as direct emissions of nitrous oxide, a powerful greenhouse gas, from certain pasture types are lower than previously thought , new findings from Rothamsted Research have revealed.
Urine from animals reared on pasture that included white clover was found to have less nitrous oxide than previously estimated. Nitrous oxide is a potent greenhouse gas that is 265 times more harmful than carbon dioxide and can account for 40% of beef supply chain emission, these findings may help farming achieve its ‘net zero’ ambition by 2040.
Benefits of growing white clover
White clover is a commonly included within grassland as it makes nitrogen more bioavailable reducing the need for additional nitrogen fertiliser. This is the first time Rothamsted scientists have quantified the climate-change related benefits of white clover, achieved both directly through lower nitrous oxide released at pasture, and indirectly by lower fertiliser requirements.
Most studies looking at the emissions from livestock arrive at their conclusions by combining data from a variety of experimental systems in addition to some estimated values. This includes data provided by the Intergovernmental Panel on Climate Change (IPCC) to scientists wishing to include it in their calculations on the climate impact of our food supply chains.
Instead, the Rothamsted researchers measured emissions from one herd on Rothamsted’s ‘farm lab’, at North Wyke Farm in Devon, in a realistic re-creation of real farming practices. The herds of 30 cattle were grazed on two types of pasture, the first a high-sugar grass commonly sown by farmers; and the other a high sugar grass and white clover mix.
Atmospheric chemist, nitrous oxide expert and co-author of the study, Dr Laura Cardena says: “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 team report 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.
Lead author of the study, Dr Graham McAuliffe and colleagues had previously reported system-wide reductions of greenhouse gas emissions associated with the inclusion of white clover in pasture. This had been primarily driven by a reduction in the need for ammonium nitrate fertiliser, which creates greenhouse gases in its production and application. The team had previously relied on figures provided by the Intergovernmental Panel on Climate Change (IPCC) which assume all cattle urine or faeces deposited to soils cause the same volume of nitrogen-based emissions irrespective of pasture type.
Climate impact of grass-fed herds less than previous estimates
The most recent IPCC figures provided to scientists estimate this ‘emission factor’ as 0.77%, the Rothamsted team 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 to explain the detailed mechanisms behind the observed complementarity between white clover and high sugar grasses – but that the data points towards an effect of sowing clover on the soil’s microbes.
“The evidence suggests that including white clover amongst high sugar grass decreases the abundance of microbial genes associated with nitrous oxide production compared with microbial communities observed under just high sugar grass.”
“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.”
McAuliffe, G.et al 2020. Elucidating three-way interactions between soil, pasture and animals that regulate nitrous oxide emissions from temperate grazing systems. Agriculture, Ecosystems & Environment. 300, p. 106978