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Grazing experiment shows IPCC climate impact estimates off

Article-Grazing experiment shows IPCC climate impact estimates off

Rothamsted Research Rothamsted cow on pasture.jpg
Nitrous oxide emissions from white clover pastures with cattle lower than previously suggested.

Direct emissions of a greenhouse gas from certain pasture types are lower than previously thought, meaning the climate impact of grass-fed cattle herds may be overestimated, according to an announcement from Rothamsted Research in the U.K.

The Rothamsted team showed that urine from animals reared on pasture where white clover — a plant sown on grazing land to reduce the need for additional nitrogen fertilizer — is growing results in just over half the amount of nitrous oxide previously assumed by scientists to be released.

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 emissions, and the researchers said these findings may help farming achieve its "net zero" ambition by 2040, the announcement said.

In perhaps the most realistic recreation of real farming practices to date, the researchers measured emissions from just one herd on their experimental beef and sheep farm in Devon, U.K., whereas 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.

Atmospheric chemist, nitrous oxide expert and co-author of the study, Dr. Laura Cardenas said such estimates are currently provided by the Intergovernmental Panel on Climate Change (IPCC) to scientists wishing to include it in their calculations on the climate impact of food supply chains.

“Due to technical and logistical challenges, field experiments which measure losses of nitrous oxide from soils usually add livestock feces 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,” Cardenas said.

Writing in the journal Agriculture, Ecosystems & Environment, the team reported 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.

The research was carried out at Rothamsted’s North Wyke Farm Platform, a unique facility where all relevant environmental, agricultural and economic data related to livestock farming are collected 24/7.

For this experiment, 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.

Lead author of the study Dr. Graham McAuliffe and colleagues had previously discovered system-wide reductions of greenhouse gas emissions associated with the inclusion of white clover in pastures. This conclusion was primarily driven by a lack of need for ammonium nitrate fertilizer, the production and application of which creates greenhouse gases, the announcement said.

However, in the absence of evidence at that time, the team relied on figures provided by IPCC that assumed all cattle urine or feces deposited to soils cause the same volume of nitrogen-based emissions, irrespective of pasture type.

The most recent IPCC figures provided to scientists estimate this "emission factor" as 0.77%. However, the Rothamsted team’s latest experiment found that 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.

“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,” McAuliffe said.

This new research 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 fertilizer requirements.

According to 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 point 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,” Cardenas said.

As the U.K. strives to achieve cross-industry net-zero carbon emissions by mid-century, improving the understanding of greenhouse gas emissions and mitigation potentials has never been more important, she added.

“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 utilization of "low-carbon" fertilizers, will be essential to maximize farming’s national and international contribution to a cooler planet,” Cardenas noted.

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