Farming emissions: What they are and why they must be tackled

Farming is responsible for 11% of total UK greenhouse gas (GHG) emissions and as much as 88% of UK ammonia emissions, according to Defra’s latest Agri-climate report.

Emissions matter for several reasons – the production of GHGs such as nitrous oxide, methane and carbon dioxide is linked to global warming and climate change, while ammonia is a damaging air pollutant with harmful effects on the environment.

Air pollution is also the leading threat to public health – harmful particulates cause up to 36,000 deaths a year and it is estimated that this will cost the NHS and social care £1.6bn between 2017 and 2025.

See also: How livestock producers can cut their carbon footprint

Downward trend

Behind these headlines, Defra’s report confirms that the past 30 years has seen a downward trend for most emissions from farming, with total GHGs falling by 16% and ammonia emissions by 10%.

Much of this fall has occurred in recent years and is the result of a drop in ruminant numbers and less use of synthetic fertilisers.

Emissions from producing 1 litre of milk have fallen by 22% since the 1990s, with 11% more milk being produced from 21% fewer cows.

Likewise, the GHG emissions from producing 1kg of pork have fallen by 44%. And while nitrous oxide emissions from wheat production are unchanged, wheat yields have increased – suggesting that more wheat is being produced from the same amount of nitrogen.

Given the current emphasis on net zero, it’s not surprising that the Defra report also reveals that 64% of farmers considered it important to factor in GHGs when making farm business decisions, with 58% of them admitting to already taking action to reduce emissions.

Targets and legislation

The government has set a goal for the UK to be net zero by 2050, while the NFU has been more ambitious and committed to a date of 2040.

Other government targets include a reduction of 16% in ammonia emissions by 2030, compared with 2005 levels.

The Clean Air Strategy 2019 aimed to reduce air pollution from agriculture, offering support for farmers to invest in infrastructure and equipment to reduce emissions, and bringing in regulations on low-emission-farming methods and fertiliser use.

The Global Methane Pledge, which came from COP26 last year, saw the UK being one of the signatories to collectively cut methane emissions by 30% by 2030, on a 2020 baseline.

In addition, the Environment Act 2021 makes air quality one of four priorities, and requires government to set legally binding targets for England, with an additional target on fine particulate matter.

When it comes to ammonia emissions, large intensive pig and poultry units already have to obtain and comply with permit conditions set by the Environment Agency – something that could be extended to big dairy and intensive beef units before too long.

Emissions types

There are three main GHGs produced from agriculture: nitrous oxide, methane and carbon dioxide.

Of these, nitrous oxide from soils and methane from livestock are the two biggest sources. Carbon dioxide from energy use contributes much less.

GHGs are presented and measured in units of carbon dioxide equivalent (CO2e), which allows comparisons to be made between them.

Carbon dioxide is the most abundant gas and is used as a benchmark.

The others are converted into CO2e by multiplying the quantity of gas emitted by the global warming potential (GWP) of that gas – its heat-absorbing ability compared to carbon dioxide.

As such, carbon dioxide has a value of 1. For methane, the value is 28 and for nitrous oxide, the value is 298.

These figures are produced when a timeframe of 100 years is used to calculate emissions of GHGs.

Some scientists believe that this method misrepresents the impact of shorter-lived GHGs, such as methane, and that a 20-year time horizon is more relevant.

Others believe that GWP should be used, as it accounts for the different properties of the gases and their impact on the climate. It also allows for the fact that methane has a shorter lifespan than the others.

© Tim Scrivener

Methane

Agriculture is responsible for 47% of the UK’s methane emissions, most of which come from livestock production via enteric fermentation.

Research has highlighted the most promising ways to reduce methane emissions from ruminants, as well as investigating their costs.

They include:

• Breeding for lower emissions animals
• Improved livestock health
• Feed additives that reduce methane production in gut bacteria
• Growing legumes in the rotation.

At the Roslin Institute, microbiome research has shown a direct correlation between the abundance of archaea in the rumen and the amount of methane produced by an animal.

It also showed that a cow’s genetics can directly affect the environment within the rumen, making it more or less hospitable for different species of micro-organism, including the methanogenic archaea.

According to Mick Watson of the University of Edinburgh’s Roslin Institute, cattle breeders are now using microbiome data in their breeding programmes to select for lower emitting animals.

“When farmers use this data in their breeding goals, together with feed additives that reduce methane emissions, there is an immediate fall in emissions.”

Feed additives show great promise and there are several options that can help to reduce methane production.

Of particular interest is Bovaer, which has been shown to give a 30% reduction in methane – or a 10-12% drop in GHG emissions for every kg of milk.  

Animals lost to disease each year also have a climate and emissions impact, notes Prof Watson.

“Resources are used and methane produced. By addressing animal health, along with genetics and nutrition, the sustainability of production is improved.”

Nitrous oxide

Farming is responsible for 68% of the UK’s nitrous oxide emissions.

A very potent GHG, nitrous oxide mainly comes from fertiliser use, with 50% attributed to the manufacturing process and 50% to its use.

The gas is released during spreading and when on the field, through processes such as volatilisation, leaching and direct loss.

There are several actions that farmers can adopt to reduce their reliance on nitrogen fertiliser, says Emma Adams of the Farm Carbon Toolkit.

“Improving soil health, incorporating cover crops, widening the rotation and using organic sources of nitrogen are all helpful.”

Other actions that can help with nitrous oxide arising from fertiliser use includes the sourcing of fertiliser products – those coming from the UK or Europe usually have a lower emissions factor than product coming from China, for example.

Inhibitors, used to slow the conversion of ammonium to nitrate, have a role with urea products and help to improve nutrient use efficiency (NUE), which remains low with many granular fertiliser products.

“The standard figure for NUE from granular products is 60%,” adds Ms Adams. “Some 40% is being lost at the outset and we need to consider how to minimise these losses.”

For most, it’s about application. “Given their very high cost, it makes sense to apply fertilisers with accuracy, when the weather and plant growth is going to maximise uptake of the product.”

Carbon dioxide

Farming is responsible for just 2% of the UK’s carbon dioxide emissions, most of which come from combustion.

That puts fuel use in the spotlight, whether it’s for operating machinery, drying or heating.

Simply minimising cultivations and tracking fuel use in real-time can help, as can minimising the time that tractors spend idling.

Further ahead, the development of autonomous machines and the use of robotics offer improvements, while the development of hydrogen technologies, biomethane combustion and battery-based electric vehicles will all help to reduce emissions.  

Actions that make a difference

The following on-farm actions will help to reduce emissions, according to a guide produced by Innovation for Agriculture in conjunction with WWF and Tesco. It stresses that taking a whole-farm approach, with multiple actions suitable to specific enterprise types, is the best approach for making progress:

• Grow cover/catch crops
• Integrate grass leys into arable rotations
• Include legumes in arable rotations
• Keep soil pH at optimum level for plant growth
• Reduce cultivations
• Improve fertiliser applications and avoid excess nitrogen
• Use enhanced efficiency or protected fertilisers
• Consider slurry store design and management
• Investigate anaerobic digestion as a means of dealing with waste
• Look at feed additives, low cellulose diets and soya replacements in livestock rations
• Consider genetic improvement of livestock
• Focus on livestock health
• Grow grass-legume mixes for grazing
• Investigate what low-carbon farm machinery can offer
• Look at low-carbon heating/cooling systems.

© Tim Scrivener

Transition Farmer case study: Andy Bason

Emissions have already fallen at Newhouse Farm in Hampshire, as manager Andy Bason makes changes to the farming system in line with future requirements for delivering public goods.

In the past 10 years, fertiliser use has fallen by 18% and fuel use by 40%, thanks to reduced cultivations and a focus on soil health.

“It wasn’t deliberate at first – we were more intent on making adjustments that secured a sustainable future for the farm,” he says.

“But a carbon foot-printing exercise carried out a few years ago helped with identifying areas that we needed to concentrate on.”

A goal of reducing greenhouse gas emissions by 30% was set when he joined the Leaf Resilient and Ready initiative, and soils have benefited from receiving chopped crop residues and growing cover crops.

As a result, organic matter levels have risen, soil function has improved and the soils are starting to cycle nutrients.

Ways of reducing total N fertiliser applications are also being investigated – although the farm doesn’t use urea, with its associated ammonia emissions, Mr Bason would like to bring fertiliser costs down and be less reliant on the bag.

“We are using tissue testing, Nmin measurements and refined application timings, so that we are only applying what the crop needs at the time that it needs it,” he says.

© Leaf