Ways to cut emissions in housed livestock systems

All livestock farms, regardless of system, must comply with UK environmental legislation, but as the sector comes under greater pressure to go further with greener farming methods, it is businesses with housed herds and flocks that arguably face the biggest challenge.

Ammonia, phosphates and carbon all pose problems for indoor livestock farming and are increasingly drawing regulatory and public scrutiny.

This has been notable in the poultry industry – in 2022, intensive units in the catchment of the River Wye were accused by Natural Resources Wales of polluting the watercourse by spreading manure.

See also: Advice on cutting ammonia emissions on poultry farms

Ammonia emissions

It is true that ammonia emissions have been steadily rising in Wales in recent years, with more than half the country said to be experiencing concentrations that are too high for some sensitive habitats.

It is an inescapable fact that housed systems do produce a lot of manure and slurry, and disposing of that, while meeting the environmental expectations of regulatory bodies and the public, is a formidable challenge.

In Northern Ireland, the Department of Agriculture, Environment and Rural Affairs (Daera) says 96% of ammonia emissions come from the housing of livestock and subsequent storage and spreading of livestock manures, particularly cattle.

Some farmers rightly worry about what can be very high costs associated with investing in solutions – for example, some models of ammonia scrubbers cost £100,000 or more – and grants from Defra and the devolved administrations only meet part of the cost of a limited number of interventions.

Air and water pollution

In England for example, Defra has committed more than £200m for infrastructure and equipment grants to help livestock farms tackle air and water pollution from slurry and to make better use of organic nutrients.

The annual budget for the Catchment Sensitive Farming (CSF) programme has also been upped to £15.1m, from £7.2m in 2020-21, to give every farmer an opportunity to access advice and support by March 2023.

CSF is said to have reduced the occurrence of serious water pollution incidents by 17% and resulted in farmers implementing an estimated 76,700 actions to reduce pollution.

But governments are also using new legislation to force farmers to make changes.

A case in point is a pledge by Defra to mandate the use of methane-reducing additives in some English cattle feed systems by 2023.

Such additives could include probiotics as well as supplements produced from seaweed and plant essential oils. Farmers warn it could significantly increase their costs because there is only a small pool of available products.

It is not just pollution that housed livestock are blamed for. Many housed units consume significant amounts of energy for ventilating, heating and cooling the indoor environment, too.

So what can farmers do? We take a look at the management strategies and latest kit that can help mitigate emissions and cut costs.

Slurry store upgrades

Ammonia emissions from outdoor slurry stores can be reduced by limiting airflow across the surface of the slurry.

This can be achieved by fitting impermeable or semi-permeable covers, as shielding the slurry surface from airflow allows ammonia concentrations to build up beneath the cover, suppressing further emissions.

Several options are available, including a tensioned cover, which is an impermeable plastic cover shaped like a tent, supported by a central column with a small vent in the top.

Plastic floating covers are also made of an impermeable plastic sheet, but these rest directly on top of the slurry, rising and falling with the slurry level.

Alternatively, there is a floating plastic tile that creates an interlocking barrier between the slurry and the air.

Also available are light expanded clay aggregates – small, pebble-like clay products with a hollow irregular internal lattice structure that float and create an airflow barrier on the slurry surface.

Small biodigesters

Methane-capturing systems not only reduce farm emissions but generate energy.

Among those that have been successful is Bennamann, a farm-scale system that captures methane from the slurry gases that bubble to the surface of the existing storage using a cover, then converts it into biogas via a processing unit.

Under this model, the farmer invests in the cover, a biogas storage bag, and associated equipment.

Bennamann carries out the processing, handling and sale of the biomethane – the profits are shared with the farmer, or the farmer can buy biomethane for their own use at a reduced rate.

Bennamann works with a cluster of six or so farms to share a processing unit.

Eirinn Rusbridge, senior project engineer at NFU Energy, says small, farm-scale anaerobic digesters can cover most of a farm’s energy demand.

These systems work best with year-round cattle housing because they maximise the slurry that will enter the digester.

But costs can be prohibitive because the digesters are modular and tend to be tailored closely to cow numbers.

Treating, drying or pelletising manure

Reducing the moisture content in manure greatly lowers the potential for water pollution from nutrient run-off and the build-up of ammonia, carbon dioxide and fine dust. There are many different methods for drying it out.

North Wales egg producer Llyr Jones opted for a muck dryer that works by capturing heat from the hens as it rises and forces that air back down onto the muck belt.

This process removes about 25% of the moisture cutting volumes and ammonia emissions.

“When we muck out the sheds, it means one load less – two instead of three – reducing tractor and diesel usage,” says Transition Farmer Llyr. There are bacterial options, too.

A trial at another Welsh free-range poultry unit, Wern Farm, near Welshpool, demonstrated that adding non-infective bacteria to the environment dried out litter and reduced ammonia levels.

Before the trial, litter had been 70% moisture, but the bacteria reduced it by more than 50% of the original levels. Frequency of muck removal from the manure belts reduced from every three days to once a fortnight.

Slurry separation

Slurry separation divides slurry into solid and liquid portions, often using screw-press technology.

This is a process that reduces emissions related to storage and spreading because the liquid fraction is easier to spread across large areas, allowing slurries to be better targeted to fields with high nutrient demand.

Selling manure off-farm

Muck-for-straw exchanges are becoming more commonplace as arable businesses seek to reinvigorate tired soils.

Such arrangements involve an arable grower providing a livestock farm with straw for bedding in return for organic nutrients to integrate into soils used for crop production.

Straw and manure are both bulky products, so keeping the exchanges as local as possible is a good thing: moving muck a long way is an expensive option.

These agreements were historically based around farmyard manure, but with more dairy units now having slurry separators, they have a material that isn’t just a low dry matter liquid slurry and one that is more transportable.

Retrofitting ammonia scrubbers on sheds

Ammonia scrubbers work by air from a shed being pushed into the device where it is sprayed with sulphuric acid; the acid attaches itself to ammonia, and water is then added to reduce the pH level to neutral.

These have a high capital cost because they are made of stainless steel.

But there are economic as well as environmental benefits – Llyr says he can produce fertiliser with a nitrogen value of 20%.

He calculates that, as a consequence, savings on purchased fertiliser easily pay for the scrubber’s running costs.

Removing ammonia by planting trees

A novel mitigation method is site-specific tree planting to capture ammonia emissions.

Geraint Jones, forestry technical officer at Farming Connect, says creating buffer zones with trees downwind from housing or slurry stores can capture emissions and dust particulates.

In a well-designed shelterbelt or buffer zone, air from nearby livestock buildings is naturally directed into the woodland and up into the tree canopy, encouraging ammonia gases to be deposited on the tree and leaf surface.

For planting near livestock housing, species choice should be dictated by site conditions and characteristics, but for a quicker impact,

Geraint suggests planting fast-growing species such as aspen, white or grey poplar, willows or hybrid varieties if conditions permit.

To aid carbon capture and biodiversity, he recommends incorporating other native species such as oak, silver birch, rowan, wild cherry and crab apple with lower storey shrub species such as hazel, hawthorn and elder.

“It is important to get advice before planting to make sure the right trees are placed in right location to get the most benefits,” says Geraint.

While planting trees for ammonia mitigation has multiple benefits, the establishment of a fully functioning main canopy and backstop can take many years.

Case study: University of Cambridge’s Park Farm, Cambridgeshire

Investment in measures that reduce negative environmental impacts of the housed dairy herd at Park Farm is being recouped with cost savings and enhanced efficiency.

The 230-cow Holstein-Friesian milking herd at the Linking Environment And Farming (Leaf) demonstration farm is housed for most of the year.

The system generates 13,000cu m of slurry annually and the business was spending up to £40,000 a year exporting it because it didn’t have sufficient storage.

“Not only was it a cost to the business, but we were losing valuable nutrients off the farm,” says dairy farm manager Paul Kelly. To resolve this, in 2021, a 6,000cu m slurry tank was installed.

Slurry is now applied to grassland and arable crops, which has reduced artificial fertiliser requirements by 9.25t of ammonium nitrate. A slurry flow meter has been purchased too, to increase confidence in the accuracy of those applications.

The biggest investment of all was a slurry-fed anaerobic digester (AD), installed in June 2021 and funded by the university’s Carbon Reduction Fund, which is governed by the university’s Environmental Sustainability Strategy Committee.

Half the slurry produced by the herd is now fed into that plant to extract methane, allowing the farm to produce more than half of its electricity requirements from slurry.

At 44kW, Paul says the plant is small scale, but grid capacity prevented installation of a bigger digester.

Changes made to yard drainage have improved the quality of water entering the wider environment.

By diverting drains into a former slurry tank, it prevents water from mixing with the slurry, improving the dry matter of slurry and increasing storage capacity.

Paul says this has also improved the efficiency of the AD plant. “Water doesn’t produce gas,” he points out.

“A change to the layout of the slats at the end of the dairy shed has cut washing-down frequency.

“We had been using a lot of water to hose the slats because the slurry wasn’t getting down them so we removed every second slat,” he explains. Hosing down is now monthly rather than twice a week.

Installing low-energy LED lighting in the dairy building has captured savings of 19t of carbon dioxide a year; these have been installed in the youngstock sheds, too.

A recent audit showed that the farm produces 1.04kg carbon dioxide/kg fat and protein corrected milk, a reduction from 1.27kg carbon dioxide/kg in four years.

It is not only changes to infrastructure that have had positive environmental benefits, but feed efficiency improvements, too.

“We no longer feed soya and only use a sustainably sourced rumen-protected fat, and precision feeding means we only give the cows what they need,” says Paul.

Transition Farmers: Tom and Karen Halton, Astbury, near Congleton, Cheshire

Meeting evermore ambitious environmental targets set by governments can run into hundreds of thousands of pounds for some farm businesses.

Even at just 40% of the total cost of projects, grants are a vital source of funding, but securing these is not easy, as Tom and Karen Halton have experienced.

© Tim Scrivener

The pair, who farm at Astbury near Congleton, Cheshire, applied for a Slurry Infrastructure grant in round one of the scheme.

They wanted the grant help pay for measures to protect yards and their slurry lagoon from rainwater in their housed dairy system.

But, despite a Natural England report supporting their application and the farm being close to a designated site of special scientific interest, they were unsuccessful.

They are now submitting an application for round two, where a further £74m has been earmarked to help farmers invest in slurry infrastructure to tackle water pollution, improve air quality and make better use of organic nutrients.

Without the grant, they will be unable to carry out the work they had planned because of the cost and current low milk price.

“Even finding 60% is going to be a struggle with the milk prices where they are and the work costing hundreds of thousands, but we definitely won’t be able to do it without a grant,” says Karen.

“If the government is so desperate for farmers to meet the environmental targets they have set us, they are going to have to help us to pay for them, or put measures in place to ensure we get a sustainable price for what we produce, whether that be milk, meat or crops.”