Stricter controls on spreading fertiliser and better livestock housing are among the proposed measures to clamp down on farm pollution outlined in the government's plans to improve air quality.

Environment secretary Michael Gove has launched Defra's Clean Air Strategy, which says air pollution is the fourth biggest threat to public health after cancer, obesity and heart disease.

The strategy, which is now out for consultation, sets out actions aimed at reducing the costs of air pollution to society by an estimated £1bn every year by 2020.

A particular focus is cutting emissions of ammonia – a gas released into the atmosphere from agricultural sources like slurry or other rotting farm waste and fertiliser – by 8pc by 2020 and by 16pc by 2030.

The strategy says: 'For the first time the government will take concerted action to tackle ammonia from farming – which is responsible for 88% of ammonia emissions – by requiring farmers to invest in the infrastructure and equipment that will reduce emissions. Farmers will be supported to achieve this through our new system of public money for public goods.'

The measures include:

? Setting maximum limits for (organic and inorganic) fertiliser application, taking account of economic efficiency and commitments to reduce ammonia and greenhouse gas emissions from agriculture and to protect sensitive habitats and water bodies.

? Extending environmental permits to large dairy farms by 2025, bringing them up to the standards currently applied to intensive pig and poultry units. Defra says ammonia emissions from dairy cattle accounted for 28% of UK agricultural ammonia emissions in 2016, but emissions from the dairy sector are not currently regulated.

? A requirement to spread urea-based fertilisers in conjunction with urease inhibitors, unless applied by injection on appropriate land, by 2020.

? Mandatory design standards for new livestock housing by 2022, covering at least poultry, pig and dairy housing.

? A requirement for all solid manure and solid digestate spread to bare land to be incorporated within 12 hours by 2022.

? A requirement to spread slurries and digestate using low-emission spreading equipment by 2027.

? The requirement for all slurry and digestate stores and manure heaps to be covered by 2027.

Tony Bambridge, chairman of the Norfolk branch of the National Farmer's Union (NFU), said: 'I am heartened that the general thrust of this is about best practice. The better farmers are already adopting these processes, and many of our protocols we have to adhere to with Red Tractor [the farm assurance scheme] already requires farmers to do many of these things.

'There is a tightening of some restrictions – for example we have always had 24 hours to incorporate manures, and now we will have 12, so that's a change. But many farmers will already be doing this.

'One of the challenges, and unintended consequences, is when you start putting restrictions on when you can spread something, or when you start to require certain equipment to do it, you do load the system with a lot of pressure. So suddenly you have got every dairy farm in the parish all going out and spreading slurry on the same day.

'And I think another challenge to the industry is where we have to have covered stores for solid manure, that is a big expense, and the type of injection or incorporation equipment they are suggesting are also big-ticket capital items.'

NFU deputy president Guy Smith added: 'Farmers have recognised there is a need to reduce their ammonia emissions and the sector has made improvements with levels dropping by 10pc in the last 30 years.

'A range of actions is needed to ensure that farmers can retain the capacity to produce food while also continuing to safeguard the environment.

'Investment in infrastructure and new technology all have a huge role to play and the NFU welcomes the commitment from government to work with the farming industry to help achieve this. It is crucial that new measures support farm businesses to be productive and competitive.'

The consultation closes on August 14. For more details see the Defra website.