Testing carried out by the Bureau of Investigative Journalism around eight dairy units in the UK has revealed ammonia hotspots at three of them.

The government does not monitor ammonia pollution from most UK farms, despite the fact that it is a major contributor to the air pollution crisis.

The Bureau tested emissions at six intensive units housing more than 700 cattle, another farm where a confinement unit permanently houses some of the herd, and one conventional farm with dairy cows grazing.

Ammonia hotspots were detected at two of the intensive units, including sites near an uncovered slurry storage tank and near a farmyard, and at various points around the outdoor dairy farm, including next to a farm building, on a road running through the farm, and by a waste lagoon.

These findings provide only a small snapshot across the south of England, and emissions readings can be influenced by many variables including the weather, but they highlight the need for mandatory emissions monitoring on farms if the ammonia problem is to be reduced.

Mark Sutton, environmental physicist at the Centre for Ecology and Hydrology, said: “Intensive systems will give bigger emissions in general, as they are associated with livestock housing, manure storage and manure spreading, all of which tend to increase emissions compared to an animal grazing out in the field.”

Muck-spreading, the agricultural practice of spraying liquid manure on fields, is a key source of ammonia pollution. Photograph: Zoonar GmbH/Alamy

Housed systems for cattle and pigs produce more ammonia as the urine and faeces mix, producing slurry. This mixture is more toxic than if the excreta were separated, as they effectively are when animals graze naturally because the urine is largely absorbed by the grass. Ammonia is emitted when slurry is stored in uncovered lagoons, and when it is sprayed on fields – muck-spreading – for fertilisation.

Intensive poultry farms, however, can have some advantages over free range farms when it comes to ammonia, because when hens wander outside the ammonia from their excrement can disperse into the air, according to Simon Bareham, principal adviser on air quality and biodiversity at the government body Natural Resources Wales.

Many farmers lack the financial resources to invest in lowering ammonia emissions. One diary farmer in north Devon said: “Pay a better price for the product we produce. If the milk price went up by 10-15% we could all look after the environment a lot better.”

He installed a cover on his slurry lagoon two years ago at a cost of £35,000 - which would have been well out of his price range had it not been for a grant from the government. But grants are hard to come by. A scheme in 2017 offered just £3.3million nationwide for ammonia reduction. There are no grant schemes in Wales or Scotland, and Northern Ireland’s small scale trial grants are under review.

Another north Devon dairy farmer with 220 cows that graze part of the year said he would like to see grants widely rolled out to improve slurry storage. “I would like to see covers and proper storage at the farm. The government should help with that,” he said. He explained that his current lagoon is too small and fills quickly, meaning he sometimes has to spread slurry on fields in wet weather when it can run off and pollute watercourses.