In field ammonia emission mitigations – Using Protected Urea Fertiliser and Low Emission Slurry Spreading Equipment to reduce ammonia emissions

Date published: 26 January 2023

The most recent ammonia emission figures for Northern Ireland (2020), show that almost 43% of the ammonia emitted from agricultural activities comes from the application to land, of livestock manures, fertiliser, or sludge and digestate. In this article CAFRE Technologist, Robert Edwards, highlights some of the options available to farmers to reduce the ammonia emissions associated with these land applications.

Nitrogen lost from agricultural activities, in the form of ammonia gas, increases ammonia concentrations in the atmosphere. The increased concentrations and subsequent deposition to land, is having a detrimental effect on our natural habitats.


Urea is a concentrated chemical nitrogen fertiliser with the NPK (nitrogen-phosphorus-potassium) ratio of 46-0-0.  While urea is a less expensive form of nitrogen fertiliser it is not recommended for use in the warmer late spring and summer months due to significant nitrogen loss from volatilisation as ammonia. Research has shown that up to 45% of urea’s nitrogen content can be lost when sown in sub-optimal weather conditions. These nitrogen losses can be minimised by using a protected urea fertiliser.

Robert explains that protected urea is treated with an ingredient called a urease inhibitor. The urease inhibitor can be either coated onto the outside of the fertiliser granule or incorporated into the urea granule during manufacture.

Urease is the enzyme which increases the speed of conversion of urea to ammonium, the process by which nitrogen within urea becomes available for plant uptake. It is during this conversion that nitrogen is lost as ammonia gas from untreated urea. A urease inhibitor blocks the active site of the urease enzyme, slowing the rate at which urea is converted to ammonium. This slower conversion occurs over a period of days rather than hours, and better matches the nutrient uptake of a growing crop.

Calcium Ammonia Nitrate (CAN) also has lower ammonia losses than urea, but it is vulnerable to loss of nitrous oxide (N2O). Nitrous oxide is a potent greenhouse gas, that has 298 times the global warming potential of carbon dioxide. Protected urea releases 71% lower N2O emissions than CAN. 

A joint study between AFBI and Teagasc has shown that protected urea can reduce ammonia and greenhouse gas losses compared to CAN, while maintaining comparable crop yields. Protected urea, containing an inhibitor such as NBPT (N-(n-Butyl)thiophosphoric triamide), has been shown to reduce ammonia losses, compared to urea, by an average of 78.5%.

Robert explains that “Urease inhibitors degrade slowly over time, but they should continue to be effective if the urea is sown within 12 months of manufacture.  To be on the safe side, it would be best to purchase what can be used in the current season and not to carry over stock to the following year.  If the inhibitor is completely degraded, then ammonia loss will be the same as urea.”

A range of protected urea fertilisers are available, some will have added potassium/potash and/or sulphur.  Phosphate will not be incorporated with protected urea because the acidity from the phosphate can affect the inhibitors efficiency.

In addition to selecting the correct fertiliser product, it is also worth remembering that fertiliser spreader settings should be adjusted to ensure the desired application rate is achieved. As urea is less dense than other fertilisers, it will not spread as far as other fertilisers during sowing. Please check with your fertiliser manufacturer and fertiliser sower settings guide for the correct settings.


The other influential field operation, in relation to nitrogen loss as ammonia, is the application of slurry. The Nutrients Action Programme 2019-2022 (NAP) has increased the number of farms required to spread slurry and digestate using Low Emission Slurry Spreading Equipment (LESSE).

Several different low emission spreading options, suitable to Northern Ireland conditions, are available for farmers. These include,

Trailing hose – This system comprises of equally spaced hoses, trailing along the ground, depositing slurry in continual parallel lines onto the ground/crop surface.  This system has an emission reduction capability of approximately 30%.

Trailing shoes – Similar to the trailing hose system, this system comprises of hoses fitted with a metal or plastic shoe which parts the crop, depositing the slurry in equally spaced continual parallel lines directly on the soil surface. This system has an emission reduction capability of approximately 60%.

Shallow injection – This system is suitable for stone free ground in either grassland or arable crops. The injector cuts slots, approximately 4-6cm apart and 25-30cm apart, allowing the slurry to be deposited below the soil surface. This system has an emission reduction capability of approximately 70%.

Deep injection - This system is most suited to arable ground, pre cultivation and sowing, due to ground disturbance of the machine. Slots are created 10-30cm deep and 50 cm apart into which the slurry is deposited. The emission reduction capability of this method is 90%

The use of LESSE has many additional benefits compared to the traditional inverted splash plate method of slurry spreading.

LESSE allows the farmer to make better use of their slurry by increasing the window during which slurry can be spread without significant crop contamination and minimising the period after spreading when livestock cannot graze.

By reducing nitrogen loss through ammonia emissions, the amount of nitrogen available to the plant increases. In less than optimal spreading conditions (mild and windy), approximately 80% of the nitrogen content of slurry spread by inverted splash plate can be lost. As previously noted, using low emission spreading equipment can reduce this loss by between 30 to 90%.

The increase in nitrogen availability allows potential savings in chemical fertiliser, as a greater percentage of the crops nutrient requirement is provided through slurry. This reduction in nitrogen loss also has environmental benefits. More nitrogen retained in slurry means that more nitrogen is available to the plant. Additional organic nitrogen reduces the demand for inorganic nitrogen application, reducing carbon emissions associated with its application. Robert explains that Table 1 offers a comparison of the various slurry application methods.

Robert concludes that Low Emission Slurry Spreading (LESS) will have an increasing role in reducing ammonia emissions from farms in Northern Ireland. In addition, the improved efficiency of nutrient use and lower crop damage provides production advantages to farmers as well as environmental benefits.


The Department of Agriculture, Environment and Rural Affairs (DAERA) has launched a consultation on its draft Ammonia Strategy and is keen for anyone with an interest to participate and provide their views. This public consultation is open for a period of 8 weeks until 23:59, 3 March 2023. We welcome as many views as possible from all interested organisations and individuals. The final information event, at CAFRE Loughry Campus is scheduled for 7.30pm on 31st January and last approximately one hour. To reserve your place this meeting please register through Eventbrite on the CAFRE website.

Download more information including details of how to respond.

Notes to editors: 

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