Sustainable use of nitrogen fertilisers on cattle farms

Mark Plunkett Francis Quigley

Agriculture has a requirement to reduce greenhouse gas emissions by 25% by 2030, relative to 2018. To date agriculture has reduced emission by 4.6% since 2018. Meeting emission reductions will be challenging over the next 5 years. The Marginal Abatement Cost Curve (MACC) developed by Teagasc sets out a menu of actions that farmers can take to reduce emissions on their farms. Most of the actions in the MACC are ‘win-wins’ i.e. they are good for the environment and good for farmers’ pockets while maintaining farm productivity. These include, reducing our reliance on chemical nitrogen (N) fertiliser, correcting soil pH, making better use of slurry, selecting sustainable fertiliser types, increasing grazing days at pasture, reducing finishing age for beef cattle, using breeding indexes to enhance beef cattle genetics, reducing age at first-calving, improving animal health and increasing farm carbon sequestration in soils and hedgerows.

The Signpost Programme, led by Teagasc, is tasked with supporting and enabling farmers to reduce greenhouse gas emissions. There are 36 Signpost beef demonstration farms (Dairy Beef500 and the Future Beef Programmes) in the country, and these farmers are the early adopters of the aforementioned key technologies to reduce emissions, which encompasses the sustainable use of N fertilisers. The Signpost Advisory Programme is a free programme designed to support all farmers, Teagasc clients and non-clients. All farmers are encouraged to get to know their local Signpost farmer and sign up for the Signpost Advisory Programme to develop a farm sustainability plan for their own farm to help meet the targets at least cost.

Where should cattle farmers start to reduce greenhouse gas emissions?

Step one on any farm should be to reduce the reliance on chemical N in grassland and cropping systems. Chemical N releases nitrous oxide (N₂O), a potent greenhouse gas, into the atmosphere when applied to land. Nitrous oxide is one of the three main greenhouse gases (the others being carbon dioxide (CO₂), and methane (CH₄)). Therefore, if a farmer reduces the amount of chemical N used on the farm the amount of N₂O emitted is reduced. According to the Teagasc MACC 2023, reducing chemical N by 25% has the potential to reduce total emissions by 0.5 million tonnes (Mt) or 11% of the total emissions reduction needed.

What are the main fertiliser reduction strategies?

There are a range of proven technologies today to reduce our reliance on chemical N fertiliser:

Use a nutrient management plan

Improving farm N use efficiency is the first step to reducing farm N requirement and reducing total farm carbon emissions. The starting point is maintaining and following a farm fertiliser plan on a regular basis to manage soil fertility and identify farm nutrient requirements annually.

Soil sampling

Soil analysis is a small cost and provides the basis to planning nutrient applications. Take soil samples to the correct sampling depth of 10 cm, every 2 to 4 hectares (ha) and take fresh soil samples every 3 to 4 years.

Soil pH

Aim to maintain soils in the agronomic range pH 6.3 to 6.5 for productive ryegrass swards on mineral soils, and pH 6.5 to 6.8 for clover-dominated swards. For successful clover establishment, aim to increase soil pH in advance of sowing. Optimum soil pH has the largest impact on improving nutrient availability, efficiency of applied organic or inorganic fertilisers and productivity of grassland swards. For example, at optimum pH soils can release up to 70 kg N/ha/year and reduce soil N₂O emissions annually.

Considerable progress was made improving soil pH through liming over the last five years with an average of 1.0 million tonnes of lime applied. Currently, ~60% of soil samples from cattle farms indicate a lime requirement. According to the Teagasc MACC 2023, the target is to use 1.85 million (m) tonnes of lime per annum in 2026 and rising to 2.5 million tonnes per annum in 2030 for all farms. This is much greater than the current national lime usage levels of ~1.0 million tonnes in 2025. There is a large requirement for lime application on drystock farms currently estimated at ~0.95 million tonnes annually. This has the potential to deliver large productivity gains while reducing farm emissions.

Soil phosphorus (P)

Aim to maintain soil P at Index 3 (5.1 to 8.0 mg/l) for optimum productivity on moderate to intensively managed farms. Increasing soil P from Index 1 to Index 3 will increase grass production capacity by ~1.5 tonnes/ha dry matter (DM) per year and reduces soil N₂O emissions. The first step to building soil P levels is correcting soil pH to the optimum pH 6.3 to 6.5 thereby reducing the need for chemical P fertilisers to build soil P levels at least cost. Liming acidic soils will increase soil P availability and increase the efficiency of applied P in organic (slurry/farmyard manure) or chemical (fertiliser) forms.

Soil potassium (K)

Aim to maintain soil K at Index 3 (101 to 150 mg/l) for optimum productivity. Increasing soil K from Index 1 to Index 3 will increase grass production capacity by ~2.0 tonnes DM/ha per year.  Apply maintenance (Index 3) levels of K in springtime based on stocking rate to reduce risk of grass tetany on grazing areas of the farm. Aim to apply ‘build-up’ rates of K in the autumn to reduce the risk of luxury uptake of K during the main growing season, especially on silage swards. Recent research from Teagasc Johnstown Castle indicates that autumn applications of K on medium-to-heavy soils improve N efficiency compared to either spring or mid-season applications. For light soils apply maintenance levels of K to feed grass yield potential during the growing season.

Use clover or multi-species swards

White clover can fix between 50-120 kg N/ha/year depending on the underlying soil fertility and sward management. Multi-species swards, which incorporate legumes (clovers) and also herb species such as plantain and chicory, may offer extra benefits in reduced N use and animal health benefits. Aim for soil pH 6.5 to 6.8 and P / K soil Index 3.

Make best use of slurry

Slurry is a valuable fertiliser for growing grass on beef farms. On the other hand, purchased inorganic (chemical) fertiliser is one of the highest variable costs on beef farms; therefore, correct use of slurry can help reduce the requirement for chemical fertiliser and thus decrease the costs associated with growing grass. Slurry provides a balance of nutrients for grass growth in terms of N, P and K along with a return of valuable organic matter, which is especially important for silage fields. Good quality cattle slurry applied through low emission slurry spreading (LESS) in the springtime can have a typical available N-P-K value of 9-5-32 per 1,000 gallons. However, the N-P-K nutrient content within slurry can vary across beef farms. The ‘quality’ of cattle slurry is primarily influenced by its DM content and the diet of the animal. Slurry DM content can be estimated using a slurry hydrometer. The N-P-K content (and DM) can be analysed by testing slurry in a laboratory. Slurry can be analysed at a relatively low-cost and the resulting information means more appropriate and targeted application rates can be applied to the grass crop being produced.

Compared to splash plate application, slurry spread using LESS substantially reduces grass contamination meaning it can be applied to grass covers of up to 1,000 kg/DM/ha, which is equivalent to a compressed grass height of 7–8 centimetres. Low P and K index soils benefit immensely from slurry. Soil fertility maps in the Teagasc Nutrient Management Plan should be reviewed to identify paddocks that are shaded pink or blue as these paddocks are index one or two, respectively, for P and K. Cattle slurry applied to silage fields with an application of 33 m³/ha (3,000 gallons/acre) supplies 33 kg N/ha (26 units/acre) which is ~one-third of the crops N requirements.

Visit the Teagasc Slurry Calibration Tool

What type of chemical fertiliser should cattle farmers use?

If inorganic fertiliser must be applied, then switching from calcium ammonium nitrate (CAN) and urea to NBPT Urea (i.e. protected urea) will directly reduce both greenhouse gas and ammonia emissions, while also being cheaper. Calcium ammonium nitrate-based fertilisers release N₂O when applied to grassland. Compared to CAN, NBPT Urea emits 71% less N₂O emissions. By switching from urea to NBPT urea, ammonia emissions reduce from 19% to 3.3%, and reduces fertiliser requirements by ~ 12%. Of the tools assessed by Teagasc, using NBPT Urea N fertiliser offers the single largest emission reduction potential to Irish farmers. On a drystock farm, switching to NBPT Urea has the potential to reduce total emissions by up 4%, with the overall level of mitigation depending on chemical N usage. In terms of cost, NBPT Urea is cheaper than CAN, and has the potential to reduce fertiliser costs by 10 to 15% (depending on fertiliser N prices).

Low-N compound fertilisers reduce N₂O emissions compared to high-N compounds. Use low-nitrate compounds such as 18:6:12 and 10:10:20 to supply farm P requirements, reduce fertiliser costs and further reduce farm carbon emissions.

How can the accuracy of fertiliser application be improved?

• Setup and calibration of fertiliser spreaders is very important to ensure even distribution of fertilisers when spreading. This involves mounting the spreader correctly on the tractor so that it is level, adjusting the spreader settings to achieve accurate application rates and uniform coverage. Proper calibration also minimizes the risks of over- or under-application, which can lead to yield losses, environmental pollution, and increased production costs.
• Keeping the machine in good condition. Regular maintenance, including cleaning, lubrication, and inspection of components, is essential to ensure proper functionality. Worn vanes can significantly impact the spread pattern and distribution uniformity. As vanes wear out over time, this will result in uneven spreading, resulting in areas of over- or under-fertilisation. By replacing worn vanes promptly, farmers can maintain consistent application rates and optimize fertiliser efficiency.
• Different fertilisers exhibit varying flow characteristics and spread patterns. Different fertiliser types have different particle sizes and densities, leading to variations in spreading behaviour. Consequently, adjustments to spreader settings are needed to maintain an accurate spread width and flow rate and achieve uniform coverage across the field. Failure to adjust spreader settings to suit the product can result in uneven distribution and suboptimal fertiliser utilization. This is particularly important when moving between heavier products like CAN to lighter fertilisers such as Urea products
• To mitigate the risk of over-application and to reduce environmental impact, farmers can utilize headland control mechanisms. These systems allow operators to adjust the spread pattern when spreading at the field’s edges, preventing excess application in headland areas. By minimizing overlap and reducing wastage, headland control mechanisms not only conserve resources but also help protect nearby hedgerows and watercourses from pollution. This proactive approach to precision farming promotes sustainable agricultural practices while enhancing crop productivity and environmental stewardship.
• Use of other technologies such carrying out tray test and use of GPS controlled fertiliser spreaders can greatly aid the accuracy of the application of products.

In conclusion, the calibration of fertiliser spreaders is a critical aspect of modern agricultural management. By maintaining equipment in good condition, replacing worn vanes, adjusting settings for different fertilisers, and employing headland control mechanisms, farmers can optimize fertiliser application, minimize waste, improve crop yields, reduce production costs and protect the environment.

Compiled and edited by Mark McGee and Paul Crosson, Teagasc, Grange Animal & Grassland Research and Innovation Centre, and first published in BEEF2026 – Driving Sustainable Performance, additional reading from BEEF2026 is available here.