As part of the Moorepark’25 Open Day, Teagasc Dairy Specialist, Patrick Gowing took a closer look at slurry investments, examining the cost of additional slurry storage with the value of nutrients in it to grow grass.

The value of slurry is dictated by its nutrient content and how we utilise this slurry on farms. Maximising the nutrient value of slurry has never been more important due to the increasing cost of fertiliser. Maximising slurry nutrient use efficiency will also help to maintain grass production levels in the face of regulatory reduction in chemical fertiliser application limits.

Sub-optimal management of slurry also represents a significant threat to water quality. We must ensure that application timings, locations and rates are optimised by considering storage capacity, management strategies, nutrient use efficiency and identify other opportunities to add value to slurry.

As we face into further reductions in chemical nitrogen allowances due to environmental regulation, the value of our slurry increases.

Valuing slurry

In the past slurry has always been compared to the cost of chemical fertiliser. However, going forward, we need to look at the value of our slurry and its ability to grow grass in the absence of chemical fertiliser.

In the past, if we didn’t get the maximum value from our slurry by spreading at the wrong time or weather conditions, we could supplement our grass growth by top dressing with chemical nitrogen (N). This also potentially created water quality issues based on spreading time. With this in mind, we need to examine the cost additional slurry storage with the value of nutrients in it to grow grass rather than being compliant.

Table 1: Storage options and estimated costs

If we look at the cost of additional slurry storage the upfront capital cost is ~ €800 per cow excluding VAT and any potential grant (Table 1). While this is a significant cost if we average the cost over a 40-year lifespan of the tank it will cost €20/cow/year excluding a finance charge. If we now look at the value of the slurry by spreading it at the correct time to grow grass.

Table 2: Value of slurry, spread in optimal and poor conditions

As we can see by spreading the slurry at optimal times it is worth an additional €43/cow by using the nutrients to grow grass (Table 2). This will have a knock-on effect of improved water quality while maintaining grass growth.

As part of the Moorepark 2025, Dr. Patrick Tuohy provided an overview of earlier findings from the Teagasc National Slurry Monitoring Programme. For key insights, watch the video below:

Slurry additives

Some farmers are also exploring the option of adding value to their slurry by adding additives. There are two main types of additives, inoculants that add bacteria to the tank and also acids. The inoculants tested in research trials did not improve the nutrient content of the slurry and therefore should not be used.

Increasing the pH through the addition of acids has potential and is used in countries like Denmark however, it requires a lot of capital investment to ensure adequate mixing, and we are not in a position in this country to recommend it yet.

Slurry drying

Other farmers are looking at the potential of drying their slurry by using a screw press. This separates the liquid portion which is high in N back to the tank and the solid portion which is high in P into a dungstead.

While there are benefits in drying if moving the material over long distances and then potentially helping balance nutrients across, the farm it needs to be spread carefully and matched to soil samples as you are spreading nutrient dense materials.

Testing

Before looking into investing in adding value on slurry storage, we should first test the value of slurry we have. There can be a massive variance in nutrient content across tanks. Knowing the value of our own slurry and spreading accordingly could be the best investment prior to looking for alternatives.

Conclusion

Spreading of slurry at the appropriate time can save you €43/cow compared to spreading it in unfavourable conditions. Additional slurry storage is essential to achieve this. Slurry storage for 20 weeks costs ~ €800/cow ex VAT.

Slurry additives have a limited role in Ireland while, screw presses have a role especially in transporting long distances. Optimised management must begin with measurement; always start with a slurry test of each independent tank. Know the value of your own slurry.

The above was first published in the Moorepark Open Day 2025 book, more insights from the Moorepark Dairy Open Day are available here.