Of all the changes proposed by DAERA to the Nutrients Action Programme (NAP) for 2026 to 2029, it is new phosphorus (P) limits that pose the greatest challenge for the dairy industry in NI.

The current plan is that the new limits will apply to any farm with annual manure Nitrogen (N) production at and above 150kg N/ha per year. In NI there are around 3,500 farms in this category, the majority of which are dairy farms.

Farms above the 150kg threshold will have to abide by a P balance (limit) of 10kg P/ha/year from 2027, reducing to a P balance of 8kg P from 2029 onwards. DAERA has warned of even lower limits from 2030 onwards.

The aim is to reduce the overall surplus of P within NI agriculture and reduce the likelihood of it escaping to waterways.

Feed

Most of the P coming onto farms is in livestock feed (around 13,000t in 2023), followed by P in chemical fertiliser (2,500t). So when calculating a P balance on the farm, knowing the tonnes of bought-in concentrate feed and P-containing fertiliser is crucial. Other P could come in by way of bought-in hay, silage, straw or livestock.

P also leaves the farm, whether it is in milk, beef, lamb, poultry and crops, or in the likes of slurry exports. Subtracting P output from P input and dividing that figure by the area farmed, will give you a P balance/ha/year.

Given all the various permutations, the P balance calculation is quite complicated, especially on farms that trade a lot of cattle or run a mix of enterprises.

However, to highlight the main principles, we have created three relatively simple dairy scenarios, based on a 200-cow herd kept on 100ha.

We assume there are no cattle bought-in, no crops sold off the farm and no slurry imports or exports.

Scenario one

In the first scenario, the 200-cow herd produces 8,000l/cow on 3t of concentrate, so it is a total concentrate input of 600t annually.

In addition, 50t of concentrate is fed to heifers.

Our calculations use the latest estimates of P in concentrates from the NI Grain Trade Association (NIGTA), recently validated by scientists at the Agri-Food and Biosciences Institute (AFBI).

Those estimates put P in dairy concentrates at an average of 4.7kg/t, with P in other concentrate feed at 4.3kg/t.

In P balance calculations done by DAERA it is assumed that milk has a P content of 0.001kg/l so the 1.6m litres of milk leaving this farm includes 1,600kg of P.

There are also 100 dropped calves sold, with a P content of 0.33kg per head and 100 cattle sold per year averaging 500kg and with a P content of 0.0066kg per kg liveweight.

As shown in Table 1, the P inputs minus the P outputs on this farm, leaves a surplus of 1072kg or 10.72kg P/ha/year, so it is just outside the P limit proposed by DAERA for 2027.

Scenario two

In the second scenario, everything has been kept the same, expect it is assumed that it is a lower input herd with 6,000l of milk produced per cow from 1.5t of concentrate per head.

As can be seen from Table 2, this herd actually has a very low P balance and in practice, it probably needs to import some P-containing chemical fertiliser to maintain soil fertility.

Looking ahead, given this farm is in and around the 170kg manure N limit per ha, it will not be able to import slurry. Under the rules as proposed, the only way it will be able to maintain soil fertility is to get an exemption from DAERA to continue to spread P fertiliser.

Scenario three

The final scenario highlighted in Table 3 is also a 200-cow herd, but with 10,000l cows fed 4.2t of concentrate per head.

With a P input of 4,163kg, but with only 2,363kg leaving the farm, it results in a P surplus of 1,800kg, or 18kg P/ha/year.

Under the current DAERA NAP proposals, if this farm wants to avoid cutting livestock numbers it will either have to look at exporting slurry and/or taking on more land.

If solely relying on land to get to a P balance of 8kg, it would need to expand from 100ha to 225ha.

If solely relying on slurry exports, it would need to find a new home for 1,917m3 of slurry, which is approximately 422,000 gallons.