Farm livestock stocking rate “is not a primary driver” of concentrations of nitrate-nitrogen (N) in groundwater and streams, according to Teagasc.

In a research paper titled The Impact of Nitrogen Management Strategies within Grass-Based Dairy Systemspublished last week – Teagasc researchers said that nitrate-N concentrations in groundwater and streams are primarily controlled by the physical characteristics of the local catchment area.

This includes soil hydrogeological factors, and meteorological conditions and agronomic practices also play important roles in N concentrations, according to the research paper.

Citing examples of this, Teagasc noted that, between 2010 and 2017, a significant increasing trend in groundwater nitrate-N concentration was observed in the tillage catchment area of Castledockerell, Co. Wexford, which has vulnerable soil hydrogeological characteristics.

By contrast, the intensive grassland catchment of Timoleague in Co. Cork saw no long-term trend, despite an increase in the proportion of land there farmed under the nitrates derogation, as well as the overall stocking rate increasing from 134kg N/ha in 2008 to 182kg N/ha in 2018.

In recent years, there have been indications that nitrate-N concentration is reducing in the Timoleague catchment.

Despite this, areas that do not see improvements in water quality may see a reduced nitrates derogation from next year.

Under EU rules, which Ireland had to agree to in order to renew the derogation for this year, we are required, before June 30, this year, to submit a review of water conditions for last year. The review will contain the results of monitoring of nitrates concentrations of groundwater and surface waters.

It must also contain maps of areas that drain into water where monitoring shows the average value of nitrate concentration is above 50mg/L, or trends of nitrate concentration are increasing. These areas will be classed as polluted or at risk of pollution.

From January 1, 2024, farms located in these areas will see the maximum stocking rate under derogation cut to 220kg N/ha.

According to last week’s Teagasc paper, this cut in derogation could see dairy farmer incomes fall by €374/ha.

“These reductions in profitability per hectare highlight the importance of grass utilisation in pasture-based systems,” the agriculture research body said.

As another new addition to the Nitrates Action Programme (NAP), the Department of Agriculture, Food and the Marine has introduced livestock excretion banding rates.

Previously, the organic N excretion rate of each cow was reckoned as 89kg N. Under banding, cows will be assigned one of three figures based on milk output. The possible excretion rates are: 80kg N; 92kg N; and 106kg N.

This means that farmers who find themselves in band 2 or 3 may now be over the maximum stocking figure of 250kg N/ha allowed under the derogation.

Reducing organic N

Teagasc research has found that, for farmers in this position, the least negative financial strategy to reduce organic N would be to contract-rear or rear fewer replacement heifers, or rent additional land.

Exporting slurry was found to be not practical due to the quantities that are to be exported; and its impact on soil fertility of the exporting farm, as most grassland is close to farm phosphorous (P) balance and therefore exporting slurry would create a deficit across the farm.

Reducing cow numbers will have the worst impact on farm profitability, and therefore farmers would be likely to exhaust other available options before a reduction in herd size is considered.

The new banding regime, combined with a potential reduction in the derogation to 220kg N/ha, would cause significant farm profitability issues.

The combined effect of both measures could, in the most extreme scenario, reduce farm profitability by between one-quarter and one-third, according to Teagasc’s research.