The Impact of Nitrogen Management Strategies within Grass Based Dairy Systems

Prepared by: Laurence Shalloo, Elodie Ruelle, Karl Richards, Daniel Hawtree, Donal O Brien, David Wall, Michael O’Donovan, Deirdre Hennessy and Pat Dillon Teagasc, Oak Park, Carlow, Ireland.

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The Department of Agricultural, Food and the Marine (DAFM) requested Teagasc to simulate the impact (environmental and economic) of a number of farm nitrogen (N) mitigation measures in order to inform policy of the best current and potential actions to deliver the catchment-based nitrate load reduction estimated by the EPA in 2021. That resulted in a report published in July 2021 (https://www.teagasc.ie/media/website/publications/2021/Nitrates-Modelling-Final.pdf). In July 2022, DAFM requested Teagasc to undertake further analysis.

The following were requested to be investigated:

1. A review of the literature on the impact of land use, farming system, stocking rate, nitrate regulations on water quality on grass based systems in Ireland.
2. Trends in water quality from the Agricultural Catchment Programme.
3. Reduction in chemical N fertilizer application of approximately 10%, 20% and 30%, i.e. chemical N application rates of 250, 225, 200 and 175 kg/ha.
4. Stocking rate reduction - 250 kg N/ha (2.74 cows/ha), 230 kg N/ha (2.50 cows/ha) versus 220 kg N/ha (2.39 cows/ha).
5. Proportion of farms and milk that were in each of the different bands from 2020 and 2021.
6. Economic consequences of banding if associated with a reduced stocking rate at farm level.