The relative impacts of dairy and non‐dairy ruminant sectors on the Olsen‐P status of grassland soils and hence water quality in Northern Ireland

Intensive dairy farming is implicated with the phosphorus (P) enrichment and eutrophication of fresh water bodies. In Northern Ireland (NI), although P surpluses on dairy farms are high, representative soil P data for both dairy and non‐dairy ruminant sector grassland are needed to help assess the extent to which dairying per se is contributing to poor water quality. Data from a recently completed European Union funded NI‐wide soil testing scheme were available to help in this assessment. A high proportion (50%) of dairy grassland was found to be over‐supplied with P (>25 mg Olsen‐P l soil−1), but surprisingly, some 25% of more extensively managed non‐dairy ruminant grassland was also over‐supplied with P. This is possibly because fertilizer‐P and/or manure‐P is being spread and/or faeces excreted by out‐wintered livestock on only a fraction of the grassland area, with most of the remaining land being too wet or too inaccessible for such management. Therefore, even though the P over‐supply situation is most acute in the dairy sector, because the non‐dairy ruminant sector occupies the majority (78%) of the ruminant grassland platform, it is responsible for almost two thirds (64%) of all P‐enriched grassland soils across the ruminant sectors, and hence potentially poses a greater threat to water quality than the more intensive dairy sector. To address this newly recognized problem, farm‐gate P surpluses may need to be redefined in terms of surplus P per unit of land suitable for manure‐P application and/or out‐wintering livestock, as opposed to land owned or managed, and a regulatory maximum limit applied to all ruminant farms regardless of their management intensity or stocking density.