Economic impacts of nitrogen and phosphorus use efficiency on nineteen intensive grass-based dairy farms in the South of Ireland

A 3 year (2009-2011) study found a mean N balance of 166 kg N/ha, P balance of 4.93 kg P/ha, N use efficiency of 0.24, P use efficiency of 0.71, and net profit of €598/ha on 19 Irish dairy farms. The increase in mean net profit with mean milk receipts and the decrease with mean expenditure on chemical fertilisers imply that increasing milk receipts while optimising the use of chemical fertiliser input can be an effective strategy to increase net profit. Mean net profit was not directly related to mean N and P balances or N and P use efficiencies. However, there was an indirect link between net profit and N and P use efficiencies, as indicated through significant relationships between N and P use in the form of chemical fertilisers and feeds and the associated expenditures on chemical fertilisers and feeds. The increase of mean expenditure on feeds (concentrate, forages) with mean SR (stocking rate) and feed N input highlights the importance of matching SR with the feed imports on grass-based dairy farms, when there is limited availability of grassland area. This can be an effective strategy to control expenditure on feeds, with potential positive impact on net profit. Results of the sensitivity analysis indicated that milk price was the main driver for changes in net profit in high and low milk price situations investigated across nine price scenarios. The decrease in mean (51.4 l/kg N) N-eco-efficiency (milk produced per kilogram N balance) with mean fertiliser N input (190 kg N/ha) implies that efficient on-farm N management is needed to achieve increases in milk production and reduce N balance per unit product (litre milk). Potential fertiliser N replacement values of €317/ha for the spring and €64/ha for summer slurry application may represent strong incentives for farmers to make increased use of organic fertilisers, as part of overall on-farm N management. This can have positive impacts on farm nutrient use efficiency and farm net profit. Eight farms exceeding the limit of 2 livestock units (LU)/ha, imposed through the Nitrates Directive, had 1.63 times higher net profit compared with the remainder, which justified the cost of compliance associated with being in derogation. The results of this study generally indicate that Irish dairy farms, as low-input production systems, have the potential to improve both economic (as indicated by net profit per hectare) and environmental (as indicated by N and P balances per hectare, N and P use efficiencies and N-eco-efficiency) sustainability.