Environmental and economic aspects of nitrogen and phosphorus use efficiency on intensive grass-based dairy farms in the South of Ireland

The large use of nitrogen (N) and the finite nature of global phosphorus (P) resources have led to increasing concerns about balancing agronomic, environmental and economic gains from N and P use on dairy farms. Nitrogen and P inputs, in the form of fertiliser and concentrates, are key drivers of increased herbage yields and milk saleable output on most dairy farms. However, N and P inputs typically exceed N and P outputs in milk and livestock exported off the farms. Increased N and P use efficiency (NUE and PUE) may be considered as a strategy to reduce the expenditures on the main N and P inputs on dairy farms. Data from a 3 year (2009-2011) survey were used to assess farm-gate N and P balances and NUE and PUE on 21 intensive grass-based dairy farms operating under the Good Agricultural Practice (GAP) regulations in Ireland, as well as the economic implications of NUE and PUE on 19 of these farms and the sensitivity of net profit to changes in milk and fertiliser N prices. Comparative profitability and sensitivity to changes in milk and fertiliser N prices of ten N fertilised grass (FN) and eight grass-white clover-based (WC) dairy systems were also investigated. Mean balances for the 21 farms were 175 kg N ha-1 and 5.09 kg P ha-1, respectively, or 0.28 kg N kg MS-1 (milk solids), and 0.004 kg P kg MS-1, respectively. Mean NUE was 0.23 and mean PUE was 0.70. Comparison to similar studies carried out before the introduction of the GAP regulations in 2006 indicates that N and P balances have significantly decreased (by 40 and 74 %, respectively) and NUE and PUE increased (by 27 % and 48 %, respectively), mostly due to decreased inorganic fertiliser input and a notable shift towards spring application of organic manures. Mean net profit was €598 ha-1 and was driven mainly by milk receipts and to a lesser extent by expenditure on mineral fertilisers. Net profit was indirectly related to N and P surplus and N and P use efficiency. The results of this study generally indicate that Irish dairy farms, as low input production systems, have the potential to achieve both economic (as indicated by net profit per ha) and environmental (as indicated by N and P balances per ha, N and P use efficiency and N-eco-efficiency) sustainability. The results of the sensitivity analysis indicated that milk price was the main driver for changes in net profit between 2009 and 2011 both in high and low milk price situations investigated across nine price scenarios. Net profit was similar for FN and WC (€1,274 ha-1) mainly due to €148 ha-1 lower expenditure on mineral N fertiliser on WC. Net profit of WC was found to be comparably less sensitive than FN in low milk price situations. A wider adoption of WC on farms offers potential to meet the twin goals of a sustainable income for dairy farmers in the context of rising fertiliser N price while decreasing N surpluses on pasture-based dairy farms.