Abstract
SUMMARYThere is increasing concern about balancing agronomic and environmental gains from nitrogen (N) usage on dairy farms. Data from a 3-year (2009–2011) survey were used to assess farm-gate N balances and N use efficiency (NUE) on 21 intensive grass-based dairy farms operating under the good agricultural practice (GAP) regulations in Ireland. Mean stocking rate (SR) was 2·06 livestock units (LU)/ha, mean N surplus was 175 kg/ha, or 0·28 kg N/kg milk solids (MS), and mean NUE was 0·23. Nitrogen inputs were dominated by inorganic fertilizer (186 kg N/ha) and concentrates (26·6 kg N/ha), whereas outputs were dominated by milk (40·2 kg N/ha) and livestock (12·8 kg N/ha). Comparison with similar studies carried out before the introduction of the GAP regulations in 2006 would suggest that N surplus, both per ha and per kg MS, have significantly decreased (by 40 and 32%, respectively) and NUE increased (by 27%), mostly due to decreased inorganic fertilizer N input and improvements in N management, with a notable shift towards spring application of organic manures, indicating improved awareness of the fertilizer value of organic manures and good compliance with the GAP regulations regarding fertilizer application timing. These results would suggest a positive impact of the GAP regulations on dairy farm N surplus and NUE, indicating an improvement in both environmental and economic sustainability of dairy production through improved resource-use efficiencies. Such improvements will be necessary to achieve national targets of improved water quality and increased efficiency/sustainability of the dairy industry. The weak impact of SR on N surplus found in the present study would suggest that, with good management, increased SR and milk output per ha may be achievable, while decreasing N surplus per ha. Mean N surplus was lower than the overall mean surplus (224 kg N/ha) from six studies of northern and continental European dairy farms, while mean NUE was similar, largely due to the low input/output system that is more typical in Ireland, with seasonal milk production (compact spring calving), low use of concentrates, imported feed and forages, high use of grazed grass and lower milk yields per ha.
Highlights
Irish dairy production systems tend to be relatively intensively managed compared with other Irish grassland agricultural production systems, and are pasturebased, with the objective of producing milk in a low-cost system through maximizing the proportion of grazed grass in the cows’ diet
The concurrent decrease in chemical fertilizer N use and shift towards later application of this chemical fertilizer N both indicate an improved awareness of the fertilizer value of organic manures and accounting for them in nutrient management planning
Farm-gate N inputs were dominated by inorganic fertilizer (186 kg N/ha) and concentrates (26·6 kg N/ha), while outputs were dominated by milk (40·2 kg N/ha) and livestock (12·8 kg N/ha)
Summary
Irish dairy production systems tend to be relatively intensively managed compared with other Irish grassland agricultural production systems, and are pasturebased, with the objective of producing milk in a low-cost system through maximizing the proportion of grazed grass in the cows’ diet. Denitrification is naturally facilitated in Ireland, due to common anaerobic soil conditions and the generally high content of organic carbon (C) in soils (between 2 and 7%; Dillon & Delaby 2009) enabling development of denitrifying bacteria. These N losses can have negative environmental impacts such as eutrophication of surface waters, pollution of groundwater aquifers, ozone depletion and anthropogenic climate change (in the case of N2O emissions) (Leach & Roberts 2002; Eckard et al 2004; O’Connell et al 2004). In Europe, N use efficiency (NUE; proportion of N imports recovered in agricultural products; Ryan et al 2012) values between 0·17 and 0·38 have been recorded (Mounsey et al 1998; Groot et al 2006; Nevens et al 2006; Raison et al 2006; Roberts et al 2007; Treacy et al 2008; Cherry et al 2012; Oenema et al 2012)
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