Nitrogen losses from outdoor pig farming systems

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Abstract
Nitrogen losses via nitrate leaching, ammonia volatilization and nitrous oxide emissions were measured from contrasting outdoor pig farming systems in a two year field study. Four 1‐ha paddocks representing three outdoor pig management systems and an arable control were established on a sandy loam soil in Berkshire, UK. The pig management systems represented: (i) current commercial practice (CCP) ‐ 25 dry sows ha−1 on arable stubble; (ii) ‘improved’ management practice (IMP) ‐ 18 dry sows ha−1 on stubble undersown with grass, and (iii) ‘best’ management practice (BMP) 12 dry sows ha−1 on established grass. Nitrogen (N) inputs in the feed were measured and N offtakes in the pig meat estimated to calculate a nitrogen balance for each system. In the first winter, mean nitrate‐N concentrations in drainage water from the CCP, IMP, BMP and arable paddocks were 28, 25, 8 and 10 mg NO3 l−1, respectively. On the BMP system, leaching losses were limited by the grass cover, but this was destroyed by the pigs before the start of the second drainage season. In the second winter, mean concentrations increased to 111, 106 and 105 mg NO3‐N l−1 from the CCP, IMP and BMP systems, respectively, compared to only 32 mg NO3‐N l−1 on the arable paddock. Ammonia (NH3) volatilization measurements indicated that losses from outdoor dry sows were in the region of 11 g NH3‐N sow−1 day−1. Urine patches were identified as the major source of nitrous oxide (N2O) emissions, with N2O‐N losses estimated at less than 1% of the total N excreted. The nitrogen balance calculations indicated that N inputs to all the outdoor pig systems greatly exceeded N offtakes plus N losses, with estimated N surpluses on the CCP, IMP and BMP systems after 2 years of stocking at 576, 398 and 264 kg N ha−1, respectively, compared with 27 kg N ha−1 on the arable control. These large N surpluses are likely to exacerbate nitrate leaching losses in following seasons and make a contribution to the N requirement of future crops.