Ammonia and odour emissions from UK pig farms and nitrogen leaching from outdoor pig production. A review.

We reviewed specific literature for emissions of ammonia (NH3) and odours from all stages of pig production together with nitrogen (N) leaching from raising pigs outdoors. Emissions of NH3 decrease with decreases in the crude protein (CP) content of pig diets, at all stages of manure management. The CPs of pig diets have been greatly reduced by matching the CP content to the protein required at each stage of the animals' growth and by using synthetic essential amino acids to minimise total CP intake. The CP contents of the dietary ingredients needed to provide energy for the animals impose further limits to reductions in dietary CP. Housing systems have been designed and evaluated which offer potential for reducing NH3 emissions. However such designs may not be applicable at all stages of the pigs' development and the careful management needed to ensure their effective working may be costly and difficult to implement on commercial farms. The factors behind odour emissions are less well characterised. Reducing diet CP to 160 g CP kg(-1) has been shown to reduce odour emissions but further CP reductions may increase them. Some reductions in odour emissions from buildings can be achieved by careful management of the ventilation rate but the most effective measures to reduce emissions of NH3 and odours are to cover slurry stores and to inject slurry into soil. Changes in the feeding and management of outdoor pigs mean that N leaching losses may be up to 50% less than previously reported. No studies have been undertaken that compare the N leached from pigs raised outdoors, versus that arising from the application of pig manure from an equal number of housed pigs. As a precursor to any field study, current models could be used to provide a first estimate of any systematic differences.