Ammonia emissions from pig houses: Influencing factors and mitigation techniques

Pig houses are important sources of ammonia (NH3) emissions. For decades, investigations were carried out in determine the influencing factors and to point out opportunities of mitigation. In Europe, current NH3 emissions associated to pig production are about 24% lower than in 1990. However, further reduction seems necessary to avoid noxious effects on ecosystems. The main factors influencing NH3 production are the floor type, the manure removal system, the climatic conditions inside the building, the diet composition and the feed efficiency of animals. In pig production, the main floor types are the slatted floor and the bedded floor systems. In both systems, numerous variants and adaptations can be found with consequently a range of emission levels for each housing condition. Therefore, decision in favour of a floor type as regards NH3 emissions is difficult, especially as effective reducing strategies are available for both systems. For litter-based systems, the nature and the amount of substrate greatly influence the NH3 production with usually lower emission in case of generous bedding. For slatted floor systems, most of the studies resulted in lower emissions with partly slatted floor on condition that the solid part of the floor remains clean. Indeed, hot conditions, high animal density or inadequate pen design can increase the soiling of the solid floor and lead to increased NH3 emissions. In any case, emissions are lower if concrete slats are replaced by smooth materials like iron cast, metal or plastic slats. Several slurry pit designs and manure removal strategies were developed to mitigate emissions. The reduction of the slurry pit surface thanks to sloped pit walls are related to proportional reductions of NH3 emissions. Frequent manure removal, flushing and separating urine from faeces by V-shaped scraper or conveyor belts reduce the NH3 releases from the buildings by about 50%. However, the emissions during the storage period outside the building have to be taken into account for a whole assessment of the technique. Climate conditions inside the building also influence the emissions which are positively correlated with ambient temperature and ventilation rate. Consequently, ammonia emissions present seasonal and nychtemeral patterns. But, reducing the NH3 production by modulation of the climate conditions is rather unpractical because the ambient parameters must primarily respect the bioclimatic requirements for animal comfort. A closer match between dietary intakes and requirement of the pigs according to the physiological and growth stage results in lower NH3 emissions. In this way, diets with reduced crude protein content are highly effective in reducing the emissions with almost a 10% reduction for every 10 g kg−1 reduction in dietary crude protein. Other dietary strategies are also effective in lowering emissions. Dietary fibre inclusion reduces NH3 emissions by about 40% by shifting the nitrogen from urine to faeces due to promotion of bacterial growth in the large intestine. Lowering the dietary electrolyte balance or supplementation with acidifying salts like benzoic acid or CaSO4 are related to significant reductions. Other feed additives like Yucca extract, zeolites, probiotics, humic substance or lactose were also validated by several experiments. Moreover, better feed efficiency obtained by genetic selection or modification of the hormonal status of the pigs is also related to reduced emissions. In conclusion, effective reduction of ammonia emissions from pig buildings can be reached operating both on housing conditions and feeding strategies. The former are very efficient but the assessment has to include the specificity of each system and involve the complete process. In some cases, investment and cost operating can hamper their development. Feeding strategies offer the advantage of being easy to implement and rapid to adapt function of particular circumstances