The ammonia emissions from housing systems for laying hens must be reduced to abate environmental damage. In traditional cage systems and alternative, welfare-oriented, housing systems for laying hens two types of waste can be distinguished, namely manure and litter. Manure is encountered as slurry or as dry manure. Litter on the floor is a dry and granular mixture of faeces and e.g. sand. It is used by the hens for dust bathing and scratching. In the processes involved in the emission of ammonia, the most important process parameters are temperature, pH and water activity, i.e. the availability of water for micro-organisms in the substrate. Air temperature, relative humidity and air velocity define climatic best. Despite the effectiveness of pH control, application of this measure has too many negative side-effects. Forced drying of manure seems the only effective and acceptable way for temporary control of ammonia emissions inside housing systems. Regular removal of manure out of the house is always necessary. Reduction of ammonia emissions can be achieved by changing over from housing systems with composting (emission rate 386 g NH3/hen a) to battery systems with manure belts underneath the cages (emission rate 34 g NH3/(hen a), which enable drying of manure and regular removal. The drying rate of the manure and the frequency of removal are crucial in relation to the emission. Minimum emission from manure is achieved if a dry matter content of 60% is reached within 50 h after excretion of the faeces. The emission form battery systems with belts can be reduced to 10 g NH3/(hen a) if the manure is removed from the house twice daily instead of twice weekly. the emissions of ammonia from alternative housing systems with litter are several times higher compared with battery cages with belts, due to the long stay of litter inside the house. A higher dry matter content of litter slows down the volatilization of ammonia, but practical means to reduce the emission from litter in this way ar not yet available. Moreover, a good understanding of water activity is still not available.
