Arbejdsrapport fra Miljøstyrelsen, 8/2004 – Reduceret ammoniakemission fra hønsestalde med dybstrøelse

Reduced Ammonia Emission from Deep Litter Hen Houses

Summary and conclusions

Nitrogen loss in deep litter houses is considerably larger than nitrogen loss from battery cage houses. The substantial ammonia loss in deep litter houses is due to the decomposition process in the manure. The ratio of carbon and nitrogen (C:N) is of significance to the loss of nitrogen through decomposition. Usually the C:N ratio in poultry deep litter is low. By increasing the C:N ratio to about 20, the nitrogen loss can be minimised. An increase in the C:N ratio necessitates use of larger quantities of straw than those used today. Another consequence of using increased quantities of straw may be that the litter is kept very dry. Poultry excrete nitrogen as uric acid, which is very stable under dry conditions.

The objective of this project was to survey the nitrogen loss in a housing system (straw-flow) of an organic hen stock in which large quantities of straw may be used. The survey was carried out as eight separate surveys in order to disclose what influence the use of various quantities of straw may have on the nitrogen loss. The nitrogen loss is determined through a unit mass balanced calculation using phosphorus as tracer. Phosphorus does not evaporate (as is the case with nitrogen) and therefore it is possible to make a mass unit balance for phosphorus throughout the entire production system. The quantity of phosphorus deposited outdoors cannot be directly measured, but is calculated as the difference between admission and elimination of phosphorus. Phosphorus is added through the litter material and feed and eliminates through eggs and manure.

The excreted nitrogen is divided between the outdoor area and the house at the same ratio as the excreted phosphorus. It is therefore possible to calculate the quantity of nitrogen that is being deposited in the house. The difference between the calculated quantity of nitrogen in the manure and the quantity measured represents the loss of nitrogen from the house.

• In houses with an increasing C:N ration a statistical significant fall in the nitrogen loss was demonstrated

A significant negative correlation between the calculated C:N ratio in the litter and the relative nitrogen loss measured as a percentage of the quantity of nitrogen deposited in the house was demonstrated (figure 6, section 5.2). The survey showed a negative, but not significant correlation between the consumption of litter and the nitrogen loss.

• In three of the eight separate surveys the quantity of phosphorus eliminated through eggs and deep litter exceeded the quantity of phosphorus in the feed

In three separate surveys a negative deposit of manure (phosphorus) outdoors was calculated (tables 4 and 5, pages 20 – 21). Although subject to some uncertainty, the reason might be that - to a larger extent than presupposed in the planning of this survey - the hens use other phosphorus (feed) sources than grass when outdoors. The survey emphasises that we our knowledge on how and to what extent hens in production systems with access to outdoor areas actually utilise these areas, is insufficient.

In spite of the uncertainty as regards the validity of the unit mass balanced calculation mentioned above, the demonstrated correlation between quantities of straw and the loss of nitrogen should be considered to be of some importance. If unknown sources of phosphorus and nitrogen are found in the outdoor area, the miscalculation will at first imply a shifting of the level of the calculated loss of nitrogen from deep litter, while the ranging of the separate surveys on nitrogen loss is unchanged.

• The large quantities of straw meant that the deep litter in all separate surveys was very dry, the dry matter content in the deep litter varied from 54.2 – 74.5 per cent.

No significant correlation between the dry matter content in the deep litter and the nitrogen loss was demonstrated (figure 7, page 26). This may be because the litter in all separate surveys was dry/very dry, with a dry matter content above 50 %, and in one separate survey above 70 % (table 10, section 5.2).

• 10 – 20 times the usual quantities of straw was handled in the straw flow house

The high percentages of solids indicate that it was possible to handle very large quantities of straw in the surveyed housing systems. In those surveys where most straw was used, the quantities were 10 – 20 times the usual quantities used in deep litter houses for table egg hens (table 11, section 5.3).

The survey demonstrated that it is possible to reduce the nitrogen loss in deep litter houses by increasing the quantity of litter. Furthermore, the survey demonstrated that the houses surveyed could handle litter quantities 10 – 20 times the usual quantities. The survey, however, also demonstrated that the applied unit mass balanced model for nitrogen and phosphorus may be insufficient. In an open and naturally ventilated house as that surveyed, the unit mass balanced calculation is the only practical method to determine the ammonia loss. The exchange of air cannot be measured, contrary to mechanically ventilated houses in which all return of air takes place through relatively few openings. In the naturally ventilated house the value of air exchange and the value of air concentration of ammonia are therefore not comparable. If, in the future, calculations are made of the ammonia loss from a naturally ventilated hen house, greater knowledge on the hens' use of outdoor areas and what sources of feed the hens use when outdoors should be achieved.