Noxious gases in rabbit housing systems: effects of cross and longitudinal ventilation

Animal welfare is a matter of increasing interest due to ethical and economical worries regarding animal rights and the sustainability of meat production. Ammonia, carbon dioxide, and methane can be produced in the livestock buildings and, if not adequately controlled by ventilation, can be dangerous for animals and farmers. The aim of the present paper is to study the effects of different ventilation systems in rabbit buildings based on the temporal patterns and the spatial distribution of these noxious gases. The experimental measurements were conducted in two rabbit farms with genetically homogeneous animals subjected to the same diet. Two buildings with different forced ventilation layouts (cross ventilation - building A and longitudinal ventilation - building B) were subjected to the monitoring of indoor environmental conditions (temperature, relative humidity, ammonia, carbon dioxide, methane) over a whole year. In both the buildings, ventilation was adjusted automatically by means of electronic control units, which were controlled by temperature sensors, located at the centre of the buildings. Gas concentrations inside the buildings followed clearly defined sinusoidal patterns on a daily basis with the highest values reached in winter during the morning hours for ammonia and during the night hours for carbon dioxide and methane. In particular, ammonia revealed a maximum concentration of 30.7 mg m^–3 in building A (cross ventilation) and 12.9 mg m^–3 in building B (longitudinal ventilation), whereas the minimum values were 6.0 and 4.2 mg m^–3, in building A and B, respectively. As a consequence, daily mean concentrations of noxious gases, solely could not be considered representative of the actual conditions of air quality in the buildings. The airflow direction clearly influenced the spatial concentration of ammonia, which showed different patterns in the two buildings. In building A, the highest ammonia concentration was in a diffuse central area, whereas in building B, it was determined to be less extended and located in the proximity of the wall equipped with extraction fans. The results of this study provide important indications for the planning and management of housing systems for rabbits including: the correct positioning of gas sensors for regulating ventilation systems must be central in case of cross ventilation, but close to the suction fans in case of longitudinal ventilation; the cross ventilation can lead to ammonia concentration higher than longitudinal ventilation, which is caused by the close and prolonged contact of airflow with manure surface in the channels; fans for longitudinal ventilation must be positioned in the same side of the building where scrapers discharge manure; furthermore, manure scraping has to be performed daily in winter during the hours of the day when ventilation rate is at its maximum.