Managing sulphur content of pig diet to control sulphides production during pig slurry anaerobic storage

In the present configuration of livestock manure management, and particularly for the pig industry, slurry is collected from barns and stored into storage tanks inside and/or outside of livestock buildings before spreading. During this storage period, in anaerobic condition, production and accumulation of sulphur (S) compounds occur. Among the molecules produced, some are of particular concern since they are responsible for odours or are dangerous for health. According to Spoelstra (1980), the major S compounds responsible for odours are hydrogen sulphide (H2S) and methyl mercaptan. Other authors (Clanton and Schmidt, 2000) extend this list to include dimethyl-sulphide, dimethyldisulphide, carbon disulfide and carbonyl sulphide. Handling these types of livestock manures can be dangerous when gaseous S forms dissolved in the liquid phase are suddenly transferred and released to the gas phase (Mackie et al., 1998; Oneill and Phillips, 1992). The most dangerous S gas compound is probably H2S that is toxic and lethal at high concentrations. In most cases, the maximum exposure to H2S occurs when the effluent is mixed before land spreading (Oesterhelweg and Puschel, 2008). The acute toxicity of H2S absorbed by inhalation is well documented. Thresholds of toxicity have been identified and regulated to protect exposed populations. For pig slurry, different sources can explain H2S production. First, in anaerobic conditions, sulphates present in pig excreta (urine and faeces) can be reduced by sulfate-reducing bacteria into sulphite and sulphide, this pathway is well known as dissimilatory sulphate reduction. The microorganisms involved are terminal degraders of organic matter and are widely present in anaerobic ecosystems (Elferink et al., 1994; Stams et al., 2005). Second, H2S formation may result from the mineralization of organic compounds containing S, such as non digested proteins which contain methionine, cystine and cysteine amino acids that can be reduced to pyruvate and H2S (Mackie et al., 1998). In the literature, much documentation is available to correlate odours emitted by pig slurries and gaseous S emissions but little is found to correlate animal feeding strategies, S excretion via faeces and urine and further sulphide production during pig slurry storage. The objectives of this study were to determine the S content of different raw materials usually used to formulate pig’s feed, to evaluate the behaviour of the ingested S and to determine the S compounds produced during pig slurry storage.