Hydrogen sulfide production in manure storages on Pennsylvania dairy farms using gypsum bedding

Recycled gypsum products can provide a cost-effective bedding alternative for dairy producers. Manufacturers report reduced odors, moisture and bacteria in the stall environment when compared to traditional bedding and agronomic benefits upon land application of stored manure. Gypsum, calcium sulfate (CaSO 4˙2H2O), provides a sulfate source that can be converted to hydrogen sulfide (H2S) under anaerobic manure storage conditions. Agitation of stored manure promotes release of volatile gases including potentially concentrated plumes of H2S. Incidents have been reported anecdotally linking human and cattle injury and death to dangerous levels of H2S emission released from manure storages containing gypsum-based bedding. But not all farms using gypsum bedding have experienced problems. This study was undertaken to determine if gypsum bedding was contributing to elevated H 2S concentrations during manure agitation and, if so, what biophysical factors or management practices contribute to unsafe conditions.;Observations at 19 farms from three dairy manure categories were compared: (1) traditional, organic bedding; (2) gypsum-based bedding, and (3) gypsum-based bedding with a commercial product added to the manure that reportedly reduces H2S emissions. Portable meters placed around the perimeter of dairy manure storages recorded H2S concentrations every minute prior to and during agitation events during fall and spring hauling seasons. Manure samples from just below the surface crust and at the bottom of the storage were collected prior to agitation with a third sample collected from the mixed manure. A detailed farm characterization documented manure storage design parameters, manure handling practices and inputs to the manure storage facility.;Results show that manure storage agitation at farms that use gypsum in bedding produced H2S concentrations that were considered immediately dangerous to life and health (above 100 ppm). Increasing gypsum use significantly increase cumulative H2S concentrations (P-value <0.0001). Farms that used an amendment exhibited lower H2S concentrations yielding a trend line having an R-squared value of 0.86. However the lower cumulative H2S concentrations were not significantly attributed to the manure additive (P-value=0.725). More data could strengthen this trend and prove statistical significance. Manure analyses support gypsum increases total sulfur content and lowers phosphorus source coefficient (PSC) values. Data did not support the claim that gypsum use promotes nitrogen retention or reduced somatic cell counts.;A statistical model did not pinpoint farm practices or environmental conditions that significantly affect H2S production and release from storage. However, empirical observations indicated low H2S concentrations from storages when manure was transferred from temporary pits before ending up in the final stage of storage. Wind direction also appeared to affect H2S concentrations near the manure storage during agitation. With wind blowing into a structure proximate to the manure storage, H 2S concentrations were observed to increase due to the structure inhibiting dissipation.;An emissions model was developed that predicted cumulative H2S concentrations after 60 min of agitation to within approximately 15% of measured H2S concentrations. Dangerous levels of H2S were predicted during periods immediately following calm wind conditions (<1.1 km h -1) at locations where physical structures inhibited dispersal of H2S plumes.;This work quantitatively documents the potentially dangerous levels of H2S release during agitation of outdoor manure storage structures containing gypsum. Greater awareness of the risks to human and animal health is needed in the dairy industry. Future work includes developing solutions to reduce H2S emissions so that the use of gypsum bedding can continue to benefit the dairy industry without creating hazardous environments.