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Dairy manure storages containing gypsum-based bedding have been linked anecdotally with injury and death due to presumed dangerous levels of gases released. Recycled gypsum products are used as a cost-effective bedding alternative to improve animal welfare and provide agronomic benefits to manure recycled back to the land. Sulfur contained in gypsum (calcium sulfate) can contribute to hydrogen sulfide (H2S) gas formation under the anaerobic storage conditions typical of dairy manure slurry. Disturbance of stored manure during agitation releases a burst of volatile gases. On-farm monitoring was conducted to document conditions during manure storage agitation relative to gas concentration and operator safety. One objective was to document operator exposure to H2S levels; therefore, each operator wore a personal gas monitor while performing tasks associated with manure storage agitation. Data from three dairy bedding management categories on ten farms were compared: (1) traditional organic bedding, (2) gypsum bedding, and (3) gypsum bedding plus a manure additive thought to reduce H2S formation and/or release. Portable meters placed around the perimeter of dairy manure storages recorded H2S concentrations prior to and during 19 agitation events. Results show that farms using gypsum bedding produced higher H2S concentrations during manure storage agitation than farms using traditional bedding. In most cases, gypsum-containing manure storages produced H2S levels above recognized safe thresholds for both livestock and humans. Farm operators were most at risk during activities in close proximity to the manure storage during agitation, and conditions 10 m away from the storage were above the 20 ppm H2S threshold on some farms using gypsum bedding. Although H2S concentrations rose to dangerous levels, only two of 18 operators were exposed to >50 ppm H2S during the first 60 min of manure storage agitation. Operators who are aware of the risk of high H2S concentrations near gypsum-laden manure storages can reduce their exposure risk by working upwind and away from the H2S plume within a closed tractor cab.

Abstract. To mitigate noxious hydrogen sulfide (H2S) gas release that has been observed from gypsum-laden dairy manure, three additives were studied in sequential investigations. Three trials with specific aims were conducted using experimental vessels containing 15 kg of dairy manure each. Trial 1 investigated two additives: iron oxide (specifically, iron oxide-hydroxide, FeOOH) and a proprietary gypsum-lime based product (DriMatt). Trial 2 investigated effective ratios of gypsum to iron oxide and a modified DriMatt additive, and trial 3 evaluated iron oxide at the most effective ratio. Manure agitation events were monitored in the first two trials, while gas releases were continuously monitored in trial 3 during and between agitations. Hydrogen sulfide concentrations were captured using electrochemical sensors or a Fourier transfer infrared (FTIR) gas analyzer assembly over an incubation period of two months for the first two trials and over 40 days for the third trial. Additionally, nutrient analyses were performed for each trial. Extremely high concentrations of H2S were observed during most manure agitation events (500 to 8000 ppm), while minimum releases (<10 ppm) were found when samples were static. Means of maximum concentrations of H2S were compared among treatments in each trial. Statistical tests showed that adding iron oxide to gypsum-laden manure reduced H2S production by an average of 94% compared to treatments without iron oxide. With a 1:1 molar ratio of iron oxide to gypsum, the level of H2S released was diminished to as low as the control manure (without gypsum). Therefore, iron oxide is a promising additive to mitigate H2S production in gypsum-laden dairy manure during agitation events. Keywords: Additive, Dairy, Gas, Gypsum bedding, Hydrogen sulfide, Iron oxide, Manure, Safety, Storage.

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