Recent deaths of farmers and cattle have raised awareness of the all-too-common dangers of working around manure storage facilities. People “being overcome” or feeling dizzy around manure storage areas happens too often. Headlines often list the reason as asphyxiation or toxic gas.

Wheelereileen fabian
Professor – Department of Agricultural and Biological Engineering / Penn State University

Many times, the toxic gas is hydrogen sulfide, with most deaths associated with below-ground, enclosed storages. More recent investigations indicate that hydrogen sulfide is also present in outdoor open storages, particularly on dairy farms where gypsum is used as bedding material.

Recent measurements at 10 dairy farms confirm highly elevated hydrogen sulfide gas concentrations surrounding open manure storages during agitation prior to manure removal for land application. A link to gypsum bedding seems clear. A big concern is respecting the everyday risks of working near any stored manure, particularly when agitating.

Gypsum bedding has become popular in regions with an affordable supply, such as in Pennsylvania. It is obtained from recycled construction wastes, such as drywall board. Bedding products range from a floury powder to granular material to pellet-sized wall-board chunks.

All versions seem comfortable to the cows, offering increased moisture absorption and low bacteria growth in the pH-neutral material, enhancing animal welfare through improved udder health and cow cleanliness.


Farmers who are fans of gypsum bedding point to the soil benefits. Manure from gypsum-bedded cows has reduced carbon to be broken down once land-applied versus wood chips and sawdust bedding. Plus gypsum-manure provides additional sulfate to soil while reducing phosphorus runoff through improved phosphorous source coefficient (PSC).

Thus, there are many good reasons for the use of gypsum as dairy cow bedding. The question now is how to raise awareness that safe manure-handling practices are just as important, if not more so, when handling manure containing gypsum as with any dairy manure.

One might ask how a bedding material choice could influence risk during manure handling months later. Gypsum is calcium sulfate (CaSO4-2H2O) that under anaerobic (no oxygen) conditions in manure storages is microbially converted to hydrogen sulfide gas.

This makes it very likely that hydrogen sulfide will be produced in dairy manure collection pits and manure storages. Some will recognize hydrogen sulfide by its “rotten egg” smell.

Hydrogen sulfide gas is particularly tricky as it is heavier than air. It can settle in low spots near manure storage. Children breathing at their low height are more susceptible to hydrogen sulfide plumes.

Equally tricky is that hydrogen sulfide overcomes the sense of smell and no longer smells like rotten eggs at dangerous levels (100 ppm). Then, at higher levels (500 ppm or more), it quickly arrests the ability to breathe properly, resulting in dizziness followed by passing out. At extremely high levels (approximately 1,000 ppm), breathing ceases quickly.

Hydrogen sulfide and other gases of concern are released in bursts during manure movement and agitation. These bursts are often accompanied by significant odor. During our measurements from a USDA Natural Resources Conservation Service-funded demonstration project, we found several features of interest.

One is with gypsum-bedded cows, the manure during agitation released hydrogen sulfide levels that were immediately dangerous to life and health (at 100 ppm or higher). This raises obvious concern. Plumes of this gas have been known to be present in dangerous levels in below-ground, enclosed storages with any species of animal manure and any dairy bedding material.

But recent observations raise the need for concern even at outdoor open manure storages during agitation. A second project finding is encouraging in that use of manure additives that break down the stored manure seemed to reduce hydrogen sulfide gas levels at agitation on farms using gypsum bedding.

We also found that the first 30 to 60 minutes of manure storage agitation are the most dangerous. Stay away during this time. Or wear a gas monitor that warns of risky gas level. Personal gas monitors are barely larger than a cell phone and cost less. The farmers at our 10 demonstration sites each wore a personal gas monitor during manure storage agitation so we could observe exposure and increase their safety.

Operators with highest hydrogen sulfide exposure during our project were very close to the manure being agitated or had leaned over the storage fence line to adjust or maintain equipment. Operators who stayed in tractor cabs or were otherwise well away from gas plumes coming off the manure were at lower risk.

It is virtually impossible for an individual to get themselves out of a manure storage accident. Every recent fatal incident in the northeastern U.S. (except two young boys in Pennsylvania who were overcome by gas and found unconscious at the edge of an in-ground open storage during agitation) involved people who were found unresponsive in a manure storage with no means of rescue or recovery in place.

The fatalities have involved farms using gypsum bedding and those that do not. We will never know if the people were overcome by gas or simply fell into the storage, as there were no surviving witnesses. These tragic reminders point to the importance of providing a life line (harness and rope, for example) and a plan that does not endanger those attempting rescue.

This leads to two strong recommendations for any and all dairy manure storages. One is to stay clear of manure being agitated for the first half-hour when most gas is released; more than an hour is even better. This includes not leaning over or within the storage confines. Secondly, keep non-essential people away during agitation, especially children.

Gypsum bedding offers benefits to cow comfort, milk quality and agronomic features. These benefits should be weighed against the risk of elevated levels of hydrogen sulfide gas. Hydrogen sulfide is likely to reach dangerous levels in locations in bursts even around outdoor open manure storages during agitation prior to land application when gypsum bedding is used. Be aware. PD

Eileen Fabian-Wheeler is an agricultural and biological engineering professor with Penn State University. She can be contacted by email.

PHOTO:Gypsum bedding offers benefits to cow comfort, milk quality and agronomic features, but its use should be weighed against the risk of elevated levels of hydrogen sulfide gas in manure storages. Photo courtesy of Robert Meinen, Penn State Extension.

Three manure storage safety tips

Outdoor dairy manure storages are open to the atmosphere but still meet the definition of a confined space in terms of danger from toxic gas and drowning.

1. If you must go into the fenced area of the open manure storage, wearing a safety harness with life line attached to a safely located solid object or anchor will enhance your chances of rescue.

2. Never work alone. The second person’s role is to summon help in an emergency and assist with rescue without entering the storage.

3. If you feel unsure or uncomfortable with what you are getting ready to do near the open manure pit, step back, contact someone and review the situation before proceeding.

Find more useful information in the publication from which these three tips were taken: Open Air Manure Storage Safety Tips. Penn State Extension. By D.J. Murphy, R. Meinen and D.E. Hill. 2014.