Since the increased use of manure storage facilities in agriculture there have been numerous instances where a farmer, family member, or employee has asphyxiated or succumbed to toxic gases from the storage. Cases have been documented where several individuals have died while attempting to rescue a coworker or family member from an underground pit or a spreader tank.
What toxic gases are present around such storage facilities? The four main gases produced from decomposing manure are Hydrogen Sulfide, Methane, Ammonia, and Carbon Dioxide. In high concentrations, each of these gases may pose a health threat to humans and livestock.
In animal housing facilities, where the manure pit is often located below the facility floor, manure gases are generally detectable in low concentrations throughout the year. When pits are agitated for pumping, some or all of these gases are rapidly released from the manure and may reach toxic levels or displace oxygen, increasing the risk to humans and livestock.
More at: http://nasdonline.org/document/48/d001616/manure-gas-dangers.html