Swine hoop structures are an alternative housing system where the pigs are raised on a bedded manure pack. This bedded pack begins a natural composting process inside the building, providing supplemental heat for the pigs, and this composting process can continue after hoop cleanout if the manure is stacked before application. Composting is known to release ammonia and greenhouse gases, but little data is available on the extent of those releases from bedded swine production systems. One of the challenges to collecting such data is the lack of controlled inlets and outlets in naturally ventilated buildings. Because emission rates are typically calculated by multiplying gas concentration by air flowrate, without a reliable flowrate measurement this traditional approach cannot be used.
An alternative measurement strategy used known inputs of an inert tracer gas to provide a benchmark to related downstream concentration measurements back to the original source. This system is considered state-of-the-art for measuring gas emissions from naturally ventilated structures, but had not been applied to swine systems in the US prior to this study. Although in principle this system should provide a reproducible measurement of emissions, it assumes complete mixing of the tracer and emitted gases in the structure and no separation of plumes downwind.
In this study, detailed profiles of downwind plumes demonstrated that these assumptions do not always hold. Because of inadequate mixing and plume separation, gas emission estimates varied with sampling location as well as time. Large numbers of samples and statistical averaging techniques provide better estimates of actual emissions, but additional development work is needed for this method to be practical and widely applied. However, the extensive data provided by this study did indicate several important results.
Methane emissions from hoop structures are lower than have been reported for other livestock housing systems, ammonia emissions are comparable, and nitrous oxide emissions are higher. Although methane and nitrous oxide emission rates were comparable, because nitrous oxide is 15 times as potent a greenhouse gas as methane (and has three hundred times the potency of carbon dioxide), nitrous oxide was the most environmentally significant air emission observed.
While there was no statistically significant relationship between gas emissions and bedded pack characteristics, some evidence suggests these emissions do fluctuate in response to bedding management and particularly the introduction of large round bales. Additional research is needed to develop bedding management strategies that reduce gas emissions and minimize their environmental impact.
