Methane and nitrous oxide analyzer comparison and emissions from dairy freestall barns with manure flushing and scraping

Continuous methane (CH₄) and nitrous oxide (N₂O) emission measurements were conducted at two crossflow-ventilated dairy freestall barns located in the state of Wisconsin, USA during a 19-month period from 2008 to 2010. The two cross-flow mechanically ventilated buildings (275 and 375 cow capacities) were evaluated in the National Air Emissions Monitoring Study. In September of 2008, the barns' manure collection systems were changed from flushing open gutter using manure basin effluent to a tractor scrape. A photoacoustic multi-gas analyzer (PAMGA) and a direct methane/non-methane hydrocarbon analyzer (GC-FID) provided side-by-side measurements of methane (CH₄) for 13 months. The PAMGA also measured nitrous oxide (N₂O), and a side-by-side comparison was performed with a gas-filter correlation analyzer (GFC) for six months. Barn ventilation rates were measured by recording run times of the 127-cm diameter exhaust fans. All 125 belt-driven exhaust fans were identical, and in situ airflow measurements using the Fan Assessment Numeration System (FANS) were conducted once at the beginning and twice during the test. Daily CH₄ and N₂O emission rates were calculated over approximately 19 and 6 month periods respectively, on per barn, head, animal unit, floor area space and barn capacity bases. The differences between the analyzers' concentration measurements were compared in conjunction with water vapor and other gases. The analyzer type had a significant impact on the average CH₄ emission rate (p<0.001) and the average N₂O emission rate (p<0.05). Based on the CH₄ measurements with the GC-FID, average daily mean CH₄ emissions were approximately 290gAU^-1d^-1 (390gcow^-1d^-1) with very limited seasonal effects. Little variation was observed in CH₄ emission rates before and after the change in manure collection method, suggesting that most of the CH₄ emissions were enteric losses directly from the cows. The average daily mean N₂O emission rates based on the GFC were very low, with an approximate rate of only 690mgAU^-1d^-1 (970mgcow^-1d^-1). The change in manure collection had no apparent effect on N₂O emission.