Greenhouse gas emissions from naturally ventilated freestall dairy barns

Greenhouse gas (GHG) emissions from two naturally-ventilated dairy freestall barns measured for a total of 21d, one week each in May, July, and September 2009, are presented in this article. The holding capacity of Barn 1 (B1) was 400 Holstein cows, while that for Barn 2 (B2) was 850 cows. Air samples were taken from inlets and outlets of the barns via a custom made multiplexer gas sampling system for measurement of gas concentrations using a photoacoustic infrared multigas analyzer. Barn ventilation rates were based on air velocity measured with arrays of 3-D ultrasonic anemometers at inlets and outlets. Gas concentrations (10min means) in the barns ranged from: 443-789ppm for CO₂, 0.0-39.4ppm for CH₄, and 0.25-0.39ppm for N₂O; with mean concentrations ranging from 6 to 20%, 0 to 4%, and 26 to 180% above the average background concentrations for CO₂, N₂O, and CH₄, respectively. The correlations between CO₂ and CH₄ enhanced concentrations were relatively stronger (R of 0.67-0.74) than between CO₂ and N₂O enhanced concentrations (R of 0.10-0.20). Environmental conditions did not significantly (p=0.46) impact the enhanced concentrations of N₂O in the barns. All three parameters (T, RH, and v) had significant (p<0.01) influences on CO₂ enhanced concentrations; while only T (p<0.01) and v (p<0.01) had significant influences on CH₄ enhanced concentrations. Enhanced concentrations of CO₂ and CH₄ correlated negatively with all three parameters. The influence of the temperature-humidity index (THI) on CO₂ enhanced concentrations was higher than that of v; while the effect v had on CH₄ enhanced concentrations was slightly higher than that of the temperature-humidity index. The average emissions, based on hourly means, ranged from 5.3 to 10.7kgd^-1AU^-1 for CO₂; 0.3 to 2.5gd^-1AU^-1 for N₂O; and between 67 and 252gd^-1AU^-1 for CH₄. Nitrous oxide emissions from the smaller barn, B1 (0.4-2.5gd^-1AU^-1), were significantly higher than from the larger barn, B2 (0.3-0.5gd^-1AU^-1) most probably because 50% of B1 was open (no stalls) loose dirt floor.