Measuring and assessing the role of deep litter to estimate the ventilation rate using the CO2 mass balance method

Carbon dioxide (CO2) produced from manure is a necessary input when using CO2 as a naturally produced tracer gas to measure ventilation rate in naturally ventilated livestock buildings. This work compares different chamber calculation methods for measuring CO2 production from solid manure and evaluates the variability, affecting factors and potential contribution of CO2 emissions from manure to total CO2 production in the building. A total of 925 static chamber measurements were used to this aim, conducted in five dairy cattle and three goat houses. Linearity (R2) and curvature (convex or concave) were the main factors explaining differences among models. CO2 emission from manure was on average 20.86 g m-2 h-1, but it was very variable in spatial terms within the same measurement day (coefficient of variation = 48%), among different measurement days in the same farm (coefficient of variation = 30%) and among farms of the same animal type (coefficient of variation = 66% and 48% for dairy and goat farms, respectively). Manure height and temperature were directly correlated with manure CO2 emission (r = +0.36 and + 0.38, respectively). For goats, a prediction equation of CO2 emission was obtained using these two variables (R2 = 0.74). Solid manure had a relevant contribution to the total farm production and needs to be quantified in each case. Models to predict CO2 manure are not available at the moment and therefore, measuring manure contribution using chambers seems the best option according to the current knowledge