Using slurry pit headspace gas concentrations to quantify pit ventilation rate in a dairy house. Comparison between a closed and slatted floor

Ammonia mitigation systems in dairy houses with a slurry pit below, often close off the pit to limit air circulation between the pit and the house. Currently, the effect of limiting air recirculation on emission is unknown. Experiments were carried out to understand the effect of closing off the pit on NH₃ emissions. This study aimed to (1) develop a method to quantify continuous pit air flow rate, (2) provide information about gas concentrations inside a slurry pit and (3) determine the effect of closing off the floor on air exchange rate between pit and house. Mass balances for CO₂ and CH₄ were set up to quantify air exchange rate between pit and house based on natural slurry production. The applicability of this approach was tested for both a conventional slatted floor as well as for an NH₃ mitigating closed floor. The ventilation rate estimates of the mass balance were compared to the ventilation rate as determined with a rate-of-decay (ROD) experiment. Quantification of pit ventilation rate based on the CO₂ and CH₄ mass balances is most appropriate to discover relative differences. Pit ventilation rate turned out to be temperature dependent and therefore showes a clear day–night pattern with ventilation rates during the night. The ROD method provided a more accurate quantification of the ventilation rate and showed replacement rates during daytime of 4.3–7.3 h⁻¹.