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Carbon dioxide balances are useful in determining ventilation rates in livestock buildings. These balances need an accurate estimation of the CO2 produced by animals and their litter to determine the ventilation flows. To estimate the daily variation in ventilation flow, it is necessary to precisely know the daily variation pattern of CO2 production, which mainly depends on animal activity. The objective of this study was to explore the applicability of CO2 balances for determining ventilation flows in broiler buildings. More specifically, this work aimed to quantify the amount of CO2 produced by the litter, as well as the amount of CO2 produced by the broilers, as a function of productive parameters, and to analyze the influence of broiler activity on CO2 emissions. Gas concentrations and ventilation flows were simultaneously measured in 3 trials, with 1 under experimental conditions and the other 2 in a commercial broiler farm. In the experimental assay, broiler activity was also determined. At the end of the experimental trial, on the day after the removal of the broilers, the litter accounted for 20% of the total CO2 produced, and the broilers produced 3.71 L/h of CO2 per kg of metabolic weight. On the commercial farm, CO2 production was the same for the 2 cycles (2.60 L/h per kg of metabolic weight, P > 0.05). However, substantial differences were found between CO2 and broiler activity patterns after changes in light status. A regression model was used to explain these differences (R² = 0.52). Carbon dioxide increased with bird activity, being on average 3.02 L/h per kg of metabolic weight for inactive birds and 4.73 L/h per kg of metabolic weight when bird activity was highest. Overall, CO2 balances are robust tools for determining the daily average ventilation flows in broiler farms. These balances could also be applied at more frequent intervals, but in this case, particular care is necessary after light status changes because of discrepencies between animal activity and CO2 production. [ABSTRACT FROM AUTHOR]