1. This work has been funded by BATFARM Interreg-Atlantic Area Project (2009-1/071): Evaluation of best available techniques to decrease air and water pollution in animal farms. September 0 50 100 150 200 0 4 8 12 16 20 24 Time, h % Ventilation rate CH4 concentration CH4 Emission December 0 50 100 150 200 0 4 8 12 16 20 24 Time, h % Ventilation rate CH4 concentration CH4 Emission RESULTS 52,000 hens were housed in a commercial laying hen unit, Gamiz-Fika (Basque Country). CH4 concentration was measured continuously by a photoacoustic infrared gas analyzer (INNOVA 1412). Ventilation rate was 5-minutes interval recorded. Each fan was individually calibrated using a hot wire anemometer (Testo 425). CONCLUSION CH4 emission is affected by hourly pattern. CH4 emission increased throughout the light hours. There was not seasonality in CH4 emission. INTRODUCTION MATERIAL & METHODS Scarce literature on methane (CH4) emission from laying hen houses. Objective: to study daily pattern of CH4 emissions in a laying hen facility adapted to the welfare Directive (1999/74/EC) in two periods: cool (December) and mild (September). Figure 2. Average hourly pattern (calculated as % of mean value for each day) for each month. Average CH4 emission (4.0 kg CH4 day-1) did not differ in both periods (P>0.05). Ventilation rate was proportional to outdoor temperature (R2= 0.8 P