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The objective of this study was to quantify the resistance to airflow of baled hay as a function of air velocity, hay density, hay moisture and stack height. The experiment was carried out with small rectangular hay bales (0.35 m by 0.45 m by 0.81 m) composed of grass and compressed with a stationary baler to obtain seven levels of density in the range of 98 to 240 kg dry matter/m. The stack height was successively 1, 2, 3 or 4 bales (0.45, 0.90, 1.35 and 1.80 m high) of same density. Bales were initially at about 18% moisture, and partially dried to 14 and 10% to measure airflow resistance at three moisture levels. Air velocity was varied at 8 levels (overall range between 0.08 and 1.06 m/s). Air was pulled through the bales by a suction fan and pressure drop was measured by a manometer between the bottom and the top of the hay stack. Pressure drop increased linearly with stack height and exponentially with air velocity and hay density. Among four models used to fit the experimental data, the ASAE equation for airflow resistance and a power equation provided the best fit to predict pressure drop per unit height of hay (P/H) The pressure drop per unit height was proportional to the hay density squared and to the air velocity raised to the power 1.84. No significant effect on pressure drop gradient was observed for moisture content in the range tested (10 to 18%). These data will be useful to design appropriate ventilation and drying systems for high density bales and biomass.