Cows: Gather their emissions and use them as biofuel?

I don’t know how long cows in mega farms spend in a barn, but even if it’s only part of the day, perhaps farmers could put in a negative airflow so that the air pressure would automatically send the emissions to a holding tank outside the building? And maybe there are converters that would grab the emissions out of the air as they pass through and would store the methane and let the rest of the gases pass through.