the solution is to flare- as they do in the oil, gas and landfill industries.
when the fart emerges a sensor should spark a spark and 'whoosh', the methane is combusted and changed into the less damaging ghg carbon dioxide.
and another cow runs around the field with its face on fire.
throw some bbq sauce on after 2 hours and tuck in.
I can see no holes in my plan if a back flash arrestor is inserted in each cows rectum.