Forget catalytic converters for cows. Feeding ruminants is a core business of Dutch giant DSM. Its subsiary Rumistar focuses on enzyme animal feed additives, and has developed one which efficiently inhibits the enzyme methyl coenzyme M reductase (MCR), which is responsible for the prodigeous amounts of methane released by cattle and other ruminants.
It appears highly effective. Trials in 13 countries indicate a gram a day can cut bovine methane emission by up to 55%.