Two solutions in one - steaks! /j

A serious answer: ultra-fancy cow-fart-diaper that allows solid matter to pass through a tube, but pulls a light vacuum with a fan whenever flatulence or waste is passing through the device to remove the methane to a very small holding tank. Pressurize holding tank, use sensors to indicate when the tank is full. Swap tanks automatically (2 on the cow diaper at all times), drop the tank, send radio beacon for a small maintenance robot to pick up the deposited container and deliver a new backup container. Repeat indefinitely, have the robot swarm deliver deposited tanks to a methane refinery on the farm to be used for heating and power a generator to charge the robots etc. Use idk one robot to 10 cow-fart-diapers (would take some research to find a good scale), scale up with the herd.