The whole cell is accurate, but quite expensive and awkward. The ground device is also expensive but not as much and only is cost effective if part of a carbon credit project and the satellites are good enough, but most of their focus is on fossil fuel plumes and other than a proof of concept, they don't seem to monitor cattle on a regular basis, even if they could.

They have a small level of aversion which can be overcome. The researchers at UC Davis used it in a molasses lick. I'm actually working on similar technology at work also in a molasses lick. https://www.ucdavis.edu/climate/news/can-seaweed-cut-methane-emissions-on-dairy-farms

There are three ways- a satellite company out of Canada has a GPS style grid of small satellites that has been able to detect methane plumes from space. The other is a mechanical device that uses the RFID embedded in the cow's tag to identify the cow and then measure the methane output. Lastly, you put the cow in a neatly sealed cell and monitor all gases going in and out of the cell.

The red seaweed used for this is very difficult to grow. I think there has been an Australian startup on the books since the early 2010s trying to commercialize this. Nobody can figure out how to make it at scale and to get the cows to eat it, since it is salty and they seem to not eat as much on a mixed ration.