Every atom of carbon emitted by grazing animals was taken from the pasture that animal consumed. Which was taken from the atmosphere. Therefore all grazing animals are carbon neutral.
This leaflet by Jim Hawes explains it simply:
[Click on each image for a larger version.]
For a print-ready PDF document click here: http://carbon-sense.com/wp-content/uploads/2015/08/methane-cow.pdf [1MB]