Grass-based milk production is a major contributor to Irish agricultural output. The study characterized the Irish milk pool using stable isotope ratio analysis of carbon, nitrogen, oxygen and sulphur. Authentic raw milk samples were collected from 50 farms on five occasions over 13 months. Mean values of −27.11, 6.79, −3.27 and 6.16‰ were obtained for δ13C, δ15N, δ18O and δ34S, respectively. δ13C values reflected a high level of grass input and values increased with increasing cereal ...