Abstract: The invention provides a biomarker, Blastocystis sp., the presence of which, in a fecal sample of a dairy cow at the heifer stage, is indicative of a future high productive longevity, of a richer and more diverse gastrointestinal microbiota, as well as an improved carbon footprint compared to a Blastocystis-negative dairy cow. The invention also concerns methods, including a qPCR method, and kits for detecting Blastocystis sp. in fecal samples obtained from heifers, for predicting productive longevity in heifers, and for improving dairy herd management.