At the crossroad between the history of agricultural ‘modernization’ and of the construction of markets and qualities, the sociology of measurement and the history of science and technology, this article explores the making of seeds and cultivars as objects of public policy, and the genesis of a government of genetic progress’ in postwar France. We analyze the crafting of reference marks, equivalences and gradings, agreed upon to stabilize exchanges and to ensure the ‘governability’ of genetic flows, from the laboratory to the fields. We show how norms of proof, trial protocols and modes of codification and assessment of the varieties, embodied and promoted the productivist model of the post-WW2 French agricultural modernization.