Abstract The three dairy breeds that dominated Dutch dairying in the twentieth century ‐ Black and White, Red and White, and Groningen Whitehead ‐ were first described by agriculturalist Iman van den Bosch on the basis of a quantitative analysis of their bodily dimensions. However, Van den Bosch did not analyse his data statistically, and a reanalysis with modern statistical tools shows that the data do not warrant a division into three distinct breeds. Historically, this is just a side note, yet it is important in light of recent conservation efforts. After the ‘Holsteinisation’ of the Dutch dairy herd in the 1970s, the three breeds declined and were ultimately listed as rare breeds. Currently they are taken to be authentic relics of centuries-old Dutch animal husbandry practices. We show, however, that the three breeds were a product of Van den Bosch and his successors’ efforts to modernize Dutch dairying. Before 1900 breed purity had never been an issue, and crossing of various types had been common practice. We discuss the implications of these findings for modern conservation practices and conclude that breed purity should not be sacrosanct.