In the 1940s and 1950s, Dutch scientists became increasingly critical of the practices of commercial dairy cattle breeders. Milk yields had hardly increased for decades, and the scientists believed this to be due to the fact that breeders still judged the hereditary potential of their animals on the basis of outward characteristics. An objective verdict on the qualities of breeding stock could only be obtained by progeny testing, the scientists contended: the best animals were those that produced the most productive offspring. Some scientists had been making this claim since the beginning of the twentieth century. Why was it that their advice was apparently not heeded by breeders for so long? And what were the methods and beliefs that guided their practices? In this paper I intend to answer these questions by analysing the practical realities of dairy farming and stock breeding in The Netherlands between 1900 and 1950. Breeders continued to employ traditional breeding methods that had proven their effectiveness since the late eighteenth century. Their methods consisted in inbreeding--breeding in 'bloodlines,' as they called it--and selection on the basis of pedigree, conformation and milk recording data. Their aims were 'purity' and 'uniformity' of type. Progeny testing was not practiced due to practical difficulties. Before World War II, scientists acknowledged that genetic theory was of little practical use to breeders of livestock. Still, hereditary theory was considered to be helpful to assess the value of the breeders' methods. For instance, striving for purity was deemed to be consistent with Mendelian theory. Yet the term purity had different connotations for scientists and practical workers. For the former, it referred to homozygosity; for the latter, it rather buttressed the constancy of a distinct commercial 'brand.' Until the 1940s, practical breeders and most scientists were agreed that selecting animals purely for production was ill-advised. Cows of the extreme dairy type were believed to be prone to bovine tuberculosis. This conviction was at the basis of the development of 'the modern Friesian,' a rather robust type of dairy cow that was also valued for its aesthetically pleasing conformation and that became a commercial success. Contrary to the scientists' claims, it was not only for commercial reasons that breeders were reluctant to give up their modern Friesians after World War II, when the introduction of artificial insemination opened up the possibility of breeding more productive types by means of progeny testing. The political economy of breeding did indeed require breeders to protect their breed as a recognisable brand. Yet the moral economy of breeding must also be taken into account: the modern Friesian was also a product of widely shared normative standards of good and responsible farming.