The ‘tsuru’ tradition involves an emphasis on female selection and is a unique Japanese ‘take’ on in-breeding. Following is an edited version of a web-published explanation authored by Dr. Kiyoshi Namikawa, an Executive Director of ZENWA, the Japanese Wagyu association.
Tsuru is a popular name for inbred strains of native cattle. It describes a group of related cattle within a strain, representing superior and common external and productive traits for that strain based on genetic make-up. EG: One tsuru group originated from one excellent cow which produced 19 calves over 23 years. Two daughters inherited superior dam characteristics, and they formed two sub-strains. A son was backcrossed to his dam to fix desired traits. Two bulls were selected among offspring produced by son and mother mating. Cows of this strain were sired with one of the bulls reciprocally in the successive generations. Most female progeny were held nearby to enable observation of performance.
Dr Namikawa emphasises that tsuru strains were founded on maternal lines, “because reproductive and growing performance records were observed only for females in some closed place from their farm.” For the original text see Namikawa, pp 3-4.
In contrast, modern Japanese joining strategies in many non-traditional breeding areas are similar to contemporary Western assortative mating selection, frequently joining superior individuals from different sub-populations. A cursory glance over the pedigrees of the Top 10 Japanese sires for any recent year (see examples on this site), also reveals the modern popularity of this strategy at the national level. Notwithstanding, it is also certain that breeders would have looked first to the prefectural origins of all genetics, and as noted above, included this pedigree information in catalogue joining recommendations, such as “good for Fujiyoshi female”, etc.