Development of genetic evaluation for milk production traits of Holsteins in Japan.

The procedure used for the genetic evaluation of dairy cattle in Japan has developed from a lactation sire-MGS model to a multiple-lactation random regression test-day animal model. Genetic evaluation of Holstein bulls in Japan began in 1989 with the use of field-style progeny testing; dairy herd improvement program data from all over Japan were used, along with a sire and maternal grandsire model. In 1993, an animal model was introduced to estimate breeding values for yield and type traits. A random regression test-day model was first applied in 2010. In the business of breeding dairy cattle, it is very important to users that estimated breeding values are reliable and stable among subsequent routine evaluations. With experience in the genetic evaluation of dairy cattle in Japan, Japanese researchers have found ways to improve the stability of estimated breeding values. These modifications involve changes in data editing, development of evaluation models, changes to the structures of unknown-parent groups, awareness of the problems of predicting lactation yield from partial test-day records, and adjustment for heterogeneity within herd variances. Here, I introduce developments in, and our experiences with, the genetic evaluation of yield traits of Holstein cattle in Japan.