Female fertility affects the culling rate as well as the direct reproduction costs and influences the calving interval and the calving season. It is therefore obvious that female fertility should be included in a Total Merit Index (TMI), which directs the genetic changes in dairy cattle selection. When the first TMI was introduced in Sweden in 1975 female fertility was included. A basic prerequisite for inclusion of fertility in the Scandinavian breeding programme was that milk recording, pedigree and AI-data were integrated in the same databases.
The paper deals exclusively with the additive genetic effects on female fertility. The heritabilities of the measures used for female fertility are low, usually below 0.05. Despite this, the additive genetic variance is high as demonstrated by large differences between progeny groups. Thus, selection must be based on progeny testing of bulls on daughter groups of 100-150 daughters or more, which is the case in all the Scandinavian countries. Different procedures for calculation of daughter fertility indices are practised in Scandinavia.
Research shows the importance of including measures of both the time interval from calving to insemination and a direct fertility trait such as number of inseminations or non-return rate (NR). It has also been shown that reproductive measures of different stages of life, i.e. heifer and lactating cow periods, must be considered. The genetic correlations between daughter fertility and different yield measures are negative (-0.2 to -0.4). Selection for yield without consideration of daughter fertility results in a deterioration of daughter fertility. A comparison of the flat genetic and phenotypic trends for daughter fertility along with strongly increased productivity in the SRB-breed, where Scandinavian bulls have dominated as sires of sons, and the negative trends in the SLB-breed where almost all sires of sons during a decade and a half have been imported from North America, illustrates the consequences of selection with and without consideration of daughter fertility in the sire-son path.
Daughter fertility will probably remain an important trait in the future, not only in countries with seasonal milk production connected to the pasture period. As dairy cattle breeding is quite international, calculation of international breeding values for daughter fertility in the same way as is practised by INTERBULL for production and conformation seems to be an important challenge for the future.