Abstract
Maximum likelihood procedures were used for the analysis of relationships between persistency and daily milk yield performance. Genetic and phenotypic (co)variances were generated from this analysis to construct full and reduced selection indices, selection subindices and reduced selection subindices. The structure of the data were half-sib families of 281 sire groups, lactation records in each group ranged between 12-24 records. Expected genetic gain in daily milk yield by selection subindices was similar to the corresponding value calculated by selection index, so the breeder can use selection subindices in the breeding program to improve the daily milk yield. Contribution of persistency to the improvement in daily milk yield was higher by the subindex than the selection index. The highest improvement of persistency was associated with the maximum improvement in daily milk yield per lactation and per calving interval and was achieved by the selection index which involved all these traits.