Relationship between daily milk yield and persistency of Hungarian Holstein Friesian through selection index and sub-indices

Multiple Trait Derivative Free Restricted Maximum Likelihood procedures were used to analyse the 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 sub-indices and reduced selection sub-indices. The structure of the data consisted of 281 half-sib families and the lactation records in each group ranged between 12-24 records. The expected genetic gain in daily milk yield by selection sub-indices was similar to the corresponding value calculated by selection index, so the breeder can use a selection sub-index in the breeding program to improve daily milk yield. The contribution of persistency to improvement in daily milk yield was higher by sub-index than selection index application. 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 a selection index which involved all these traits.