Genetic parameters and genetic trends of female fertility in Icelandic dairy cattle

Fertility, represented by calving interval, has been included in the genetic evaluation of Icelandic cattle since 1993. In spite of this an unfavourable genetic trend is seen and, recent implementation of test-day models for the genetic evaluation of milk production has changed the premise of using calving interval to represent fertility. The aim of this study was to estimate genetic parameters and genetic trends of different female fertility traits in Icelandic dairy cattle and suggest new traits for the genetic evaluation. Insemination records for the first three lactations and the heifer period of 52,951 Icelandic cows were used to analyse the traits: conception rate at first insemination (CR), number of inseminations per service period (AIS), interval first to last insemination (IFL), interval calving to first insemination (ICF), interval calving to last insemination (ICL) and calving interval (CI). Correlations between fertility and production traits were also estimated. Five different linear animal models were used to estimate (co)variance components. Breeding values were compared, and genetic trends of fertility traits were investigated. Estimated heritabilities for the fertility traits were low, ranging from 0.01 (IFL) to 0.08 (ICF). Genetic correlations between heifer and cow performances within traits ranged from 0.23 to 0.81. Between lactations within traits, the genetic correlations ranged from 0.36 to 1.00. Genetic correlations between different heifer traits were strong, and between different cow fertility traits they ranged from weak (-0.17) to very strong (0.97). Genetic correlations between the fertility and production traits were generally unfavourable, but more pronouncedly so for AIS, IFL, ICL and CI than for ICF and CR. Estimated genetic trends seemed unfavourable for the traits AIS, IFL and CI and favourable for ICF and ICL. In a revised genetic evaluation for fertility in Icelandic cattle, the traits ICF and IFL should be included to represent cow fertility, and CR to represent heifer fertility.