Genetic relationship between heifers and cows fertility and milk yield traits in first-parity Iranian Holstein dairy cows

Relationship between reproductive characteristics of heifers and cows and milk production traits in Iranian primiparous Holstein dairy cows were investigated under a Bayesian approach using data collected from 1995 to 2014. The studied reproductive traits in heifers were success or failure at first insemination (SFH), number of inseminations for conception (INSH), and days to calving (DC). Considered reproductive traits in cows were success or failure at first insemination (SFC), number of insemination for conception (INSC) and days open (DO). Milk production traits were included 305-day adjusted milk yield (MY), fat yield (FY) and protein yield (PY). Linear models were used for the analysis of INSH, DC, INSC and DO, while threshold models were used for SFH and SFC. Direct heritability estimates were0.012, 0.029, 0.049, 0.006, 0.010, 0.039, 0.252, 0.167 and 0.214 for INSH, INSC, OD, DC, SFH, SFC, MY, FY and PY, respectively. Genetic correlation estimates between reproductive traits were high and negative for SFH and SFC with INSH, INSC, OD and DC; varying between −0.991 for INSC-SFC and −0.627 for OD-SFH. Genetic correlation estimates between INSC, INSH, OD and DC were high and positive, varying from 0.638 for INSH-OD to 0.932 for INSC-DC. A positive estimate of 0.613 was obtained for the genetic correlation between SFC and SFH. Genetic and phenotypic correlations between milk production traits were positive and high. Unfavorable genetic correlations were obtained between MY, FY and PY with reproductive traits, varying from −0.617 for SFC-PY to 0.593 for OD-MY. High genetic correlations were found for SFH-SFC (0.613) and INSH-INSC (0.862). The reproductive traits considered in heifers (except for DC) had medium to high genetic correlations with milk production and cow's reproduction traits. Furthermore, reproductive traits of heifers can be measured relatively early in the life. Therefore, it can be concluded that including reproductive traits of heifers in genetic evaluation programs can be useful of Iranian Holsteins.