Reproductive traits are one of the important traits in dairy cattle which affecting profitability. This study was carried out to estimate the components of (co) variance and heritability of reproductive traits using linear models in Holstein dairy cows under heat stress conditions in Isfahan province. The traits were calving to first insemination and days open. Records from 18,128 cows (from 1996 to 2012) with at least 4 parity information in 34 herds were used based on their distance (less than 40 km) from the provincial meteorological stations. After correcting for the fixed effects, genetic parameters for calving to first insemination and days open were estimated using restricted maximum likelihood model (AIREML). Genetic and phenotypic trends were considered by regression of the estimated breeding values on year of the birth. Results showed that herd-year-season, parity, milk production level and age at first calving had no significant effect (p 0.01) on the calving to first insemination. However, herd-year-season, parity, age at first calving and milk production has a significant effect (p 0.01) on the days open. Heritability of calving to first insemination and open days were 0.063 ± 0.006 and 0.023±0.005, respectively. The averaged days open and calving to first insemination were 112.58 ± 37.42 d and 60.66±18.15 d, respectively. The phenotypes of days open and calving to the first insemination on heat index (HI) for the months under heat stress were considered. The estimates of genetic and phenotypic correlations for calving to first insemination and days open were 0.2 and 0.07, respectively. Phenotypic trends based on the birth year for calving to first insemination and open days were -0.226 and -0.042, respectively. Although, genetic trends for calving to first insemination based on birth year was high and undesirable (0.039) and for open days was close to -0.042 and desirable. Keywords : Genetic and phenotypic trends, heat stress, calving to first insemination and days open
