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SUMMARY Reproductive records of Holstein cows raised at a commercial farm during the period from 1991 to 2006 were used. A total of 3656 records of 1611 cow daughters of 413 sires and 588 dams were used to study genetic and phenotypic trends of age at first calving (AFC, mo) and calving interval (CI, d). Two animal models were applied to estimate the genetic parameters and breeding values of the two traits using VCE program. Genetic trend was estimated for the studied traits. Regression analyses by XLSTAT- Pro (2009) procedures were used to estimate genetic and phenotypic trends. Means of CI and AFC were 484 d and 27.5 mo, respectively. Heritability estimates for CI and AFC were 0.07 ± 0.01and 0.12 ± 0.04, respectively, while the corresponding estimate for repeatability of CI was 0.08. Genetic trends were 0.06 d/yr and 0.02 mo/yr for CI and AFC, respectively. Phenotypic trends were -0.48 d /yr for CI -0.04 mo/yr for AFC.

GENETIC AND PHENOTYPIC TRENDS OF CALVING INTERVAL AND AGE AT FIRST CALVING IN A COMMERCIAL HOLSTEIN HERD