the objective of this study was to estimate genetic parameters coupled with genetic and phonotypic trends for age at first calving (afc), open days(od) and milk yield(my) of first lactation in iranian holstein cows. records of reproduction and production for genetic evaluation from 1984 to 2005 for afc, od and my were 71736, 20126 and 20126, respectively. single and two-variable animal models w...