Genetic evaluation of first calving Iranian Holstein dairy cattle at different test days.

Using test day records of first lactation of 38547 Holstein dairy cows in 163 herds, the genetic parameters of the population estimated. The analyze conducted with a random regression Test Day model with fixed effects of herd- year- season and age at calving as covariates. Direct genetic and permanent environmental effects were included in the models as random effects. The Random direct genetic and permanent environmental effects were fitted on milk production curve using Leagendre orthogonal polynomial with power four in random regression model. The highest and the lowest heritability estimates were at first (0.07±0.001) and Ninth (0.24±0.01) test days, respectively. The Genetic correlations of the adjacent records were high and its value was low with increasing distance between the months of recording. Genetic correlations between the first and the tenth month of lactation was determined at least. Maximum additive genetic correlations between adjacent months and its value decreased with increasing distance.Minimal phenotypic correlation between the first and the tenth month of lactation (0.23) were estimated. Maximum additive phenotypic correlations between adjacent months of lactation and it was (0.7) for ninth and tenth months.