Genetic and phenotypic trends for dry days in the first three lactations of Iranian Holsteins

This study was undertaken to estimate the genetic parameters and genetic and phenotypic trends of the dry days in the first three lactations of Holstein cows in Iran during 1985 to 2006. Calving records from 1114 dairy herds comprising 264297 calving events of Holstein cows were analyzed using univariate and bivariate linear animal models. Genetic trends were estimated by regressing yearly mean estimates of breeding values on calving year. Average dry days were from 73.3 to 82.7 days and increased over the parities. Heritability estimates for dry days varied from 0.02 to 0.09 and increased over the parities. Estimates of genetic correlations between dry days in different parities were medium and ranged from 0.26 to 0.34. Also, estimates of phenotypic correlations between dry days in different parities were low and varied from 0.07 to 0.10. The average annual phenotypic trends were estimated from fitting linear regression of annual mean dry days were negative for the first three parities of Iranian Holsteins (P<0.01). On the other hand, there were significant and decreasing genetic trends for dry days at parities 1, 2 and 3 over the years (P<0.01). The low estimates of heritability for dry days obtained in the current study could be assigned to the high phenotypic variance arising from large environmental variation. Therefore, this implies that improvement in dry days could be achieved by improving the non-genetic factors such as management and production environment.