Evidence of the genetic trend for secondary sex ratio in Iranian Holsteins.

Calving records from the Animal Breeding Center of Iran collected from January 2000 to December 2007 and comprising 520964 Holstein calving events from 2135 herds were analysed using animal and sire models to estimate variance components, heritabilities and genetic trends for secondary sex ratio (SSR) in the first, second and third parities. Direct heritabilities for SSR ranged from 0.00004 to 0.001 over the parities. The linear sire model estimate of heritabilities for SSR in the present study was from 0.0002 to 0.0008 over the parities. There were positive phenotypic and genetic trends for female rate in Iranian Holsteins over the years. Exploitable genetic variation in SSR can take advantage of sexual dimorphism for economically important traits, which may facilitate greater selection intensity and thus greater response to selection, as well as reducing the replacement costs. On the other hand, increasing genetic and phenotypic trends for female rate in the population under study could be partly assigned to increased use of new reproductive technologies in dairy herds; therefore, dairy farmers have the option to select from among their herd's potential dams and produce dairy replacement heifers from only the genetically superior animals, thus promoting enhanced rates of genetic gain.