genetic analysis of milk yield and open days traits of holstein dairy cattle in iran for first five lactation

a data file containing 607662 (317608) milk (open days) records belonging 294417 (177838) holstein cows that were calved from 1983 to 2008 was used. the bivariate repeatability model was used for data analyzing so that herd, calving year, calving month, days in milk (only for milk trait) and calving age for each parity, were assumed as fixed effects and additive genetics and permanent environmental effects were considered as random. phenotypic (genotypic) trends were estimate based on mean of phenotypic values (breeding values) on birth year by simple (segmented) regression. heritability and repeatability for milk yield (open days) were obtained 0.20 (0.054) and 0.42 (0.120), respectively. phenotypic trend of milk yield was 116.65 kg per year (p0.05). genetic trend of milk yield before 1996 was estimated -4.15 per year (p>0.05), but after 1996 milk yield has uptrend and was observed 43.74 kg per year (p