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 (P<0. 05) but open days wasn’t followed any trend (P>0. 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<0. 05). Genetic trend of open days before 1996 wasn’t significant، but after 1996 genetic trend of this trait was estimated 0. 36 day per year (P<0. 05). Phonotypic correlation between these two traits was low but positive (0. 041) and genetic correlation estimated 0. 46 that shows more open days for high productive cows. Genetic correlation compared to the phenotypic correlation explains that the strong relationship between these traits was covered by environmental effects. According to the results of this study، it is suggested to pay more attention to the reproductive traits in addition to productive traits in breeding programs.