Effect of age at first calving on calving interval and dry period length of dairy cows in semiarid climate of Iran

This study was carried out to investigate the effect of age at first calving (AFC) on calving interval (CI) and dry period length (DP) of dairy cows in semiarid climate of Iran. Records of 57870 first lactation dairy cows were collected from 601 herds during 1995 to 2009 by the Animal Breeding Center of Iran. The analysis was performed using mixed models procedure. Variance components were estimated via restricted maximum likelihood method in a univariate animal model using DMU software. The data were divided in two groups as AFC<=26 mo (group 1) and AFC>26 mo (group 2). The average AFC for all data, group 1 and group 2 were 26 (±2.5), 24 (±1) and 28 (±2.5) mo, respectively. The average CI and DP were 375 (±47), 67 (±20) day for group 1 and 380 (±50), 69.5 (±22) day for group 2, respectively. The estimates of heritability of CI and DP were 0.037 (±0.008), 0.022 (±0.006) for group 1 and 0.026 (±0.009), 0.00 for group 2, respectively. Phenotypic trend of CI and DP was negative and significant for two groups (P<0.05). The difference between average estimated breeding values of CI and DP for group 1 were lower than group 2 and significant (P<0.01). The results of this study showed that cows calving at a young age in semiarid climate of Iran had lower CI and DP during their first lactation.