Effect of age at first calving on milk production traits of dairy cows in semiarid climate of Iran

This study was carried out to investigate the effect of age at first calving (AFC) on 305-d milk production traits (milk, fat and protein yield,)of Holstein cattle in semiarid climate of Iran. Records from 58872 first lactation dairy animals collected by the Animal Breeding Center of Iran between 1996 to 2011were used. The analysis was performed using mixed procedure in SAS 9.2. Variance components and breeding values were estimated via restricted maximum likelihood (REML) method in a univariate animal model using DMU package. Based on AFC the data were divided in two groups as AFC26 mo (group 2). The average AFC for all data, group 1 and group 2 were 26.22 (±2.64), 24.5 (±1) and 28.5 (±2.5) mo, respectively. The least squares mean for milk, fat and protein yield were 6616.54 (±22.5), 208.28 (±0.79) and 207 (±1.3) kg for group 1 and 6679.97 (±21.9), 211.5 (±0.77) and 208 (±1.3) kg for group 2, respectively (P