impact of birth weight of iranian holstein calves on their future milk production and reproductive traits

the objective of this study was to investigate the effect of birth weight (bw) on future milk production and reproductive performance in iranian holstein cattle. records of holstein cows between 1967 and 2010 were obtained from the animal breeding center of iran. birth weights within each herd were grouped into five categories based on the standard normal distribution curves (z ≤-0.8416, -0.8416 < z ≤ -0.2533, -0.2533 < z ≤ 0.2533, 0.2533 < z ≤ 0.8416, 0.8416 < z). the results indicated that bw significantly affected (p < 0.001) the lactation parameters (e.g. milk yield,  protein yield, fat yield), age at first calving (afc), interval between calving to first insemination (icfi), calving intervals (ci), calving ease (ce), and first service to conception length (fstc)  during the first three lactation periods. the bw positively affected the lactation performance, but had a negative effect on the reproductive performance. increase in bw was unexpectedly associated with increased dystocia. the present study indicated that bw could impact economical traits of holstein dairy cattle and need to be considered in breeding programs.