Abstract: The objective of h s study, was to evaluate the effect of Age at First Calving (AFC) on cumulative first 60 and 200 days milk production and also reproductive performance in first-parity Iranian Holstein dairy cows. Data regarding to all calving of cows were collected during 1996 until 2006 in seven large commercial Holstein farms. Each cow has been characterized by demographic data, production and reproduction data. The dependent variables analyzed were the cumulative first 60 and 200 days milk production, days open and calving class. Data were analyzed using General Linear Models. The dstribution analyze for the variables was done using the statistical software package JMP. The effect of the dfferent levels of AFC on calving class was evaluated by nominal logistic analysis. The result showed that AFC averaged 27.23+3.37 month. Age at first calving decreased from 1996-2006 (p<0.01). Age at first calving had a significant impact on cumulative first 60 and 200 days milk production @<0.001) but days open was similar among dfferent levels of AFC. The AFC has impacted the calving class (p<0.01) and by increasing the AFC probability of eutocia increased and the probability of calving dfficulty decreased (p<0.01). Cumulative first 60 and 200 days milk productiom were also affected by calf weight at calving, calving class, herd and calving year and season (p<0.001). Days open was impact by the calving class, herd and calving year and season and increased over the years (p<0.001). The result of this study demomtrated that the milk yields increased by increase in AFC but the AFC had no impact on the interval from calving to conception. Age at first calving had significant impact on calving class and the probability of calving dfficulties decreased by increase in AFC. The result also demonstrated that mean AFC is higher than the optimum age at first calving and can be decreased by 4 months.

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