Effect of Age at First Calving on Calving Interval and Dry Period Length Traits of Holstein Cows

The objectives of present study were to study of the first calving age effect on calving interval and dry period length and the estimation of variance components, breeding values and genetic and phenotypic trends of these traits in different classes of first calving age (20-23.5, 23.5-26 and ≥26 month) of Holstein cows in Isfahan province. Calving records of 62796 cows calved in 72 herds during 1994 to 2015 were used for analyses. The analysis of data was performed in Mixed procedure of SAS9.2 software and estimation of variance components and breeding values of traits was done with REML method of DMU package. The results showed the lowest least squares mean of calving interval and dry period length were for the first age class and the first (20-23.4) and second (23.5-26) age classes, respectively, and the greatest least squares mean was for the third age class in both traits (P<0.01). The lowest breeding value mean of calving interval were calculated in the second age class and for dry period length in the first age class (P<0.01). For genetic trend, the lowest amount in both traits was observed for the first and second classes of first calving age. Also, decreasing trend with reduction in first calving age was observed for phenotypic trend in recent years (P<0.01). Therefore, according to these results, the first and second classes of first calving age (<26 month) could be suggested for Holstein dairy herds in Isfahan province.