Effect of age at first calving (AFC) on yield, variance components and genetic trend of productive traits of Holstein cows in Isfahan province

The objectives of present research were investigating the effect of AFC on production traits, estimation of variance components and genetic and phenotypic trends of these traits in different classes of AFC of Holstein cows in Isfahan province. The records of 49925 cows related to 64 herds, had calved during years 2006 to 2015, were used in this study. AFC that was between 20 to 36 months was divided to 3 classes (20-23.5, 23.5-26, 26≤). The analysis of data was performed using Mixed procedure of statistical in software SAS9.2. DMU package was used for estimation of variance components and breeding values of milk production traits with REML based on univariate animal model. Least squares mean of milk production, fat and protein yield was 10270±38.2, 249.2±1.5 and 270.9±1.4 kg for the first class, 10179±39.1, 243.5±1.6 and 267.3±1.4 kg for the second class and 10017±44.9, 237.4±1.8 and 264.5±1.6 kg for the third class of AFC, respectively, and their differences were significant (P<0.01). The correspondeing heritabilities were estimated 0.12±0.008, 0.34±0.008 and 0.23±0.012 for the first class, 0.14±0.005, 0.31±0.005 and 0.23±0.007 for the second class and 0.11±0.006, 0.24±0.008 and 0.21±0.011 for third class, respectively. The highest breeding values mean of milk production traits was for the first and second classes of AFC that they had significant difference with the third class (P<0.01). The results of present study on Holstein cows in Isfahan province showed that cows calving at a young age can increase milk, fat and protein yield but can decrease genetic trend for these traits.