M. Forutan - Department of Animal Science, College of Agriculture, Isfahan University of Technology, Isfahan ۸۴۱۵۶-۸۳۱۱۱, Islamic Republic of Iran.
S. Ansari Mahyari - Department of Animal Science, College of Agriculture, Isfahan University of Technology, Isfahan ۸۴۱۵۶-۸۳۱۱۱, Islamic Republic of Iran.
M. A. Edriss - Department of Animal Science, College of Agriculture, Isfahan University of Technology, Isfahan ۸۴۱۵۶-۸۳۱۱۱, Islamic Republic of Iran.

This study was conducted to estimate the additive genetic components of calf mortality in the first month of age, calving difficulty, and birth weight in Holstein dairy cows in the central regions of Iran. The records comprised ۶۱,۲۰۰ calves born between ۱۹۹۰ and ۲۰۱۱ from ۶۰ dairy herds. Different threshold-linear models in three groups of univariate, bivariate, and multivariate models were used. The frequency of calf mortality was ۲.۶%. Distribution of calving difficulty score was ۶۵.۱۲% in the first category (no assistance), ۳۰.۶۶% in the second, ۳.۱۲% in the third, and ۱.۱% in the fourth (major assistance). Averages of birth weight and dam age were ۴۰.۳۴ kg and ۷۶۹.۴ days, respectively. Direct Heritability estimation for calf mortality varied from ۰.۰۰۵ to ۰.۰۲۷. The estimated heritability for calving difficulty ranged from ۰.۰۳۲ to ۰.۰۵۰. Heritability for birth weight was estimated about ۰.۲۲. The results of this study showed that there were genetic variations for all traits. Although there was no strong additive genetic correlation between the traits, an environmental correlation between mortality and other traits was observed. Results suggested that implementation of threshold models for mortality trait was more favorable, but they were not reflected in genetic analysis of calving difficulty records. Furthermore, current findings indicated that benefit from the use of multi-traits models for genetic evaluation of postnatal mortality depended on the methodology (linear or threshold model) used for mortality trait.

Birth weight, Calf mortality, Calving difficulty, Genetic parameters, Linear- Threshold models