Improving reproduction performance is of great significance because of the reduction in milk production and the number of calves per cow can lead to irreparable damage to dairy cow herds. The objectives of this study were to investigate the genetic aspects of body condition score (BCS) during transition period and its effect on fertility traits in dairy cattle of Isfahan province using random regression test day records. Moreover, to study the transition period data, the range of days was evaluated from 100 days before to 70 days after calving. Accordingly, the data of 66305 records from 20276 cows with more than 2 calving were used. The data were collected during 2004 to 2017 from 15 dairy cattle herds by Vahdat IndustrialAgriculturists and DairymeCooperative. The fertility traits consisted of open days, first insemination after calving, and number of inseminations per pregnancy. The genetic and environmental variance-covariance were estimated based on the random regression animal model using the DMU software and AIREML method. Results showed that heritability estimates of BCS at different days were varied in the range of 0.09- 0.26 with an average of 0.12. Likewise, genetic correlation between BCS and open days according to bi-variate random regression model was close to zero. In general, BCS during the first two weeks after calving showed a very small and close to zero correlation with open days. Genetic correlation between BCS and the interval of calving and first insemination was estimated between -0.149 and 0.55 with an average of -0.282. Besides, the range of genetic correlation changes between body condition score and the number of insemination per pregnancy was between -0.043 and -0.12 with an average of 0.032. Consequently, according to the results of this study, it can be stated that tense of decreasing BCS during the first weeks after calving will increase number of inseminations per pregnancy which leads to reduce conception rate in dairy cows. Furthermore, Holstein dairy herds in Isfahan can be genetically improved for fertility traits because of their high genetic variations. Keywords : Body Condition Score, Random Regression, Heritability, Genetic Correlation, Fertility Traits, Holstein Cattle.
