The objectives of this study were to investigate population parameters (variances, heritabilities and correlations) in the first five calving numbers of fertility traits. Studied fertility traits were calving interval and number of service per conception. Calving and insemination records were 782 and 1062, 602 and 787, 458 and 604, 368 and 455, 277 and 366 in first, second, third, fourth and fifth lactation/calving, respectively. Crossbreed cows’ data were spans over 43 years (through 1974 to 2017) recorded at Holetta dairy research farm, Ethiopia. Multivariate animal model that included breed and year (season was not significant) as fixed effects and animal and error as random effects were applied. The larger genetic and phenotypic variance for calving interval was observed at first calving (3435 and 26248 days²)but for service per conception, it was at second calving (0.66 and 0.60), respectively. The estimated heritabilities for calving interval in the five consecutive calving numbers were 0.13, 0.09, 0.18, 0.11 and 0.16. The estimated heritabilities for service per conceptions were also found to be 0.003, 0.04, 0.03, 0.11 and 0.006, respectively. Genetic correlations for calving interval in the five calving’s were found to be in the range of -0.38 to 0.69 while phenotypic correlations were positive but lower in the range of 0.06 to 0.27. The genetic correlations for service per conception in different calving numbers were found in the range of -0.76 to 0.78 whereas phenotypic correlations were about -0.026 to 0.16. As the variances, heritabilities and correlations of calving interval and service per conception are different in each lactation, both fertility traits should be interpreted as different traits across lactations rather a repeated measures along lactations.

Key words: calving number; correlations; heritability; variance