In BREEDPLAN analyses, contemporary groups for birth weight and 200d weight are sliced at 45 day intervals to avoid possible inaccuracies in age adjustment, rapid changes in environmental conditions and confounding of sires and seasons biasing the estimated breeding values. However age slicing can have a significant impact on data structure and effectiveness of records. The aim of this study was to re-estimate the variance components and examine the effectiveness of the data for birth and 200day weights from Angus cattle with different contemporary group slicing intervals. The result showed that altering the slicing interval from 15 to 60 days did not have a significant impact on the variance components for birth or 200day weight. However increasing slice interval significantly improved the average effectiveness of the data for both traits. On average the animal record effectiveness increased by 10% and the average total effective progeny records per sire more than doubled. The results suggest that the age slicing interval could be altered from 45 to 60 days to increase the effectiveness of the data available without impacting on the genetic parameters. However altering the contemporary group structure will cause changes to the existing estimated breeding values.