There are political problems in changing the base because for most traits there is a positive genetic trend over years. Assuming a trait in which bigger is better, changing the base to a more recent year will result in lowering of the EPD's and some that were positive with the more distant base year will be negative with a nearer base year. Producers don't like negative EPD's. Because many environmental factors affect weaning weight and mask and obscure the breeding value, the BLUP methodology must attempt to account for these factors. Assigning animals to contemporary groups does this.
Contemporary groups are groups of animals that are raised under similar environmental conditions.
Things such as herd, year of birth, sex, age of dam, age of calf should be considered when forming contemporary groups. For example, calves born in one herd of the same sex, same age of dam, same year and same season of the year could form a contemporary group. Factors defining contemporary groups may vary for different traits depending on whether the factors affect the trait or not. Contemporary groups must be connected to one another with some common sires in order to have valid comparisons. There are all levels of connectedness.