First calf heifers are the single, largest parity group in most herds. Therefore, it is important to breed virgin heifers to the best genetic material available - high PD, AI bulls. The best bred calves will be of no use at all if they don't survive and enter the milking string. Genetics really can't be separated from overall management. Neither one is much good without the other.
Genetic improvement is a long term proposition.
Results are additive and cumulate over time.