For example, Breeding Disaster might have been a bull selected because of his type. When sire selection is not effective, progress can be slowed and even reversed. Selection for only one trait in a breeding program can be a problem because of undesirable genetic correlations. For example, there is a positive genetic correlation between yearling weight and birth weight. This means that selection for yearling weight also will increase birth weight if no negative emphasis is placed on birth weight.
Birth weight is highly related to calving difficulties.
A producer, who is only interested in producing high EPD values for yearling weight, might also generate high EPD values for birth weight if he selects sires strictly on their yearling weight EPD values as illustrated in figure 3. The problem occurs because there is distribution of EPD values around the expected average. Not only has a high EPD value been built up for yearling weight, but the same has been done for birth weight. Bulls with a combination of high EPDs for birth and yearling weights may be useful in terminal sire mating systems. Females used on these bulls should be mature cows to minimize calving difficulties.