Birth weight in calves is influenced by several factors, including breed, genetics of the sire and dam, length of gestation, age and size of the dam, sex of the calf, environmental factors, and nutrition and health of the dam. Bull calves are typically larger than heifer calves, and gestation length is heritable, with low birthweight cattle having shorter gestation lengths and high birthweight cattle having longer gestation lengths.

The effects of gestation length are important to consider when selecting a bull for breeding. Unexpected factors can influence birth weight, such as genetics of the cattle, maternal effects, and environmental factors like nutrition and weather. Birth weight tends to be lower in hot seasons and higher in cold seasons, with winter-born calves having a slightly higher incidence of calving problems due to changes in blood flow and body heat regulation.

A study from the mid-1990s found that warmer temperatures during winter resulted in lower average birth weights and fewer calving problems in heifers. Environmental effects, such as diet balance and mineral deficiencies, can also impact calving difficulty. It is important to consider all of these factors when selecting bulls for breeding to ensure optimal birth weights and calving ease.