Female Reproductive Traits and Their Heritabilities in Beef Cattle
– K. M. Cammack M. G. Thomas and R. M. Enns, published in The Professional Animal Scientist 25 ( 2009 ):517–528, and condensed from the original manuscript by Steve Boyles, OSU Beef Extension Specialist
Female Reproductive Measures
Biological and economical efficiencies of cow-calf production are largely dependent on successful reproduction. Improvements in reproductive performance can be up to 4-fold more important than improvements in end-product traits in a conventional cow-calf operation selling market calves at weaning. Whereas estimates of heritability for many reproductive traits are low, some exist that have moderate heritabilities, and there are important genetic correlations between reproductive traits and other production traits that are moderately to highly heritable.
Beef female fertility has been recorded and measured in a multitude of ways, including age at first calving, calving date, first insemination conception (nonreturn rate), days to first breeding (days open), pregnancy rate, calving interval, longevity, and stayability.
Age at Puberty
Age at puberty is used as a measure of heifer fertility and may influence subsequent reproductive trait performance. Reproductively efficient heifers reach puberty earlier, and therefore can potentially conceive earlier in the breeding season. Cumulatively, it should be noted that age of puberty had a heritability estimate much greater than the other female reproductive traits discussed herein. However, even with extensive knowledge of the physiology of puberty, age at puberty is a difficult trait to observe in field populations.
Age at First Calving
Age at first calving is routinely recorded and is highly genetically correlated with age at subsequent calvings and the interval between subsequent calvings. Because of these relationships, this measure is often used to evaluate heifer fertility.
Calving date is defined as the day within the calving season in which the heifer or cow calved, and is similar to measurements of days to calving. Calving date is an easily recorded trait and has been proposed as a strong indicator of beef cow fertility
because of its economic importance. Earlier calving dates are associated with higher weaning weights because weaning typically occurs on a specific day of the calendar rather than on a weight-constant or age-constant basis. Calving date is generally moderately
heritable; however, estimates in the literature are variable, ranging from 0.03 to 0.21.
Heifer pregnancy is a measure of reproduction indicative of sexual maturity and is often included in breeding objectives. Heifer pregnancy, like cow pregnancy, is a binary trait (1 =pregnant; 0 = not pregnant).
Replacement heifers require significant time and resources; thus, having heifers bred and calved by 2 yr of age contributes to the economic success of a beef cattle operation.
Therefore, lifetime pregnancy rate is an economically relevant trait.