Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/8/2007
Publication Date: 9/20/2007
Citation: Cushman, R.A., Allan, M.F., Thallman, R.M., Cundiff, L.V. 2007. Characterization of biological types of cattle (Cycle VII): Influence of postpartum interval and estrous cycle length on fertility. Journal of Animal Science. 85(9):2156-2162. Interpretive Summary: Reproductive failure is the primary reason that a beef cow leaves the production herd. Therefore, better characterization of reproductive phenotypes in the postpartum cow is paramount to improving diagnosis of fertility issues and developing markers of reproductive efficiency. One trait that is affected by genetics and has been shown to influence fertility is the postpartum interval (PPI), the length of time it takes a cow to return to estrus following calving. In the present study, we examined breed differences in the PPI in Cycle VII of the germplasm evaluation program. Additionally we were able to examine breed differences in two novel reproductive phenotypes, number of estrous cycles displayed prior to breeding and the length of the estrous cycle immediately prior to breeding. Sire breed influenced the length of the PPI such that Limousin sired cows had the longest PPI and Simmental sired cows had the shortest PPI. Number of estrous cycles displayed prior to mating was also influenced by sire breed; Simmental sired cows demonstrated more estrous cycles prior to breeding. Dam breed influenced length of the estrous cycle; cows from Angus dams had longer estrous cycles than cows from Hereford dams while cows from USMARC III composites had intermediate estrous cycle length. Pregnancy rates decreased in a linear manner as the length of the estrous cycle immediately prior to natural service increased. Breed differences in PPI and estrous cycle length indicate genetic components to these reproductive traits and support further investigation of the genes controlling these differences.
Technical Abstract: Genetic improvement in reproductive efficiency through selection is difficult, because many reproductive traits are binomial and have low heritabilities. Before genetic markers can be generated for reproductive efficiency of cows, greater characterization of reproductive phenotypes is needed to understand the components of reproductive efficiency. The present study tested the hypotheses that 1) breeds vary in postpartum interval (PPI) and estrous cycle length, 2) a longer estrous cycle immediately prior to breeding increased pregnancy rates, and 3) a greater number of cycles prior to breeding increased pregnancy rates. PPI, estrous cycle length, and number of cycles prior to breeding were examined in F1 cows (n = 519) obtained from mating Hereford, Angus and USMARC III cows to Hereford, Angus, Simmental, Limousin, Charolais, Gelbvieh, and Red Angus sires. Cows were classified as having 0, 1, 2, or 3 observed estrous cycles prior to breeding. Sire breed of the cow influenced the length of the PPI and number of cycles prior to the start of breeding (P < 0.001). Simmental-sired cows had the shortest PPI and greatest number of cycles prior to breeding; while Limousin-sired cows had the longest PPI and least number of cycles prior to breeding. However, cows with a greater number of cycles prior to breeding did not have greater pregnancy rates than cows that had not exhibited standing estrus prior to breeding (P = 0.87). In cows that cycled prior to breeding, the length of the estrous cycle immediately prior to breeding was influenced by dam breed and body condition score (BCS; P < 0.01). Cows out of Hereford dams had shorter estrous cycles than cows out of USMARC III or Angus dams, and estrous cycle length increased as body condition score increased. Pregnancy rate decreased as length of the estrous cycle immediately prior to breeding increased (P = 0.05; -2.2% per day of cycle length). Therefore, previously anestrous cows were just as likely to conceive as cows that had cycled prior to breeding and an increased number of observed estrous cycles prior to breeding did not increase pregnancy rates. There may be an influence of the length of the estrous cycle immediately prior to breeding on conception rates, possibly because a longer estrous cycle results in a prolonged preovulatory follicle with greater potential for a lower quality oocyte. Breed differences in PPI and estrous cycle length suggest that there are genetic components to these traits.