Submitted to: Biology of Reproduction Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 3/30/2007
Publication Date: 7/20/2007
Citation: Cushman, R., Echternkamp, S., Allan, M., Freetly, H. 2007. Identification of indicator traits of fertility in the postpartum beef cow [abstract]. Biology of Reproduction. Special Issue:187-188.
Technical Abstract: Reproductive failure is the primary reason that a beef cow leaves the production herd; however, the low heritability of reproductive traits makes identification of genetic markers for fertility difficult. Postpartum interval is a moderately heritable trait that influences fertility because cows that ovulate earlier postpartum may have higher first service conception rates. However, in most production settings, the time of first ovulation is unknown. The objective of the current study was to identify single measurement factors that could be indicator traits of fertility in beef cows prior to the start of breeding. The hypothesis tested was that cows that become pregnant earlier in the breeding season have longer intervals from calving to the start of breeding (CSBI), more follicles detectable by ultrasound, greater circulating concentrations of progesterone, and decreased Period1 mRNA expression in the leukocytes. Second (n = 20) and fifth (n = 20) parity crossbred beef cows were subjected to an ovarian ultrasound scan six days prior to the start of a 55-day breeding season. Blood samples were collected at ovarian ultrasound and at approximately 2-week intervals throughout the breeding season. Nine days after the bulls were removed, pregnancy was diagnosed by ultrasound and fetal age was estimated. Parity did not affect any of the endpoints examined. On the basis of CL detected by ultrasound and serum progesterone concentrations, 36 of 40 cows had ovulated prior to the start of breeding. There was no difference in CSBI between cows that were bred in the first 21 days (n = 17), the second 21 days (n = 18), or the last 13 days of the breeding season (n = 5; CSBI = 73 ± 1.6 days). Cows that conceived in the first 21 days of the breeding season had greater serum progesterone concentrations at ovarian ultrasound prior to breeding than cows that conceived in the second 21 days or cows conceiving in the last 13 days of the breeding season (7.0 ± 0.7 vs. 2.8 ± 0.6 and 3.6 ± 1.6 ng/ml, P = 0.0005). By 42 days of breeding, serum progesterone concentrations in cows conceiving during the first 42 days of breeding were greater than in cows conceiving in the last 13 days of breeding (7.9 ± 0.3 and 7.3 ± 0.6 vs. 2.5 ± 1.1 ng/ml, P < 0.0001). The number of antral follicles prior to the start of breeding did not differ among the three groups, and there was no relationship between CSBI, number of antral follicles, or serum progesterone concentrations prior to breeding. Relative levels of Period1 mRNA measured in the leukocytes by real-time RT-PCR and corrected for GAPDH were lower in cows that conceived in the first 42 days of the breeding season as compared to cows that conceived in the last 13 days of breeding (6.6 ± 3.5 and 3.9 ± 3.4 vs. 26.8 ± 6.2 arbitrary units; P = 0.009). Sub-functional corpora lutea in the postpartum period are associated with decreased fertility; therefore, greater serum concentrations of progesterone in cows that conceived early may indicate that these cows had ovulated earlier postpartum, and that their reproductive axis was better primed for maintenance of pregnancy. Lower relative amounts of Period 1 mRNA in cows that conceived in the first 42 days of breeding indicates that the biological clock may be set higher in cows that conceive earlier in the season.