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Title: Association between Period 1 mRNA levels in leukocytes and initiation of estrous cycles in peripubertal beef heifers

Author
item FIELDS, SARAH - SO DAK ST UNIV, BROOKINGS
item PERRY, BRANDI - SO DAK ST UNIV, BROOKINGS
item PERRY, GEORGE - SO DAK ST UNIV, BROOKINGS
item Cushman, Robert - Bob

Submitted to: Biology of Reproduction Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 2/20/2008
Publication Date: 9/1/2008
Citation: Fields, S., Perry, B., Perry, G., Cushman, R. 2008. Association between Period 1 mRNA levels in leukocytes and initiation of estrous cycles in peripubertal beef heifers [abstract]. Biology of Reproduction. Special Issue:209 (Abstract #654).

Interpretive Summary:

Technical Abstract: Initiation of fertile estrous cycles is one of the largest reproductive obstacles that a beef cow must overcome. In beef cows, postpartum interval and age at puberty are moderately heritable reproductive traits that have been reported to have a positive genetic correlation (between 0.45 and 0.70). Therefore, there may be a common set of genes involved in initiation of fertile estrous cycles in both postpartum cows and peripubertal heifers. Previously, we demonstrated that mRNA levels of the clock gene, Period 1, were greater in the leukocytes of beef cows that did not conceive as compared to cows that did conceive during the breeding season when sampled 6 d before the start of breeding. Cows that did not conceive also had lower progesterone concentrations prior to the start of breeding, suggesting that Period 1 mRNA levels in the white blood cells might be an indicator of reproductive status (ovulatory or anestrus) in the beef cow. The hypothesis of the present experiment was that among beef heifers, those that became pregnant during the breeding season would have lower Period 1 than those that did not become pregnant. Blood samples were collected from beef heifers (n = 96) by caudal venipuncture to determine reproductive status prior to the start of the breeding season. A sub-group (n = 86) of the heifers were hormonally synchronized and artificially inseminated. Following artificial insemination, heifers were placed with fertile bulls for 60 days or observed for estrus for 42 days and re-inseminated if observed in estrus. Based on progesterone concentrations, heifers were categorized as anestrus (n = 33) or cycling (n = 63). Total cellular RNA was extracted from leukocytes and relative mRNA levels for Period 1 and GAPDH were determined by real-time RT-PCR using previously validated primers. Relative levels of Period 1 mRNA corrected for GAPDH tended to be greater in the leukocytes of heifers determined to be anestrus as compared to heifers determined to be cycling (170.1 ± 34.5 vs. 81.0 ± 25.5 relative units; P = 0.06). There was no difference in pregnancy rates between cycling heifers and anestrous heifers (64.2 % vs. 56.7%; P = 0.50). Anestrous heifers that conceived during the breeding season (n = 17) did not differ from cycling heifers (n = 53) in relative levels of Period 1 mRNA (63.4 ± 34.1 vs. 97.9 ± 50.8 relative units; P = 0.57); however, relative levels of Period 1 mRNA were greater in heifers that did not conceive during the breeding season (274.3 ± 57.7 relative units; P < 0.03) than in either the cycling heifers or the anestrous heifers that conceived. The tendency for greater Period 1 mRNA levels in the leukocytes of anestrous heifers agrees with our previous results and continues to support the hypothesis that Period 1 mRNA levels may be a biomarker for reproductive status. Additionally, Period 1 mRNA levels in the leukocytes may be an indicator of depth of anestrus at the start of breeding and the potential for conception during the breeding season.