The aim of this study was to investigate the effect of short term progesterone (P4) supplementation on circulating P4 concentrations, corpus luteum (CL) size, and conceptus development in cattle. The oestrous cycles of crossbred beef heifers were synchronised using a 7-day PRID® Delta (1.55 g P4) treatment with administration of a PGF2α analog (Enzaprost®) the day before PRID® Delta removal. Only those recorded in standing oestrus (Day 0) were used. In Experiment 1, heifers were randomly assigned to 1 of 5 groups: (1) control: no treatment, (2) placebo: insertion of a blank device (no P4) from Day 3 to 7, (3) insertion of a PRID® Delta from Day 3 to 7, (4) insertion of a PRID® Delta from Day 3 to 5, or (v5) insertion of a PRID® Delta from Day 5 to 7. In vitro produced blastocysts were transferred to each heifer on Day 7 (10 blastocysts per heifer) and conceptuses were recovered at slaughter on Day 14. In Experiment 2 heifers were artificially inseminated at oestrus and randomly assigned to 1 of 3 treatment groups (1) placebo, (2) PRID® Delta from Day 3 to 5, or (3) PRID® Delta from Day 3 to 7. All heifers were slaughtered on Day 16, and recovered conceptuses were incubated in synthetic oviduct fluid medium for 24 h; spent media and uterine flushes were analysed for interferon-tau (IFNT). In both experiments, daily blood samples were taken to measure serum P4 concentration. Data were analysed using the PROC MIXED procedure of SAS (SAS Institute Inc., Cary, NC, USA). Insertion of a PRID® Delta resulted in an increase (P < 0.05) in serum P4, which declined following removal. In Experiment 1, serum P4 concentration was significantly lower from Day 9 to 14 (P < 0.05) and Day 14 CL weight was lower in the PRID® Delta Day 3 to 7 group than the placebo or control groups. P4 supplementation from Day 3 to 5 (17.0 ± 1.4 mm) or Day 3 to 7 (11.3 ± 2.3 mm) increased conceptus length compared to the placebo (2.1 ± 1.8 mm). In Experiment 2, serum P4 was significantly lower in the two supplemented groups following PRID® Delta removal compared with the placebo (P < 0.05) and was associated with a lower CL weight in the Day 3 to 7 group. Supplementation from Day 3 to 5 (94.0 ± 18.8 mm) or Day 3 to 7 (143.6 ± 20.6 mm) increased conceptus length on Day 16 compared to the placebo (50.3 ± 17.4 mm). Conceptus length was strongly correlated with the concentration of IFNT in the uterine flush (r = 0.58; P = 0.011) and spent culture medium (r = 0.68; P < 0.002). These findings highlight the somewhat paradoxical effects of P4 supplementation when given in the early metoestrus period in terms of its positive effect on conceptus development and its potentially negative effects on CL lifespan.
Supported by CEVA Sante Animale and Science Foundation Ireland (07/SRC/B1156).
Interferon-Tau Regulates a Plethora of Functions in the Corpus Luteum
