110 EFFECT OF HUMAN CHORIONIC GONADOTROPIN (hCG) ADMINISTRATION ON DAYS 1, 2, 3, OR 4 POST-OESTRUS ON CORPUS LUTEUM DEVELOPMENT AND CIRCULATING PROGESTERONE CONCENTRATIONS IN BEEF HEIFERS

The steroid hormone progesterone (P4) plays a key role in the reproductive events associated with the establishment and maintenance of pregnancy. Elevated concentrations of circulating P4 in the immediate post-conception period have been associated with advanced conceptus elongation, increased interferon-tau production, and greater pregnancy rates in cattle. Several strategies have been employed to supplement the endogenous P4 supplied by the corpus luteum (CL). One such strategy is the administration of human chorionic gonadotropin (hCG) during the early luteal phase (typically Day 5) that induces ovulation of the first wave dominant follicle and formation of a functional accessory CL, in turn leading to increased circulating concentrations of P4. In addition to this effect, hCG also stimulates hypertrophy of the original CL, leading to a significant deviation in P4 concentrations 2 days after administration. The aim of this study was to examine whether administration of hCG earlier during the metoestrus period would lead to an earlier rise in P4 by stimulation of the developing endogenous CL. The oestrous cycles of cross-bred beef heifers (n = 50) were synchronised and those recorded in standing oestrus (Day 0) were assigned randomly to one of 5 treatments and administered a single IM injection of saline on Day 1 after oestrus (Control) or 3000 IU hCG on Day 1, 2, 3, or 4 after oestrus. Daily blood samples were collected from Day 0 to 14 by coccygeal venipuncture to measure serum P4 concentration. The dimensions of the original CL and induced CL, when present, were measured by daily ultrasound scanning beginning on Day 3 after oestrus until Day 14. Data were analysed using the PROC MIXED procedure of SAS (SAS Institute Inc., Cary, NC, USA). In terms of effects on the CL, administration of hCG on Day 1 had no effect on CL area. Administration on Day 2 increased CL area from Day 6 to 12 (P ≤ 0.05). hCG on Day 3 increased CL area from Day 9 to 11, while administration on Day 4 increased CL size on Day 9 and 10 (P ≤ 0.05). Administration of hCG on Day 4 induced the formation of an accessory CL in 89% of heifers, resulting in a significant increase in total luteal tissue area on the ovaries compared to all other groups. Consistent with the effects on the CL, hCG on Day 1 did not affect P4 concentrations. Administration on Day 2 significantly increased P4 compared to the control from Day 6 to 11 (P ≤ 0.05). Administration on Day 3 resulted in a nonsignificant increase in P4, while hCG on Day 4 increased P4 from Day 8 to 13 compared to the control (P ≤ 0.05). In conclusion, administration of hCG as early as Day 2 post oestrus results in increased P4 in circulation from Day 6 which should have beneficial downstream effects in terms of uterine receptivity and conceptus elongation. Funded by Science Foundation Ireland (07/SRC/B1156) and the Spanish Ministry of Science and Innovation AGL2009-11810.