Summary. The experimental objective was to evaluate how a spontaneously formed corpus luteum (CL) differed in its response to prostaglandin (PG) F-2α, given during the first 5 days after ovulation, from a CL induced during dioestrus with hCG. Sixteen Holstein heifers were used during each of 2 consecutive oestrous cycles. During the first cycle (sham cycle), heifers were given no PGF-2α (control) or PGF-2α (25 mg, i.m.) on Day 2, 4 or 6 (oestrus = Day 0). During the second cycle (hCG-treated cycle), heifers were given hCG (5000 i.u., i.m.) on Day 10, followed by no PGF-2α (control) or PGF-2α on Day 12, 14 or 16, corresponding to 2,4 or 6 days after the ovulatory dose of hCG. A new ovulation was induced in 13 of 16 heifers given hCG on Day 10. Luteolysis did not occur immediately in heifers given PGF-2α on Day 2 or 4 during the sham cycle, but concentration of progesterone in serum during the remainder of the cycle was lower in heifers given PGF-2α on Day 4 than in sham controls or heifers given PGF-2α on Day 2 (P < 0·05). Luteolysis occurred immediately in heifers given PGF-2α on Day 6 of the sham cycle or on Day 12,14 or 16 of the hCG-treated cycle, with concentration of progesterone in serum decreasing to < 1 ng/ml within 2 days. Intervals between periods of oestrus were shorter in heifers given PGF-2α on Day 6 of the sham cycle (8·3 ± 0·3 days) than in control heifers (20·5 ± 0·9 days) or those given PGF-2α on Day 2 or 4 (20·8 ± 0·6 and 18·4 ± 0·5 days; P < 0·05) of the sham cycle. During the hCG-treated cycle, interoestrous intervals were shorter in heifers given PGF-2α on Day 12, 14 or 16 (19·5 ± 0·5, 18·8 ± 0·5 and 20·8 ± 1·3 days) than those of hCG-treated controls (25·5 ± 1·8 days; P < 0·05). It was concluded that CL induced by treatment with hCG on Day 10 of the oestrous cycle regress after exogenous PGF-2α given within 2 days after ovulation, whereas spontaneously formed CL respond to PGF-2α only after Day 5.
Keywords: cattle; progesterone; gonadotrophin; corpus luteum; prostaglandin F-2α