Administration of gonadotropin-releasing hormone during metoestrus in cattle: influence on luteal function and cycle length

Gonadotropin-releasing hormone (GnRH) has been used to warrant the success of artificial insemination by accurately timing occurrence of ovulation. In practical conditions, GnRH may be administered too late, after ovulation, with an eventual reduction in pregnancy rate. The aim of this study was to investigate whether GnRH administration after ovulation would have a negati ve effect on luteal function. Three cows and six heifers of the Finnish Ayrshire breed were used. Oestrus es were synchronised. After detection of ovulation, one of the following tr eatments was implemented: gonadorelin (250 mug, i.m.) at either 0-24 h (TI) or 24-48 h (T2) post-ovulation or control (no gonadorelin, C). Every anima l was assigned once to each of these three manipulations. Ultrasonography w as performed on days 1, 4 or 5, 7 or 8, 11 or 12, 14 or 15 post-ovulation a nd daily from the beginning of the next oestrous signs until ovulation (day 0 = day of ovulation). Blood samples for progesterone (P-4) determinations were collected daily from day 1 after the occurrence of ovulation until re cording of the next oestrus. Administration of GnRH during metoestrus did not induce ovulation of either large or small follicles and, thus, no accessory corpora lutea (CL) were f ormed. In T1, on day 14 or 15, the diameter of CL was 1.3 +/- 0.3 mm smalle r than in C (P < 0.01), but no differences were found either on days 1 I or 12 or on the same days of the T2 and C treatments. No significant differen ces in levels or profiles of P-4 curves were found between GnRH treatments and control. Neither had the treatments any effects on the length of the oe strous cycle. In conclusion, GnRH treatment during metoestrus does not seem to alter subs equent luteal function and, thus, this does not explain previous reports of reduced fertility post-treatment. (C) 2000 Elsevier Science B.V. All right s reserved.