162 J. Taponen et al. Animal Reproduction Science 64 2000 161–169
In conclusion, GnRH treatment during metoestrus does not seem to alter subsequent luteal function and, thus, this does not explain previous reports of reduced fertility post-treatment.
© 2000 Elsevier Science B.V. All rights reserved.
Keywords: Cattle-endocrinology; Gonadotropin-releasing hormone; Corpus luteum; Progesterone; Cycle length
1. Introduction
Gonadotropin-releasing hormone GnRH has been indicated as a tool for control of ovarian function in cattle when applied close to the time of insemination to induce ovulation
and enhance conception rate review by Thatcher et al. 1993, Morgan and Lean 1993 and Shaw 1999. During recent years, emphasis has been put on the use of GnRH during the
luteal phase to support CL-function and thus, possibly, to avoid early embryonic mortality around the time of maternal recognition of pregnancy review by Thatcher et al. 1993 and
Shaw 1999. In most of these studies, GnRH has been given during early or mid-dioestrus as single or multiple injections.
As Taponen et al. 1999 have described earlier, in tie-stall conditions, especially during the long wintertime in the north, occurrence of silent heat is one of the most important causes
for sub-fertility. Therefore, GnRH treatment is often used by practitioners to induce ovula- tion as close as possible to artificial insemination AI hoping to enhance conception rates.
Under these circumstances, GnRH treatment may be clearly given before the physiological luteinizing hormone LH surge, but also after ovulation, e.g. during metoestrus.
The effects of GnRH given during the luteal phase on luteal function and oestrous cycle length have been studied intensively review by Thatcher et al. 1993. In some studies,
increased luteal lifespan and cycle length andor progesterone P
4
production have been found Milvae et al., 1984; Macmillan et al., 1985; Stevenson et al., 1993, but the results
have been inconsistent. This is probably due to different experimental designs natural or synthetic GnRH, dose, cycle day of treatment, single or repeated injections. Despite this
intensive research, very few studies have been published on GnRH treatments given during metoestrus in cattle. Macmillan et al. 1986 reported a reduction in pregnancy rate by
10.9 units when buserelin 5 microg was administered 1–3 days after insemination. Ford and Stormshak 1978 found a significantly reduced P
4
concentration when GnRH 100 microg was given 55 h after the detected oestrus. Later, Rodger and Stormshak 1986 and
Martin et al. 1990 studied the influence of GnRH 100 mg given on day 2 post-oestrus day of oestrus = day 0 on subsequent luteal function. In both studies, a tendency for a
reduction in P
4
concentration was found. It has been shown that early embryonic loss is probably associated with a complex endocrine syndrome, which could be manifested by
a lower capacity to synthesise andor release CL-progesterone Lamming et al., 1989. It is, however, questionable that the reduced fertility reported by Macmillan et al. 1986 can
be explained by an impaired luteal function, because the reasons behind the decline of P
4
concentration caused by the GnRH given during metoestrus are yet to be disclosed. In all the above-mentioned studies, GnRH was given at least 24 h post-ovulation. Since
under veterinary praxis GnRH is probably applied during the first 24 h post-ovulation oc- currence, the aim of the present study was to investigate the influence of exogenous GnRH
J. Taponen et al. Animal Reproduction Science 64 2000 161–169 163
given slightly post-ovulation on the development and function of the corpus luteum and the length of the oestrous cycle in cattle.
2. Materials and methods
