Bioscientifica Proceedings

Maturation processes that culminate in puberty and sexual maturity are initiated before birth, continue through prepuberty (> 50 days before puberty) and peripuberty (50 days before puberty) and are completed early after puberty. The hypothalamus is the primary site of change during transition to sexual maturity. Maturation of the hypothalamus results from decreased negative feedback of oestradiol that leads to increased frequency of release of LH pulses. Increased tonic r...

This article reviews experiments performed to investigate the importance of thyroid hormones to the expression of the seasonal reproductive cycle of ewes. Thyroidectomy was found to block the transition from the breeding season to anoestrus and to cause ewes to exhibit oestrous cycles all year round. Mechanistically, thyroidectomy produced this effect by preventing the seasonal increase in responsiveness to the negative feedback action of oestradiol on episodic GnRH secretion,...

The pineal gland has a major role in the translation of scotophase duration into a hormonal signal by the hormone melatonin. Animals such as sheep, goats and deer use the seasonal variation of this signal to coordinate reproductive behaviour with the environment. Despite intensive research over more than 30 years the site(s) of action of melatonin and the resultant intracellular responses are still not clear. This review discusses recent work that has localized the site of act...

Changes in the nutrition of mature rams and goat bucks lead to profound responses in testicular size and therefore the rate of production of spermatozoa. These effects are largely due to changes in the size of the seminiferous tubules and in the efficiency of spermatogenesis. With the exception of severe undernutrition, the effects on spermatogenic function are not accompanied by similar changes in endocrine function of the testes, as measured by the production of testosterone...

Transport of post-partum cows or sheep before an oestradiol-induced LH surge delayed gonadotrophin secretion possibly by affecting hypothalamic activity but not via an opioid mediated mechanism as the effect could not be reversed by naloxone. In addition, reduced LH responses to GnRH were observed in cattle during transport. In sheep, adrenocorticotrophic hormone (ACTH) also diminished the LH response to GnRH, but only when GnRH was administered 3 h after ACTH, not after 0.5 h...

A major limiting factor in the resumption of postpartum ovarian cycles in cattle is the inhibitory influence of the suckling calf on central regulatory elements controlling the release of GnRH from the median eminence. This inhibitory influence occurs only as a consequence of specifically defined behavioural interactions between the cow and calf in the presence of a maternal bond, cannot be simulated experimentally using thermal, electrical or mechanical stimuli, and is not de...

The focus of this review is the physiological effects of undemutrition on ovarian follicle growth and the occurrence of ovulation in postpartum cows. Evidence suggests that moderate levels of underfeeding, before or after calving, may interfere with the mechanism(s) of final follicle maturation and ovulation, whereas more pronounced nutritional deficiencies may affect the mechanism(s) regulating dominant follicle size and the dynamics of dominant follicle growth and regression...

We have used a model to study infertility in postpartum cows. In mated cows with short luteal phases, daily supplementation with progestagen, beginning on day 3, failed to maintain pregnancy, despite the fact that fertilization, early embryo development and transport of the embryo into the uterus appeared to be normal. Normal embryos were transferred, on day 7 after oestrus, into cows with short luteal phases that received daily supplementation with progestagen, and embryos fr...