Control of seasonal reproduction in sheep and goats by light and hormones

Abstract

Seasonal variations of reproductive ability have been demonstrated in male and female sheep and goats. For the female, there is a distinct breeding season. Whilst there are large breed differences in the duration of the sexual season, oestrous cycles generally start when daylength is decreasing and end when daylength is increasing. In some breeds, ovulation without oestrus occurs during the non-breeding season. Although males are able to mate all the year round, seasonal variations occur in the weight of the testis and seminal vesicles and in the fructose concentration of the seminal vesicles. Sexual behaviour and semen characteristics also show seasonal variations. These seasonal variations in males and females are related to hormonal levels. FSH and LH concentrations in the pituitary are reduced to 50% during the non-breeding season. In the peripheral blood, LH concentrations vary throughout the year whilst prolactin concentrations follow the pattern of daylength. Steroid hormones and their feedback actions at the hypothalamo-pituitary axis play a role in the regulation of seasonal reproduction. Three different ways are proposed to overcome these seasonal variations. (1) Males can be introduced into a flock of females before the onset of the breeding season. This leads to induction of ovulation and oestrus, although the first ovulation is frequently followed by a short luteal phase. Teasing does not necessitate contact or sight and is only effective after a period of isolation of the females from the males. (2) Oestrus and ovulation may be induced during the anoestrous period by hormonal treatments. Inducers of LH release have to be associated with a progestagen treatment; PMSG is still the most efficient inducer of LH release. Artificial insemination is useful in this context to avoid subfertility due to seasonal variations of the male. (3) The period of reproduction may be controlled by artificial light regimens. There is a photoinducible period in the circadian cycle of the ewe and it may soon be practical to breed out of season by timed exposure of ewes to short periods of light during the normal hours of darkness. Reproduction in wild animals is often linked to the seasons to ensure that the young are born at a time which maximizes their chance of survival. In domestic animal reproduction, however, seasonality of breeding is a limiting factor because farmers can provide additional food during periods when natural food is short and build shelters to protect the mother and young from the elements. A knowledge of the biological mechanisms determining seasonality is therefore important for the management of domestic sheep and goats.

Regulation of uterine function during estrous cycle, anestrus phase and pregnancy by steroids in red deer (Cervus elaphus L.)

“…An essential finding reported here was the marked seasonal variation of LH, FSH, P4, E2, and T4 concentrations in hinds’ blood plasma as described before 39 . Our results have clearly shown that LH concentration in hinds’ plasma was at a similar level during all analyzed seasons, but we did not analyze the preovulatory outflow period.…”

“…Yuthasastrakosol et al 40 noted the high LH levels in anestrus in ewes, and they suggested an association of these values with the increase in P4 levels. Low levels of P4 during anestrus in goats and high levels of P4 in the luteal phase in sheep, during the middle of reproductive season, were observed by Thimonier 39 . In turn, Li et al 9 did not observe a luteal P4 peak in sheep during anestrus.…”

Subtropical goats ovulate in response to the male effect after a prolonged treatment of artificial long days to stimulate their milk yield

“…In some small ruminants, such as goats and sheep, animals restrict their breeding season to a short period of the year, allowing births to occur during favourable environmental temperatures and high availability of pastures (Bronson, ). The reproductive seasonality in goats and sheep is mainly modulated by changes in the day length; natural decreasing or artificial short days stimulate ovulations, whereas natural increasing or artificial long days inhibit it (Thimonier, ). In goats from subtropical regions, ovulation is always stimulated during artificial short days (10 hr light/day) when exposed to alternations between 3 months of long and 3 months of short days (Duarte, Nava‐Hernández, Malpaux, & Delgadillo, ).…”

Studies of the GnRH pulse generator. Developmental and location-specific regulation of GnRH secretion

“…The GnRH pulse generator is a critical mechanism that underlies cyclical changes in the HPG axis activity that helps drive the female reproductive cycle as well as male reproductive function, which is not cyclic, although it may undergo seasonal changes (57,58). Pulse generation appears to be an integration point for various inputs and mechanisms involved in the role of CNS in regulating fertility, but its composition and mechanisms underlying coordinated production of GnRH pulses remain poorly understood.…”