Male-induced short oestrous and ovarian cycles in sheep and goats: a working hypothesis

Abstract

-The existence of short ovulatory cycles (5-day duration) after the first male-induced ovulations in anovulatory ewes and goats, associated or not with the appearance of oestrous behaviour, is the origin of the two-peak abnormal distribution of parturitions after the "male effect". We propose here a working hypothesis to explain the presence of these short cycles. The male-effect is efficient during anoestrus, when follicles contain granulosa cells of lower quality than during the breeding season. They generate corpora lutea (CL) with a lower proportion of large luteal cells compared to small cells, which secrete less progesterone, compared to what is observed in the breeding season cycle. This is probably not sufficient to block prostaglandin synthesis in the endometrial cells of the uterus at the time when the responsiveness to prostaglandins of the new-formed CL is initiated and, in parallel, to centrally reduce LH pulsatility. This LH pulsatility stimulates a new wave of follicles secreting oestradiol which, in turn, stimulates prostaglandin synthesis and provokes luteolysis and new ovulation(s). The occurrence of a new follicular wave on days 3-4 of the first male-induced cycle and the initiation of the responsiveness to prostaglandins of the CL from day 3 of the oestrous cycle are probably the key elements which ensure such regularity in the duration of the short cycles. Exogenous progesterone injection suppresses short cycles, probably not by delaying ovulation time, but rather by blocking prostaglandin synthesis, thus impairing luteolysis. The existence, or not, of oestrous behaviour associated to these ovulatory events mainly varies with species: ewes, compared to does, require a more intense endogenous progesterone priming; only ovulations preceded by normal cycles are associated with oestrous behaviour. Thus, the precise and delicate mechanism underlying the existence of short ovulatory and oestrous cycles induced by the male effect appears to be dependent on the various levels of the hypothalamo-pituitary-ovario-uterine axis.male-effect / ovulation / corpus luteum / cycle / oestrus / uterus

Reproductive performance of Lacaune dairy sheep exposed to artificial long days followed by natural photoperiod without and with additional progestagen treatment during the nonbreeding season

“…After introduction of the ram in a flock of anovulatory ewes, a twopeak pattern of estrous activity between 18 to 20 days and 24 to 26 days after joining has been reported in the literature [3,12]. In this study, however, estrous activity was noted in six ewes already within 4 days and in the remaining 29 ewes between 7 and 24 days after joining.…”

“…breeds [3], but similar to goats [17,18], estrous behavior can be expressed without a previous luteal phase of normal duration.…”

“…However, the high estrous activity around Day 10 after joining indicates that ovulation did occur in most ewes immediately after exposure to the rams and that the following luteal phase was shortened. As it is known that the occurrence of short luteal cycles is negatively correlated to the percentage of females cycling before joining [3] and because persistently low P4 levels were measured at the end of the light program in more than 70% of the ewes, it can be assumed that most of the sheep were noncyclic before. In a study [16] performed 1 year before with the same flock in spring, nearly half of the pluriparous ewes showed elevated serum P4 concentrations.…”

“…In sheep and goats, the introduction of males in a flock of anestrous females provokes an increase in LH pulsatility followed by synchronized ovulations [3]. This phenomenon, called "male effect", works well in Merino sheep for induction of fertile estrus in the nonbreeding season; however, in other breeds, it is less effective [2].…”

Environmental and social cues can be used in combination to develop sustainable breeding techniques for goat reproduction in the subtropics

“…We did not test the effect of these treatments in females because, in subtropical Mexico, most local goats remain in precarious open pens at night, making it difficult to expose them to photoperiodic treatment. In females, it was rather decided to stimulate their sexual activity by acute exposure to males, a technique known as the 'male effect' (Chemineau et al, 2006;Delgadillo et al, 2009;Bedos et al, 2010). Indeed, exposure of anestrous does or ewes to rams or bucks, respectively, induces the rapid activation of LH secretion leading to estrous behavior and ovulation.…”