Sexual experience and temperament affect the response of Merino ewes to the ram effect during the anoestrous season.

In seasonally anoestrous ewes of many breeds, the introduction of rams triggers an increase in gonadotrophin secretion that induces ovulation, a phenomenon known as the 'ram effect'. The ram effect is a practical method for mating ewes outside the natural breeding season, and also can provide synchronised lambing, but the variability of the response, especially in young animals, reduces its potential for widespread application. The aim of our study was to assess two factors that are thought to contribute to the variability in young ewes: temperament and sexual experience. We used anovulatory ewes from a flock that had been genetically selected for 'calm' or 'nervous' temperament and compared the endocrine and ovarian responses to the ram effect in four groups (each n=15): 'calm' and parous (3-6 years old); 'calm' and nulliparous (2 years old); 'nervous' and parous; and 'nervous' and nulliparous. Parous ewes, independently of their temperament, exhibited a faster endocrine response and a higher proportion of females cycling after ram introduction than nulliparous ewes. 'Nervous' ewes exhibited a higher proportion of females cycling after ram introduction than calm ewes, but only in the nulliparous group. We conclude that temperament exerts little influence on the response to the ram effect in sexually experienced ewes, and that females of 'nervous' temperament appear to respond better when sexually 'naive'. Both sexual experience and temperament need to be taken into consideration when flock management involves the ram effect. Finally, some ewes were cyclic at ram introduction, yet exhibited an increase in LH secretion even in the presence of high concentrations of progesterone. The mechanism by which the inhibitory effect of progesterone on LH secretion was bypassed needs to be clarified.

Can audio-visual or visual stimuli from a prospective mate stimulate a reproductive neuroendocrine response in sheep?

Stimuli from a prospective mate increase the secretion of luteinising hormone (LH) in sheep. This 'male effect' in ewes and 'female effect' effect in rams is predominantly mediated by olfactory signals, though it is thought that non-olfactory signals play synergistic or substitutive roles. In this study, we tested whether exposure to visual or audio-visual stimuli from a prospective mate would stimulate an increase in LH secretion in ewes (Experiment 1) and rams (Experiment 2). In Experiment 1, groups of eight Merino ewes were exposed to one of three stimuli midway through a frequent blood-sampling regimen: full ram contact, still images of rams, a video of ewes and rams mating. Control ewes (n = 8) were completely isolated from rams. Exposure to still images of rams appeared to stimulate an increase in mean LH concentrations (P < 0.05) and tended to increase LH pulse frequency (P < 0.1), but the response was significantly smaller than that observed in ewes exposed to rams (P < 0.01). Audio-visual stimuli had no effect on any parameters of LH secretion (P > 0.1). In Experiment 2, Merino rams were allocated to either an Exposure (n = 7) or a Control (n = 7) group. Exposure rams underwent two exposure periods midway through a frequent blood-sampling regimen; exposure to still images of ewes and audio recorded during mating of ewes and rams (audio-visual exposure); exposure to oestrous ewes (ewe exposure). Control rams were sampled at the same frequency but remained isolated from ewe stimuli. Exposure of rams to the audio-visual stimuli did not affect any parameters of LH secretion (P > 0.1). In contrast, exposure to oestrous ewes increased LH pulse frequency (P < 0.05) and advanced the onset of the next LH pulse (P < 0.05). In conclusion, visual signals appear to be involved in eliciting the neuroendocrine response of ewes to rams and are of greater importance to this phenomenon in ewes (male effect) than rams (female effect). However, overall the visual and audio-visual signals used in this study were far less effective than stimulus animals, suggesting that these stimuli are less important than olfactory signals, or a combination of olfactory and audio-visual signals.

Do cyclic female goats respond to males with an increase in LH secretion during the breeding season?

The male effect is currently only used during seasonal or lactational anoestrus because the response is thought to be blocked in cyclic females by periods of elevated progesterone. In this study, we tested whether cyclic, female goats would respond to male exposure with an increase in pulsatile LH secretion. During May (breeding season; Southern Hemisphere) the cycles of 16 Australian Cashmere goats were synchronised using intravaginal progesterone pessaries. Pessary insertion was staggered to produce groups in their early luteal (EL; n=8) and late luteal phases (LL; n=8). The LL group was retrospectively subdivided into mid-luteal (ML; n=4) and late luteal (LL; n=4) groups due to differences in oestrous cycle length that emerged during the study. Male exposure stimulated an increase in LH pulse frequency in the EL and LL groups (P<0.01) but not in the ML group (P>0.1). This increase was accompanied by an increase in basal and mean concentrations of LH in the LL group (P<0.05) but not in EL (P<0.1) or ML (P>0.1) group. There was no effect of male exposure on LH pulse amplitude (P>0.1). Progesterone concentrations differed among all groups on the day of male exposure (P<0.05) and declined significantly over the 12-h sampling period in the LL group (P<0.05). Prolactin concentrations declined in the EL group but did not change significantly in the ML or LL group. In conclusion, male exposure induced an increase in pulsatile LH in goats in the early and late luteal phases of the oestrous cycle. The high concentrations of progesterone in females in the mid-luteal phase appeared to block the male effect.

The introduction of rams induces an increase in pulsatile LH secretion in cyclic ewes during the breeding season.

Application of the ram effect during the breeding season has been previously disregarded because the ewe reproductive axis is powerfully inhibited by luteal phase progesterone concentrations. However, anovulatory ewes treated with exogenous progestagens respond to ram introduction with an increase in LH concentrations. We therefore tested whether cyclic ewes would respond to ram introduction with an increase in pulsatile LH secretion at all stages of the estrous cycle. We did two experiments using genotypes native to temperate or Mediterranean regions. In Experiment 1 (UK), 12 randomly cycling, North of England Mule ewes were introduced to rams midway through a frequent blood-sampling regime. Ewes in the early (EL; n=3) [corrected] and late luteal (LL; n=6) phase responded to ram introduction with an increase in LH pulse frequency and mean and basal concentration [corrected] of LH (at least P<0.05). In Experiment 2 (Australia), the cycles of 32 Merino ewes were synchronised using intravaginal progestagen pessaries. Pessary insertion was staggered to produce eight ewes at each stage of the estrous cycle: follicular (F), early luteal (EL), mid-luteal (ML) and late luteal (LL). In all stages of the cycle, ewes responded to ram introduction with an increase in LH pulse frequency (P<0.01); EL, ML and LL ewes also had an increase in mean LH concentration (P<0.05). In conclusion, ram introduction to cyclic ewes stimulated an increase in pulsatile LH secretion, independent of ewe genotype or stage of the estrous cycle.