The role of socio-sexual cues in sheep reproduction

Abstract

Exposure of previously isolated anoestrous ewes to a ram induces an almost instantaneous rise in luteinising hormone (LH) pulse frequency. This physiological response, a phenomenon coined 'the ram effect' is commonly sufficient to override the seasonal suppression of the hypothalamic-pituitary axis and induce a synchronous first ovulation. The objective of the first series of experiments in this thesis was to develop a nonpharmacological method of oestrus synchronisation, using socio-sexual cues, for natural mating of mule ewes during the breeding season. Initially two experiments were conducted to investigate the effect of short-term fence line and vasectomised ram exposure repeated every 17 days on three occasions during the transition into the breeding season. Ewes repeatedly exposed to the ram had a significantly compacted mating period compared to ewes maintained in isolation from rams prior to mating. This compaction persisted through to lambing with no significant negative effect on litter size. Artificially inseminated ewes synchronised using the above method of ram synchronisation had higher conception rates than progestagen synchronised ewes. The second objective was to compare the efficacy of different durations and frequencies of ram exposures as methods of oestrus synchronisation. Ewes maintained continuously with rams over the pre-mating period had a more compacted mating and lambing period than ewes exposed intermittently to rams. Maiden ewes typically show a poorer level of reproductive competence than adult ewes. Similarly maiden ewes induced to ovulate using the ram effect have been found to have a lower ovulatory response. The next objective of this thesis was to determine if pre-exposure to the ram during anoestrus or the breeding season would modulate the hormonal and behavioural responses of maiden ewes when re-introduced to rams during the breeding season or anoestrus. There was no major effect of prior experience of the ram on any parameters of the LH response to ram introduction. However ewes with prior ram experience did have more positive interactions with the rams and demonstrated more ram seeking behaviour. Incorporation of socio-sexual cues with artificial methods of reproductive control has to date been restricted to ram exposure post progestagen sponge withdrawal. Therefore the final objective of this thesis was to investigate the effect of ram exposure towards the end of a progestagen synchronisation protocol on ewe fertility. There was no significant difference in conception rates between ram exposed and control ewes, however ram exposed ewes had a significant depression in mean litter size due to a greater number of ewes having single lambs. The studies in this thesis show a robust and repeatable endocrine response to ram introduction in mule ewes exposed to the ram during the transition between anoestrus and the breeding season. The potency of the socio-sexual cues from the ram permits modification of the distribution of oestrus within randomly cycling ewes. The findings in this thesis highlight the potential for application and development of pre-mating strategies using socio-sexual cues within seasonal breeds of sheep.