The effects of ram exposure during progestagen oestrus synchronisation and time of ram introduction post progestagen withdrawal on fertility in ewes

Abstract

Three experiments were undertaken to investigate the effect of a pre-mating ram exposure during progestagen synchronisation treatment on time of breeding, ovulation rate, embryo quality and fertility and any interaction with time of ram introduction for breeding post sponge withdrawal. Crossbred ewes in experiment 1a (n = 348), 1b (mule; n = 133) and 2 (n = 58) underwent a 12–14 days synchronisation protocol. Three days prior to sponge withdrawal ewes were divided into Control (ewes in continued isolation from rams) or +Ram (ram-exposed) groups. Rams were introduced to +Ram ewes and remained with ewes until sponge withdrawal. Ewes in experiments 1a and 2 received eCG at sponge withdrawal and were reintroduced to rams at either 36 or 48 h post sponge removal (PSR). In experiment 1b, ewes did not receive eCG and were reintroduced to rams at 24 h PSR. In experiments 1a and 1b time of breeding, date of lambing and litter size were recorded. In experiment 2, ewes were slaughtered 5 days post breeding, reproductive tracts flushed and corpora lutea, ova and embryos assessed. Fewer +Ram ewes were mated by 96 h PSR (P < 0.001) than Control ewes in experiment 1a but not when rams were introduced earlier in experiment 1b. In experiment 1a, ram introduction at 36 h PSR improved conception to first service compared to introduction at 48 h PSR (P < 0.01) in both +Ram and Control groups. In experiments 1a and 1b, +Ram ewes had reduced litter size caused by more single births (1a; P < 0.001, 1b; P < 0.01). In experiment 2, +Ram ewes had fewer corpora lutea than Control ewes (P < 0.001) but embryo quality was similar. However, more good embryos were produced when rams were introduced for breeding at 36 h compared to 48 h PSR (P < 0.001). We conclude that a pre-mating ram exposure during the synchronisation treatment reduced the number of ewes mated at and conceiving to the first service. This was partially overcome by introducing rams for breeding earlier (24 or 36 h compared to 48 h PSR) but the most dramatic decrease in fertility was due to a reduction in ovulation rate in the ram-exposed ewes.

Introduction

Intravaginal progestagen sponges are commonly used to synchronise the oestrous cycles of ewes during both the breeding and non-breeding season [1]. The sponge mimics the action of a corpus luteum through provision of an artificial source of progesterone sufficient to suppress gonadotrophin production. Removal of the sponge removes the progestagen block and induces synchronous re-instatement of gonadotrophin release and subsequent ovulation in treated ewes.

Exposure of previously isolated anoestrous ewes to a ram induces an instantaneous rise in luteinising hormone (LH) pulse frequency [2], [3], [4]. 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 [3]. The ram effect is mediated predominantly through olfactory cues [5] with maximal affect through a combination of olfactory, behavioural, visual and audible stimuli [2], [4], [6]. The influence of the ram upon LH secretion and the application of this response to out-of-season breeding are well documented within the literature [3], [7]. However, less is known regarding the influence upon or possible incorporation of the ram effect with progestagen synchronisation treatments.

The majority of research investigating the interaction between exposure to rams and controlled breeding programmes has focused on the influence of the ram after removal of the artificial progestagen [8] and have been undertaken predominantly during anoestrus [9]. Several studies have identified an earlier onset of oestrus and shorter duration of oestrus as a result of continuous ram presence post sponge removal [9], [10], [11].

Previous work has shown that ewes under the influence of an artificial progestagen show an increase in LH pulse frequency in response to ram introduction [12]. The persistent endocrinological ability of the ewe to respond to the ram, even under the influence of the artificial progestagen, denotes a possible novel opportunity to influence follicle development prior to sponge withdrawal. Previous work in cattle has shown that removal of the progestagen block at the end of the synchronisation protocol does not reliably result in a synchronous onset of ovulation due to varying stages of follicle development at sponge removal [13]. Similar observations have been made in sheep synchronised during the breeding season, due to an impact of stage of the oestrous cycle at sponge insertion on follicle size at sponge withdrawal [14].

We had originally hypothesised that a ram-induced increase in LH prior to sponge withdrawal may result in uniform growth of follicles and an optimum stage of follicle development at sponge removal, due to the importance of LH in follicle deviation and growth and development of the ovulatory follicle(s) [15]. Therefore, within a previous study [16], we exposed ewes to rams during the last 3 days of a progestagen synchronisation programme, undertaken during the breeding season, and examined the impact of this pre-mating ram exposure on LH concentrations (whilst still under the influence of the progestagen) and the timing of the LH surge, oestrus and ovulation post sponge withdrawal. The study [16] did identify a significant increase in LH concentrations in response to the pre-mating ram exposure and the ram-exposed ewes had a more rapid onset of oestrus, shorter oestrous period and earlier LH surge and ovulation compared to Control ewes isolated from rams prior to breeding. However, the number of ram-exposed ewes mated at and conceiving to the synchronised service was lower than that of control ewes and ram exposure appeared to result in an overall depression in lambing percentage induced by significantly fewer multiple births. We suggested that the LH response induced by the pre-mating ram exposure had accelerated follicle development, as hypothesised, but that this had resulted in some of the ewes reaching the end of their fertile oestrous period when the rams were introduced for breeding at 48 h post sponge removal [16]. We speculated that the reduction in lambing percentage and overall reproductive performance may have occurred due to a negative effect of the pre-mating ram exposure upon the functional competency of the oocyte, embryo or corpus luteum [16].

The objectives of the studies within this paper were to investigate further the negative effects of a pre-mating ram exposure (during the last 3 days of an artificial synchronisation protocol) upon fertility. Specifically, our aim was to determine the impact of this type of ram exposure on time of mating and subsequent fertility and any interaction with time of ram introduction for breeding at 24, 36 or 48 h post sponge withdrawal. Furthermore we hoped to determine the cause of the previously observed [16] depression in litter size by investigating the effect of this type of pre-mating ram exposure upon ovulation rate and embryo quality and any interaction with time of ram introduction for breeding.