The average pregnancy rate in this work
The average pregnancy rate in this work was 60.1%; similar results have been reported with the same estrus synchronization treatment (58% pregnancy rate) . Later studies also showed that the pregnancy rate was 56.9 and 51.1%, with FTAI and estrous detection, respectively [34,35]. Although that the pregnancy rate in the present study was similar to the reference values, treatments with GnRH or hCG did not improve the pregnancy rate. Administration of hCG reduced pregnancy losses on Day 33 post FTAI compared to GnRH and Control groups. It was recently proposed that hCG treatment used at early luteal phase in sheep showed a positive effect on the reduction of pregnancy losses. It was probably related to the formation of accessory corpora lutea and the increase of serum P4 concentration observed after the administration of hCG on days 4 post FTAI . In addition, other researchers showed that hCG treatment improved luteal function and pregnancy establishment [27,29]. It could be hypothesized that the luteotrophic action of hCG produce an increase of P4 concentration required for an adequate intrauterine environment, improve embryonic survival and therefore reduce embryonic losses in ruminants. In the present study, GnRH or hCG administration did not increase embryo CRL. In fact, previous studies in sheep showed that GnRH administration on Day 12 post AI did not affect growth and fetal or embryo weight [11,33]. In contrast, Cam and Kuran  demonstrated that hCG administration on Day 12 post mating increase weight and fetal CRL. A later study also showed that the administration of GnRH or hCG increased embryo weight and CRL . The reasons for the stimulatory effects of GnRH or hCG treatment on conceptus growth are unclear. However, it seems unlikely that GnRH or hCG can directly stimulate conceptus growth, because there is no evidence to suggest that the ovine conceptus has S1RA for GnRH or hCG . The results of the present study showed that GnRH administration on Day 4 post FTAI marginally improved litter size when compared to the hCG and Control groups. This agrees with previous results that reported an increase in litter size after GnRH treatment at Day 11 or 13 post AI . In contrast, the administration of GnRH or hCG post AI showed no increment in litter size [11,12]. In sheep it is believed that buserelin causes the development of endometrial oxytocin receptors to be delayed by reducing oestrogen concentrations. Luteal oxytocin binds with these receptors to promote the formation and secretion of prostaglandin. Any delay in the release of PGF2α would allow an underdeveloped embryo extra time to produce and release the maternal recognition of pregnancy signal, which might otherwise be lost following luteolysis . The beneficial effect of GnRH or hCG administration on luteal function may be exerted through the effect of P4 on changes in the uterus necessary for embryo or fetal growth and development [38,39]. However, in the current study single lamb weight did not increase after treatment with GnRH or hCG, as has been reported after GnRH administration . In contrast, other authors showed an increased single lamb birth weight after GnRH or hCG administration [13,21,40]. Whereas, twin lambs birth weight tended to increase by 400 g in GnRH treatment compared to Control group. It is possible that hormonal treatment with GnRH prevented twin embryos death by stimulating fetal growth . Although ram fertility and body condition score on the pregnancy rate were not initially considered variables in the study, interesting results were found on the fertility rate. The variation (15% in the conception rate) in male fertility is often observed when AI is used in bovine reproduction systems . In this study, hCG treatment in ewes inseminated with high fertility rams increased pregnancy rate by 41.5% points (pp) compared to ewes inseminated with low fertility rams, while the difference was lower in GnRH and Control groups (19.0 and 3.8 pp, respectively). Although the proportion of rams according to ram fertility was similar in each group, these differences could be due to variable rates of embryonic loss between rams . It is noteworthy that the effect of male fertility as a factor to be considered in hormonal treatments post-AI on reproductive efficiency in ewes has not been found in previous literature.