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    • Many studies support that the increase of

    2022-08-09

    Many studies support that the increase of P4 improves embryonic survival and reduces embryonic losses in ruminants [6,[18], [19], [20]]. However, experimental results are variable, contradictory and often inconclusive results. Certain studies showed that administration of GnRH or hCG after artificial insemination (AI) or natural mating improves pregnancy rate [11,13,21] and stimulates bcl-xl inhibitor or fetal growth [19,22]. Furthermore, the administration of hCG or GnRH in the luteal phase decreases embryonic losses and increases litter size and birth weight lambs [9,23,24]. However, some researchers have not reported improvements in pregnancy rate, fertility or litter size [7,12,14]. Therefore, it is important to evaluate the potential and efficacy of therapeutic strategies on reproductive parameters in each ovine productive system. The hypothesis of this study was that treatment with GnRH or hCG at Day 4 post fixed time artificial insemination will improve pregnancy rate and litter size and reduce pregnancy loss in sheep. The objective of this study was to determine the effect of the administration of GnRH or hCG at Day 4 FTAI on pregnancy rate, pregnancy loss, embryo size, litter size and birth weight of lambs in Merino sheep during the reproductive season in Northern Patagonia, Argentina. We also evaluated the interaction between hormonal treatment and ewes body condition score or ram fertility, on the pregnancy rate.
    Material and methods
    Results Results on performance reproductive in the GnRH, hCG and Control groups are presented in Table 1. There was no difference between experimental groups in pregnancy rate on Days 33 and 90 post FTAI (P > 0.05). Pregnancy losses on Day 33 post FTAI were lower in the hCG group compared to GnRH and Control groups (P < 0.05). Pregnancy losses from Day 33 to Day 90 after FTAI were negligible (P > 0.05). Embryo CRL on Day 33 post FTAI was not increased in GnRH and hCG groups compared to Control group (P > 0.05); however, it was lower in GnRH group compared to controls (P < 0.05). Litter size tended to be greater in the GnRH group compared to the hCG and Control groups (P < 0.1). The birth weight of twin lambs in the GnRH group tended to be higher than in the Control group (P < 0.1). Birth weight of single lambs did not differ between groups (P > 0.05). There was an effect of rams used in the FTAI on pregnancy rate (46.5, 59.6, 67.6, 69.4 and 76.0% for rams A, B, C, D, E, respectively; P < 0.05). Due to this effect, ewes were grouped into those inseminated with high or low fertility rams used in the FTAI (high, > 65%, n = 3; low, ≤ 65% of pregnancy rate, n = 2). There was a significant difference in the pregnancy rate between ewes inseminated with high or low fertility rams in the hCG group (P < 0.05; Table 2). However, this difference was not observed in the GnRH or Control groups (P > 0.05). In addition, ewes were grouped into high or low BCS at FTAI (high, > 2; low, ≤ 2; Table 3). Pregnancy rate in ewes with high BCS was not different between experimental groups. However, ewes with low BCS and treated with GnRH had higher pregnancy rate than ewes treated with hCG and control ewes (interaction of BCS by treatment; P < 0.05). The pregnancy rate in ewes with high BCS tended to be higher compared to ewes with low BCS (52.4 and 60.2%, respectively; P < 0.1).
    Discussion The results of this study do not support the hypothesis that GnRH or hCG treatment at Day 4 after insemination improves pregnancy rate. In agreement, Fukui et al. [7] reported that administration of hCG on day 4 after insemination, did not improve fertility. Other authors observed similar results after the application of GnRH, hCG or eCG at mid-luteal phase [14,29]. In contrast, other researchers observed an increase on pregnancy rate by 10–20% when ewes were treated with hCG or GnRH at mating or in luteal phase (7, 11 or 13 Days post mating or AI) [10,13,19,21,[30], [31], [32], [33]]. Variability between results could be attributed to different protocols, management systems, nutritional status or physiological status resulting from diverse experimental conditions. Furthermore, may indicate that breed, timing of treatment or doses used in these treatments produce critical effects on fertility rate and embryo survival.