Estrus Characteristics of Menz Breed ewes and Effect Of Ostrus Cycle Phases at Initiation of Ovulation Synchonization On Pregnancy Rate
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Date
2024
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Addis Abeba University
Abstract
The study was conducted from January to April 2024 with twofold objectives: to characterize estrus signs and the time of occurrence of estrus events in natural and PGF2α-induced estrus; and to assess the effect of estrus phase at the initiation of ovulation synchronization on plasma progesterone, plasma estradiol, and pregnancy rate in Menz breed ewes. To attain the objectives, two experiments were conducted. Briefly, in the 1st experiment, ewes (n = 12) were kept with vasectomized ram to get data from natural estrus, after which ewes were estrus induced by PGF2α to gather induced estrus data. In the 2nd experiment, ewes (n = 12) were estrus induced by PGF2α, and when ewes showed estrus (taken as D0 of the experiment), they were divided into two groups (n = 6 per group),
balancing for age, live weight, and parity. In the 1st part of experiment two, ewes were treated with intramuscular (IM) injection of 50µg GnRH agonist on day 16th of estrus (at follicular phase of estrus, designed as follicular group); at day 7 of GnRH injection, ewes were IM injected with 250µg PGF2α; and at 48 hours of PGF2α (day 9), ewes were IM injected with 50µg GnRH. In 2nd part of experiment two, ewes were treated in a similar protocol as in the 1st part of experiment two, but the 1st GnRH injection was started on the 8th day of the estrus (at the luteal phase of the estrus, designed as a luteal group). In both protocols, ewes were kept with a vasectomized ram, and they were bred by Dorper rams standing estrus. The behavioral and physical signs of estrus were similar to those of natural and induced estrus. Estrus events were not affected (P > 0.05) by estrus source, although mean estrus duration was slightly longer in natural estrus (34.1±2.56 hrs) than induced estrus (32.85±3.540 hrs). The mean duration of natural estrus, interval from the onset of estrus to standing estrus, duration of standing estrus, and interval from standing estrus to estrus end were 34.1±2.56hrs, 17.14±3.52hrs, 4.3±0.5357hrs, and 12.07±1.08hrs, respectively. The mean estrus cycle length was 15.46±1.22 days. The luteolysis rate to PGF2α was 100% in the follicular group within 24hrs of PGF2α injection (P4 changed from ≥1ng/ml to <1ng/ml within 24hrs), however, in the luteal group, all ewes underwent spontaneous luteolysis before PGF2α (P4 <1ng/ml on D7). All ewes in the luteal group did not ovulate to D0 GnRH (P4 remains ≥1 ng/ml) while in the follicular group all ewes were spontaneously ovulated before D0 GnRH (P4 <0.5ng/ml at D0 GnRH). In the luteal group, the ovulation rate to D9 GnRH was 83.33% (P4 changed from >0.5 ng/ml to P<0.5 ng/m), while in the follicular group, all ewes were ovulated (100%) to D9 GnRH. The mean ±SE P4 concentrations were 0.448 ± 0.325 ng/ml at D0 and gradually increased to IX 1.348±0.162 ng/ml on D7 in the follicular group; however, the mean ±SE P4 concentrations were 2.464 ± 0.355 ng/ml at D0 and gradually decreased to 0.252±0.103 ng/ml on D7 in the luteal group. Mean plasma E2 concentration was significantly lower (P<0.05) on D7 (15.48±0.87pg/ml in luteal vs. 23.84±2.641pg/ml in follicular) and D8 (16.685±0.95pg/ml luteal vs. 28.104±2.93pg/ml in follicular, P<0.05) luteal group than follicular group. The pregnancy rate was significantly lower (50%) in the follicular group than in the luteal (83.33%) group (P<0.05). Based on this preliminary study, it could be concluded that the estrus phase at the intuition of ovulation synchronization affects the pregnancy rate in Menz ewes. Moreover, when the protocol was started on the 16th day of estrus (follicular group), ewes were already ovulated, which led to a plasma average P4 level below mid-luteal stage (1.348±0.162 ng/ml) 7 days later (at PGF2α), low pre-ovulatory P4, and a low pregnancy rate. When protocol was started on the 8th day of estrus (the luteal group), ewes underwent spontaneous luteolysis 7 days later (at PGF2α), although the protocol resulted in higher ovulation to D9 GnRH, higher pre-ovulatory P4, and a higher pregnancy rate. To get the best starting day for ovulation synchronization, we recommend a detailed study on the length of estrus stages (pro-estrus, estrus, met-estrus, and di-estrus) and the pattern of corresponding hormonal dynamics per stage (P4, E2, FSH, LH pulse/surge) throughout the estrus cycle of Menze ewes. To overcome the spontaneous ovulation and spontaneous luteolysis encountered in this experiment we recommend a study that asses preferable days to start ovulation synchronization that would provide better pregnancy rate in Menz breed ewes.
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Keywords
: Estradiol, Estrus cycle, Menz ewes, Ovulation, Progesterone, Prostaglandin