The development of this reproductive biotechnology in sheep has had a similar development in the past, especially in important breeds of sheep and goats (like the Suffolk breed). Small ruminant ET is a well described and yet underexploited animal breeding technology. The size of sheep and goats, aspects of their anatomy and seasonal reproductive behavior, present challenges not common to cattle.
Those considerations have not deterred serious breeders and ET practitioners in sheep and goat producing countries. The success of an ET protocol in sheep depends on many factors, but in the end, what matters is the number of embryos obtained. Recovery rate is an essential step in ET. Biotechnology of Embryo Transfer is applied to females of superior genetic and aims to increase the frequency of their genes by increasing their progeny. ET allows the transfer of embryos from superior females (donors) to recipients with low genetic value or embryo freezing for later use. Obtaining embryos is influenced by the development dynamics of the ovarian follicles, but also by their ovulation [1].
Response to superovulation: approximately 25% of programmed donors will not respond to superovulation treatments. Some never respond, some may respond on a subsequent program. If the donor responds, she may produce from 1 to over 30 embryos, with average 8 to 12 depending on the breed, time of year, condition of animal [3, 9, 15].
The superovulatory ovarian response to our donors was 83.3%, a fairly high percentage given that the medication used was long lasting and used several hormones involved in reproductive function. The result is at the upper limit of those published by other authors in similar experiments [3, 5, 12, 21]. A 77.8% of females showed ovulation with a mean of 9.6 CL and 3,3 viable embryos [10].
In such an ET Direct/in vivo program, (donor flushing and embryo transfer in the same day), it is recommended that 8 to 10 recipients per donor be synchronized to ensure that at least 6 to 8 recipients will be suitable for direct embryo transfer. The number of recipients initially programmed per donor may be adjusted if there is prior information about the expectation of donor response and potential embryo yield [17].
An effective estrus synchronization regimen (recipient group) is expected to synchronize the estrus of the treated animals within a 12 to 24-h period, stimulate high rates of estrus and ovulatory response, and enable the achievement of a high pregnancy rate. The most common methods used for the hormonal control of the estrus cycle in ewes include the establishment of an artificial corpus luteum (CL) function through the administration of progestogens for a certain time period, the stimulation of luteolysis by means of the administration of luteolytic agents, and the synchronization of ovulation by means of the combined administration of gonadotropin-releasing hormone (GnRH).
In this method, the function of the corpus luteum is simulated by application of analogous progesterone compounds. The release of gonadotropins is inhibited by progesterone, and, hence, the ovulation is also inhibited until progesterone is removed (figure no 2). Is applied for a group of females and withdrawn simultaneously, this will synchronize the estrus and ovulation in recipient group.
Progesterone was initially delivered for a period equal to the length of the natural luteal phase. There are various administration time protocols such as: Long-term progesterone treatments (18 to 21 days), and Short-term progesterone treatments (7 to 12 days). Therefore, it is crucial to include a luteolytic agent in combination with short-term progesterone treatments in order to get rid of any natural corpus luteum. This technique is applicable for cycling and non-cycling ewes during the breeding and non-breeding season, but in this case, ovulation induction is required e.g. administration of 200 IU eCG.
An injection of PGF2α, or one of its analogs, during the mid-luteal phase of the estrus cycle can induce a premature CL regression and ewes, therefore, can be expected to exhibit estrus symptoms approximately 50 hours later.
Following the hormonal protocol in the local sheep breeds (cross breed Țurcana, Țigaia) the growth of mature follicles and synchronization of estrus was induced. We consider that the veterinary drugs that were administered to recipients (P4, PGF-PMSG) provided a favorable response (estrus and ovulation synchronization) of 84% (42/50), Synchronization was perfect in 92.9% (39/42) of recipients in heats.
Worldwide, the number of IVDs in sheep increased in 2019, compared to 2018, 22.374 vs 17.353 (+ 28.8%). Among European countries, Spain reported the highest number of IVD embryos (111), with an average of 9.25 embryos / protocol, followed by Portugal with 18 embryos, an average of 6 embryos / procedure and the UK with a number of 905 embryos and an average of 5.45 embryos / procedure. In 2019 the number of IVD embryos in N America, S America has increased, compared to 2018 reports, 8295 vs 2158 (+ 73,98%) in N America, and 10196 vs 5239 (+ 48,62%) in S America. In Australia there was a slight decrease in the number of IVD embryos, from 2970 in 2018 to a number of 2805 in 2019 (− 5. 56%) [5].
These studies show that the veterinary drugs used to stimulate follicular growth, induce estrus and synchronize ovulation in sheep, are topical and in increasing use worldwide.
The superovulation of Black Suffolk ewes may be affected by the seasonal changes. Generally, the ewe's ovulation rate was higher in May, whereas the viability rate of embryos was higher in September [21]. In another similar study CR in Suffolk was 59% [8].
Results indicate that during the late compared to peak breeding season, there is an increased incidence of fertilization failure as a possible consequence of seasonal shifts in LH secretion and (or) associated effects on follicular function. Frozen-thawed embryos produced at contrasting stages of the breeding season are equally viable in vivo but those produced during the late, as opposed to the peak breeding season have lower viability following in vitro culture [18].
This study sought to evaluate the ovarian response to superovulation protocol (SOP), embryo recovery rate, in vivo embryo production (IVD) in Suffolk sheep acclimatized in Romania as well as conception rate (CR) in local recipients. Results proved that the MOET protocol applied had a positive effect on the production of in vivo embryos (IVD) in Suffolk sheep and can guarantee the success of ET activity to ewes with lower genetic merit.