Application of Exogenous GnRH in Food Animal Production: History
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AHM Musleh Uddin
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Kiro Petrovski
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Yunmei Song
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Sanjay Garg
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Roy Kirkwood

Exogenous GnRH and agonists have been employed for controlling reproductive cascades in animals, and treating some reproductive morbidities. The administration of GnRH is used in animals to counter ovarian dysfunction, induce ovulation, and to increase conception and pregnancy rates. GnRH and its agonists are used in the treatment of cystic ovarian degeneration and repeat breeder syndrome. The development of protocols for GnRH administration by intramuscular injection, intramuscular or subcutaneous implants, and intravaginal deposition has empowered their clinical use worldwide. 

  • GnRH agonists
  • artificial insemination
  • reproduction

1. Follicular Development and Ovulation Induction

Large doses of exogenous GnRH (100–500 µg) have been administered for the purpose of stimulating acyclic cows, and have shown greater efficacy in dairy cows [1] than in beef cows [2]. Interestingly, Ball [3] reported that these protocols had no effect on the calving to conception interval in dairy cows, while Beckett and Lean [4] found that the administration of GnRH before day 40 post-partum may reduce the time to first estrus but had no relation to the final reproductive performance of dairy cows.
In pigs, follicle growth from about 5 mm to ovulatory size (10 mm) primarily requires LH [5]. Due to the relatively low and batch-dependent LH-like activity, the estrus response to eCG injection has been noted to be dose-dependent. Injected doses of 363 to 600 IU resulted in 25% to 52% gilt estrus rates [6], while the injection of 725 to 1000 IU resulted in 70% to 100% estrus rates [7]. Similar to eCG, optimal GnRH dosages for inducing an ovulatory LH surge are product-dependent, being 10 µg for buserelin [8], 50 µg for gonavet [9], 10 µg fertilan [9], 25 µg for lecirelin [10], and 100–200 µg for intravaginal triptorelin [11][12][13][14][15][16]. Additionally, the efficacy of GnRH for ovulation induction is influenced by follicle size with sows having smaller follicles (<6.5 mm; [17]; <5.0 mm; [18]) at the time of treatment having poorer ovulation responses, presumably reflecting follicular immaturity.

2. Timed Artificial Insemination (TAI)

Timed artificial insemination (TAI) is an insemination timed relative to estrus detection with an assumed or controlled subsequent timing of ovulation, and it is commonly used in animal production [19]. In cyclic females, TAI protocols were established to control luteal and subsequent follicular dynamics to control estrus onset with subsequent satisfactory pregnancy rates. Protocols vary with species, reflecting species-specific follicular dynamics, corpora luteal sensitivities, and treatment logistics. The various TAI protocols involve the administration of exogenous gonadotrophins (equine chorionic gonadotrophin [eCG] and/or human chorionic gonadotrophin [hCG]) to drive ovarian follicular growth, GnRH to drive endogenous LH release, exogenous progesterone/progestin to suppress/synchronize estrus, prostaglandin F2α (PGF) for luteolysis, or combinations of these hormones.
The artificial insemination of cattle after estrus detection resulted in a higher pregnancy per AI (P/AI) compared with an insemination after an ovulation synchronization protocol based on GnRH and PGF alone [20][21]. The stage of the estrous cycle [22][23] and the cyclic status [24] at the time of GnRH administration have also been shown to affect results [25]. Interestingly, due to prolonged luteal function with consequent impaired uterine involution and pyometra, an early resumption of postpartum ovarian cyclicity has been associated with reductions in the fertility of some dairy cows. In attempts to prevent these problems, Padula et al. [26] determined the potential for down-regulating pituitary gonadotrophin release using a deslorelin ear implant, and successfully delayed the resumption of postpartum ovarian cyclicity in recently calved Holstein cows.
In buffalo, GnRH was effective in inducing ovulation in around 80% of cows, making it suitable for use in timed AI [27][28][29][30]. The size of the ovarian follicle was important in timed artificial insemination protocols, as it affected the size of the subsequent corpus luteum, with larger corpora lutea having greater progesterone secretion. This, in turn, increased the chances of maintaining pregnancy and improving fertility in buffalo and cattle [31][32][33][34]. However, while cows have larger ovarian follicle diameters compared to heifers, this did not always result in improved reproduction, as in the older individuals hepatic steroid clearance could affect the result [35][36].
Buffalo pose a challenge for artificial insemination due to weak estrus signs and delayed ovulation [37]. However, there are still opportunities to overcome this challenge by utilizing timed artificial insemination during a specific ovulation window using protocols such as Ovsynch or a progesterone-based approach [38]. Research indicates that ovulation in beef and buffalo cows occurs between 65 and 75 h after CIDR removal, and that knowing this window is crucial for achieving optimal results [39][40]. However, administering GnRH/TAI at 72 h after a 7-day CIDR Co-synch may result in inadequate oocyte development when smaller follicles are subjected to ovulation, which could negatively affect embryonic and fetal survival because oocytes from small follicles have lower developmental competence compared to those from larger follicles [41]. In contrast, buffalo heifers receiving GnRH/TAI at either 72 or 84 h after CIDR removal showed good estrus intensity in 7-day CIDR Co-synch [37]. This is significant because estrus intensity positively affected conception rates in conventional farming systems [42].
The fertility of sows subjected to TAI has been variable, possibly due to variability in the wean-to-estrus interval affecting the synchrony of ovulation with insemination. Indeed, the interval between sperm deposition and ovulation is known to impact sow fertility [43]. It was demonstrated that the 24 h before ovulation was the optimum time for insemination, although the optimum window was also dependent on the age of the inseminated sperm [44]. A single dose of pLH, GnRH, or hCG administered at estrus detection can synchronize ovulation and allow a single insemination with acceptable fertility; there was no difference between treatment and control sows in terms of pregnancy rate and subsequent litter size [45]. Bai et al. [46] allocated primiparous sows to 1000 IU eCG at weaning followed by GnRH at estrus detection or 96 h after weaning, and also had a group of untreated controls. Treated sows were inseminated twice, about 8 and 32 h after GnRH, and control sows were inseminated at estrus detection and 24 h later. More sows receiving eCG were bred. The farrowing rates of bred sows were not different for eCG sows receiving GnRH at estrus detection and controls, but were lower for sows receiving GnRH at 96 h. No effects on subsequent litter sizes were evident [46]. These results lend support to the suggestion that, in sows, insemination without evidence of estrus is more likely to depress sow reproductive performance, with estrus expression being indicative of the presence of larger pre-ovulatory follicles.
Exogenous GnRH agonists in non-injection form, such as intravaginal implants of goserelin acetate or deslorelin [47][48] or a gel containing triptorelin acetate [11][17][49][50][51][52][53], have been used to control ovulation and so improve the management of artificial insemination in swine. Knox et al. [54] used mature gilts following estrus synchronization using the orally active progestogen, altrenogest, to perform a dose–response study with the vaginal deposition of different doses of triptorelin gel administered at different times after the last altrenogest feeding. They noted that the estrus response did not vary among treated gilts but that ovulation intervals were shorter in GnRH treatment groups, and showed the highest ovulation responses when deposited at 120 h after the last altrenogest feeding.

3. Fixed Timed Artificial Insemination (FTAI)

Fixed timed artificial insemination (FTAI) protocols based on exogenous GnRH are commonly used in cows [55][56][57][58], and, to a lesser extent, pigs [45][59][60]. The protocols consist of GnRH administration to initiate LH release and the ovulation of the follicle, and, in sows, insemination without estrus detection, while, in cattle, initiation of renewed follicular dynamics 1–2 days later. After an initial GnRH injection in dairy cattle, PGF2α was administered at 7 days to induce luteolysis, followed by a second dose of GnRH given to provide ovulation synchronization [61][62]; this protocol is known as Ovsynch. A variation, Co-Synch, is usually used in beef cattle because the cows are inseminated at the time of the second GnRH to minimize animal handling [63]. The first GnRH was responsible for a 44–54% ovulation rate in dairy cows [64][65], 60% in beef cows, and 56% in beef heifers [23]. If the first GnRH failed to synchronize a follicular wave, the ovulation following the second GnRH may be poorly synchronized [66], resulting in unexpectedly low pregnancy rates. To increase the number of ovulated cows, pre-synchronization with one or two doses of PGF followed by the first GnRH dose was introduced [67][68]. The efficacy of the pre-synchronization is yet to be firmly established.
FTAI protocols in sows and gilts are used to control ovulation and allow insemination without the detection of estrus while maintaining their fertility. Martinat-Botté et al. [12] treated altrenogest-synchronized gilts with an injection of buserelin at different times following the last altrengest feeding, and sows at different times after weaning. They noted an improved synchronization of ovulation in gilts treated at 104 h compared to 120 h, and in sows treated at 94 h compared to 104 h. Clearly, the timing of treatment relative to degree of follicular maturation will influence the response to GnRH administration. Non-injection GnRH treatment using triptorelin in gel has also been used extensively in sows [69][70][71]. This involves vaginal deposition 96 h after weaning, with insemination 24 h later, and the reproductive performance has been commercially acceptable. Of interest, however, is that while farrowing rates were generally comparable, in sows not exhibiting standing estrus at the time of insemination the farrowing rate was very low [72][73]. The lack of estrous behavior presumably reflected inadequate follicular development resulting in non-ovulation or the ovulation of poor quality oocytes, and thus poor farrowing rates.

4. Embryo Survival and Pregnancy Rate

Embryo mortality is an important reproductive constraint in farm animals, resulting in pregnancy loss or reduced litter sizes, depending on the number of embryos originally present. Cattle first-service embryo mortality rates ranged from 10% to 40% and were up to 65% for repeat breeders [74]. Injecting GnRH at the time of artificial insemination may increase the pregnancy rate in cows. However, effects have been equivocal due to inadequate animal numbers and differences in experimental design. It is noteworthy that Peters [2] reported that a meta-analysis indicated improved pregnancy rates in GnRH-treated cows.
A recent comparison was made between two GnRH products, the agonist dephereline and natural GnRH, administered in cows following the removal of a 5-day progesterone-containing CIDR treatment [75]. Thereafter, FTAI was performed in cows that were heat-stressed, or not, at the time of insemination. Cows treated with dephereline showed greater ovulation and pregnancy rates compared to those receiving the natural GnRH, especially under conditions of heat stress. Although speculative, these findings may suggest the heat stress inhibition of hypothalamic/pituitary function with a reduced GnRH release adversely affecting reproductive function, and that exogenous GnRH can override this. In another stress situation, excitable Nellore cows had higher blood cortisol levels [76], and a clear link between stress and cortisol levels has been established [77]. In excitable Nellore cows, it has further been observed that the administration of GnRH at 7 days after TAI improved pregnancy rates at 30 days but not gestational losses assessed at 60 days [76]. This effect was presumably due to GnRH-induced LH release providing early luteal support. In contrast, an evaluation of the impact of GnRH treatment in dairy cows experiencing heat stress during AI, and 5 days after AI, noted no improvement in pregnancy rates, gestational loss, or P4 concentrations [78].
It has been noted that the injection of GnRH into sows at artificial insemination increased the numbers of embryos in sows underfed in their previous lactation, and in primiparous sows, and this was associated with increased blood progesterone concentrations [79]. The mechanism underpinning this effect was not determined, although it has been previously documented that a lower magnitude LH surge was associated with subsequently lower blood progesterone concentrations, with the suggestion that a smaller surge will lead to poorer follicular luteinization [80]. Although speculative, it is possible that a GnRH injection at mating could enhance an otherwise attenuated LH surge, improve luteal quality, and promote improved pregnancy maintenance.
It is known that GnRH injections in the mid-late luteal phase in non-gravid cows will elicit increased progesterone secretion [81]. The increased progesterone secretion also stimulates the production of estradiol-17β, with Mann et al. [82] concluding that the GnRH attenuated or weakened the luteolytic signal allowing embryos more time to develop their own luteotrophic capability.
GnRH was examined as a protector of pregnancy in buffalo cows, related to a suboptimally functioning corpora lutea. Its administration in the various stages of pregnancy, usually after day 25, improved final pregnancy rates [83][84], probably by decreasing early embryonic loss [85]. The role of GnRH in improving pregnancy rates is an ongoing area of research and is of particular interest in some physiologic states, such as in dairy cows that suffer from heat stress-induced early embryonic losses. In sheep, it was reported that GnRH administered to ewes 12 days post-mating resulted in improved embryo survival [86]. However, in another study, the administration of GnRH and progesterone 4 and 12 days post-breeding improved embryo survival but did not increase the pregnancy rate in ewes [87]. Unfortunately, there is a dearth of data concerning the influence of exogenous GnRH for improving embryo survival in sheep and goats.
Several studies have investigated the effects of the LH agonist, hCG, in reducing embryo mortality in pigs. Injecting 750 IU on day 12 of the estrous cycle resulted in prolonged luteal survival and increased progesterone production [88][89]. Additionally, exogenous hCG improved luteal viability, as well as angiogenesis in endometrial and luteal tissues. It also had a positive effect on the PGE2–PGFM ratio during the estrous cycle, which decreased luteal cell apoptosis during early pregnancy in sows and gilts [88]. It remains to be determined whether GnRH, via the stimulation of endogenous LH, can substitute hCG, although, presumably, it will require a more sustained activity than what is currently available.
Several researchers have reported that a single injection of GnRH between 11 and 13 days after insemination resulted in an elevation of progesterone and an improved pregnancy rate of about 10% in cows [90][91][92][93]. However, others found no relationship between exogenous GnRH administration and pregnancy performance [94]. The lack of a GnRH effect on pregnancy rates does not necessarily mean a lack of efficacy. The result of any study is a comparison between a treatment group and a control group, but in the event that the control group fairs well, the treatment ‘does not work’. This is well illustrated in a study on the effects of GnRH injected into buffalo cows on day 35 after insemination [84]. Cows with a blood serum concentration of pregnancy-associated glycoproteins (PAGs) of less than 2.5 ng/mL were considered at a higher risk of embryo mortality. Only within this subpopulation, treatment with GnRH significantly increased PAGs between the 28th and 60th days of pregnancy, and markedly reduced embryo/pregnancy loss.
For cows, the placentation process begins around days 28–32 of pregnancy and concludes between 40 and 45 days [95][96]. The elevation in PAG levels indicates the expansion of trophoblastic tissue as its quantity is linked to the growth of the placenta [97][98]. The trophoblastic cells play a crucial role in prostaglandin metabolism and they can convert luteolytic PGF2α into luteotrophic PGE2, which ensures the maintenance of the corpus luteum and adequate levels of progesterone necessary for the survival of the conceptus [99][100]. When GnRH was administered on the 5th or 15th day [101], or on the 12th day [102], there was no noticeable improvement in pregnancy rates. Conversely, administering GnRH via subcutaneous implantation on the 27th day following insemination significantly reduced embryonic loss between 45 and 90 days in cows that had developed accessory corpora lutea [85].
The establishment of pregnancy in buffaloes requires a gradual increase in progesterone secretion during the first 2–3 weeks following mating. To support pregnancy, various treatments prolonging the lifespan of the corpus luteum have been employed, including GnRH treatment on the 5th or 25th day after insemination [103][104]. The GnRH treatment on the 5th day after AI increased progesterone secretion and enhanced the chances of a successful pregnancy in buffaloes bred during periods of increasing photoperiod, while treatment on the 25th day resulted in a higher pregnancy rate [103][104].

5. Treatment of Reproductive Morbidity

The use of GnRH as a treatment for some reproductive morbidities in food animals is not well established [105][106], but is well recognized for others, such as cystic ovarian degeneration in cattle [107][108][109]. Early studies found that serial administrations of different doses of GnRH were unable to initiate ovulation in anestrous cows [110][111]. Repeat breeding syndrome, i.e., a requirement for repeated inseminations without detectable abnormalities, is a major reproductive problem in dairy and beef cows, and increases the costs of production. The use of GnRH agonists to modify the condition of repeat breeding cows has been investigated [106]. Various studies reported that GnRH agonists administered coincident with insemination had positive effects on conception and pregnancy rates, finding a 10% to 18% more successful pregnancy in repeat breeder cows compared to untreated groups [2][106][112]. Further, a buserelin dose–response effect on the conception and pregnancy rates of repeat breeder cows has been identified, with 20 µg resulting in better pregnancy rates than 10 µg [106]. Similarly, the injection of 2.5 times the label dose (250 µg) of dephereline at 5 to 7 days after insemination increased pregnancy rates in repeat breeder cows more than 100 µg did [113], with both doses resulting in accessory corpora lutea. Taken together, these data suggest that repeat breeding may be a consequence of inadequate GnRH stimulation, and that this is a compensable effect that can be addressed by administering higher than label exogenous doses.
The inclusion of eCG in a controlled ovulation protocol prolonged stimulation during a follicle’s dominance, and estradiol secretion elevated the concentrations of follicular cytochrome enzyme P450 17α hydroxylase [114][115]. This increase induced the LH surge, leading to ovulation in cattle [114]. Therefore, combining eCG on day 5 with GnRH on day 9 of a FTAI protocol can enhance the induced LH release effect, resulting in the ovulation of the dominant follicle in a shorter time in suckled beef cows. This also resulted in a significantly larger corpus luteum on day 9 after ovulation, evidenced by a higher concentration of serum progesterone on day 12. Similar findings were reported by other researchers, who observed greater luteal volume and progesterone concentration in anestrous cows that received eCG two days before intra-vaginal device removal in an 8-day FTAI protocol, at 10 and 15 days after ovulation [116]. These results align with the concept that the progesterone concentration plateau occurs between 10 and 14 days after ovulation [117], and that luteal size is closely related to luteal function. The measurement of luteal area had the strongest association with luteal function during its development [118]. Furthermore, cows having higher progesterone concentrations sooner after ovulation are generally associated with greater fertility [119].
Following a progesterone-based protocol for estrus induction, the reproductive performance of anestrous ewes receiving treatment with nanoencapsulated GnRH was evaluated as a potential alternative to eCG [120]. The authors developed a nano-GnRH formulation with distinct physicochemical features that yielded favorable outcomes on ovulation and luteal function in various animal species, including goats, sheep, and rabbits [121][122]. Their results illustrated that GnRH administration, as a component of ovulation control protocols, resulted in a significant increase in conception, lambing, and fecundity rates with a tendency towards larger litter sizes compared to those treated with eCG. However, various studies have indicated potential adverse impacts of eCG on embryo survival, particularly during the pre-implantation stage, when utilizing either in vivo [123] or in vitro [124][125] techniques. For instance, in Chios-breed ewes, the use of eCG for superovulation, and, to a lesser extent, for estrus synchronization, can impact glycosidase activity in the genital tract, potentially leading to adverse effects on embryo survival and pregnancy maintenance before the maternal recognition of pregnancy [123]. As a substitute for eCG, nanoencapsulated GnRH can be employed in progesterone (CIDR)-based estrus induction regimens for sheep [120].
The use of GnRH agonists in the treatment of cystic ovarian degeneration is established in cows [107][108][109][126], and there is an indication that double injection has an effect on this morbidity in sows [127]. In fact, in cows, GnRH is the treatment of choice for the undifferentiated cystic ovarian degeneration, independent of the type (i.e., follicular or luteal cyst/s). Responsiveness to the first administration of GnRH has been suggested to be an indicator for prognosis, with cows not responding to the first treatment having a lower likelihood of return to fertility [126].

This entry is adapted from the peer-reviewed paper 10.3390/ani13121891

References

  1. Bulman, D.C.; Lamming, G.E. Milk progesterone levels in relation to conception, repeat breeding and factors influencing acyclicity in dairy cows. Reproduction 1978, 54, 447–458.
  2. Peters, A. Veterinary clinical application of GnRH—Questions of efficacy. Anim. Reprod. Sci. 2005, 88, 155–167.
  3. Ball, P.J.H. Milk progesterone profiles in relation to dairy herd fertility. Br. Vet. J. 1982, 138, 546–551.
  4. Beckett, S.D.; Lean, I.J. Gonadotrophin-releasing hormone in postpartum dairy cattle: A meta-analysis of effects on reproductive efficiency. Anim. Reprod. Sci. 1997, 48, 93–112.
  5. Driancourt, M.A.; Locatelli, A.; Prunier, A. Effects of gonadotrophin deprivation on follicular growth in gilts. Reprod. Nutr. Dev. 1995, 35, 663–673.
  6. Gama, R.; Vianna, W.L.; Pinese, M.E.; De Campos Rosseto, A.; De Sant’anna Moretti, A. Different doses of porcine luteinizing hormone in precocious puberty induction in gilts. Reprod. Domest. Anim. 2005, 40, 433–435.
  7. Brüssow, K.; Wähner, M. Biological and technological background of estrus synchronization and fixed-time ovulation induction in the pig. Biotechnol. Anim. Husb. 2011, 27, 533–545.
  8. Driancourt, M.; Cox, P.; Rubion, S.; Harnois-Milon, G.; Kemp, B.; Soede, N. Induction of an LH surge and ovulation by buserelin (as Receptal) allows breeding of weaned sows with a single fixed-time insemination. Theriogenology 2013, 80, 391–399.
  9. BrüSsow, K.; Schneider, F.; Tuchscherer, A.; Rátky, J.; Kraeling, R.; Kanitz, W. Luteinizing hormone release after administration of the gonadotropin-releasing hormone agonist Fertilan (goserelin) for synchronization of ovulation in pigs. J. Anim. Sci. 2007, 85, 129–137.
  10. Fries, H.; Souza, L.; Faccin, J.; Reckziegel, M.; Hernig, L.; Marimon, B.; Bernardi, M.; Wentz, I.; Bortolozzo, F. Induction and synchronization of ovulation in sows using a Gonadotropin-releasing Hormone Analog (Lecirelin). Anim. Reprod. 2010, 7, 362–366.
  11. Stewart, K.; Flowers, W.; Rampacek, G.; Greger, D.; Swanson, M.; Hafs, H. Endocrine, ovulatory and reproductive characteristics of sows treated with an intravaginal GnRH agonist. Anim. Reprod. Sci. 2010, 120, 112–119.
  12. Martinat-Botté, F.; Venturi, E.; Guillouet, P.; Driancourt, M.; Terqui, M. Induction and synchronization of ovulations of nulliparous and multiparous sows with an injection of gonadotropin-releasing hormone agonist (Receptal). Theriogenology 2010, 73, 332–342.
  13. Driancourt, M. Fixed time artificial insemination in gilts and sows. Tools, schedules and efficacy. Soc. Reprod. Fertil. 2013, 68, 89–99.
  14. Brüssow, K.; Schneider, F.; Kanitz, W.; Ratky, J.; Kauffold, J.; Wähner, M. Studies on Fixed-Time Ovulation Induction in the Pig; Nottingham University Press: Nottingham, UK, 2009.
  15. Von Kaufmann, F.; Holtz, W. Induction of ovulation in gonadotropin treated gilts with synthetic gonadotropin releasing hormone. Theriogenology 1982, 17, 141–157.
  16. Langendijk, P.; Soede, N.; Kemp, B. Synchronisation of ovulation with GnRH or hCG in weaned sows, without pre-treatment with eCG. J. Reprod. Fertil. 2000, 26, 35.
  17. Knox, R.V.; Willenburg, K.; Rodriguez Zas, S.L.; Greger, D.; Hafs, H.; Swanson, M. Synchronization of ovulation and fertility in weaned sows treated with intravaginal triptorelin is influenced by timing of administration and follicle size. Theriogenology 2011, 75, 308–319.
  18. Lopes, T.P.; Padilla, L.; Bolarin, A.; Rodriguez-Martinez, H.; Roca, J. Weaned Sows with Small Ovarian Follicles Respond Poorly to the GnRH Agonist Buserelin. Animals 2020, 10, 1979.
  19. Baruselli, P.S.; Sales, J.; Sala, R.V.; Vieira, L.; Sá Filho, M.F.D. History, evolution and perspectives of timed artificial insemination programs in Brazil. Anim. Reprod. 2012, 9, 139–152.
  20. Pursley, J.R.; Wiltbank, M.C.; Stevenson, J.S.; Ottobre, J.S.; Garverick, H.A.; Anderson, L.L. Pregnancy rates per artificial insemination for cows and heifers inseminated at a synchronized ovulation or synchronized estrus. J. Dairy Sci. 1997, 80, 295–300.
  21. Rivera, H.; Lopez, H.; Fricke, P.M. Fertility of holstein dairy heifers after synchronization of ovulation and timed AI or AI after removed tail chalk. J. Dairy Sci. 2004, 87, 2051–2061.
  22. Moreira, F.; De La Sota, R.L.; Diaz, T.; Thatcher, W.W. Effect of day of the estrous cycle at the initiation of a timed artificial insemination protocol on reproductive responses in dairy heifers. J. Anim. Sci. 2000, 78, 1568–1576.
  23. Martinez, M.F.; Adams, G.P.; Bergfelt, D.R.; Kastelic, J.P.; Mapletoft, R.J. Effect of LH or GnRH on the dominant follicle of the first follicular wave in beef heifers. Anim. Reprod. Sci. 1999, 57, 23–33.
  24. Bisinotto, R.S.; Chebel, R.C.; Santos, J.E.P. Follicular wave of the ovulatory follicle and not cyclic status influences fertility of dairy cows. J. Dairy Sci. 2010, 93, 3578–3587.
  25. Wiltbank, M.C.; Sartori, R.; Herlihy, M.M.; Vasconcelos, J.L.; Nascimento, A.B.; Souza, A.H.; Ayres, H.; Cunha, A.P.; Keskin, A.; Guenther, J.N.; et al. Managing the dominant follicle in lactating dairy cows. Theriogenology 2011, 76, 1568–1582.
  26. Padula, A.; Borman, J.; Wright, P.; Macmillan, K. Restoration of LH output and 17β-oestradiol responsiveness in acutely ovariectomised Holstein dairy cows pre-treated with a GnRH agonist (deslorelin) for 10 days. Anim. Reprod. Sci. 2002, 70, 49–63.
  27. Carvalho, N.; Nagasaku, E.; Vannucci, F.; Toledo, L.; Baruselli, P.S. Ovulation and conception rates according intravaginal progesterone device and hCG or GnRH to induce ovulation in buffalo during the off breeding season. Ital. J. Anim. Sci. 2007, 6, 646–648.
  28. Carvalho, N.; Soares, J.; Souza, D.; Vannucci, F.; Amaral, R.; Maio, J.; Sales, J.; Sá Filho, M.F.D.; Baruselli, P.S. Different circulating progesterone concentrations during synchronization of ovulation protocol did not affect ovarian follicular and pregnancy responses in seasonal anestrous buffalo cows. Theriogenology 2014, 81, 490–495.
  29. Murugavel, K.; Antoine, D.; Raju, M.; López-Gatius, F. The effect of addition of equine chorionic gonadotropin to a progesterone-based estrous synchronization protocol in buffaloes (Bubalus bubalis) under tropical conditions. Theriogenology 2009, 71, 1120–1126.
  30. Neglia, G.; Natale, A.; Esposito, G.; Salzillo, F.; Adinolfi, L.; Campanile, G.; Francillo, M.; Zicarelli, L. Effect of prostaglandin F2α at the time of AI on progesterone levels and pregnancy rate in synchronized Italian Mediterranean buffaloes. Theriogenology 2008, 69, 953–960.
  31. Monteiro, B.M.; De Souza, D.C.; Vasconcellos, G.S.F.M.; Corrêa, T.B.; Vecchio, D.; De Sá Filho, M.F.; De Carvalho, N.a.T.; Baruselli, P.S. Ovarian responses of dairy buffalo cows to timed artificial insemination protocol, using new or used progesterone devices, during the breeding season (autumn–winter). Anim. Sci. J. 2016, 87, 13–20.
  32. Carvalho, N.; Soares, J.; Porto Filho, R.M.; Gimenes, L.; Souza, D.; Nichi, M.; Sales, J.; Baruselli, P.S. Equine chorionic gonadotropin improves the efficacy of a timed artificial insemination protocol in buffalo during the nonbreeding season. Theriogenology 2013, 79, 423–428.
  33. Vecchio, D.; Neglia, G.; Gasparrini, B.; Russo, M.; Pacelli, C.; Prandi, A.; D’occhio, M.; Campanile, G. Corpus luteum development and function and relationship to pregnancy during the breeding season in the Mediterranean buffalo. Theriogenology 2012, 77, 1811–1815.
  34. Vasconcelos, J.; Sartori, R.; Oliveira, H.; Guenther, J.; Wiltbank, M. Reduction in size of the ovulatory follicle reduces subsequent luteal size and pregnancy rate. Theriogenology 2001, 56, 307–314.
  35. Sartori, R.; Haughian, J.; Shaver, R.; Rosa, G.; Wiltbank, M. Comparison of ovarian function and circulating steroids in estrous cycles of Holstein heifers and lactating cows. J. Dairy Sci. 2004, 87, 905–920.
  36. Wolfenson, D.; Inbar, G.; Roth, Z.; Kaim, M.; Bloch, A.; Braw Tal, R. Follicular dynamics and concentrations of steroids and gonadotropins in lactating cows and nulliparous heifers. Theriogenology 2004, 62, 1042–1055.
  37. Haider, S.; Chishti, G.A.; Mehmood, M.U.; Jamal, M.A.; Mehmood, K.; Shahzad, M.; Tahir, M.Z. The effect of GnRH administration/insemination time on follicular growth rate, ovulation intervals, and conception rate of Nili Ravi buffalo heifers in 7–day-CIDR Co-synch. Trop. Anim. Health Prod. 2021, 53, 558.
  38. Haider, M.; Hassan, M.; Khan, A.; Husnain, A.; Bilal, M.; Pursley, J.; Ahmad, N. Effect of timing of insemination after CIDR removal with or without GnRH on pregnancy rates in Nili-Ravi buffalo. Anim. Reprod. Sci. 2015, 163, 24–29.
  39. Martinez, M.; Kastelic, J.; Adams, G.; Mapletoft, R. The use of GnRH or estradiol to facilitate fixed-time insemination in an MGA-based synchronization regimen in beef cattle. Anim. Reprod. Sci. 2001, 67, 221–229.
  40. Naseer, Z.; Ahmad, E.; Singh, J.; Ahmad, N. Fertility following CIDR based synchronization regimens in anoestrous Nili-Ravi buffaloes. Reprod. Domest. Anim. 2011, 46, 814–817.
  41. Lonergan, P.; Monaghan, P.; Rizos, D.; Boland, M.; Gordon, I. Effect of follicle size on bovine oocyte quality and developmental competence following maturation, fertilization, and culture in vitro. Mol. Reprod. Dev. 1994, 37, 48–53.
  42. Mehmood, M.U.; Qamar, A.; Sattar, A.; Ahmad, L.; Ahmad, N. Incorporation of estradiol benzoate to CIDR protocol improves the reproductive responses in crossbred dairy heifers. Trop. Anim. Health Prod. 2017, 49, 347–351.
  43. Soede, N.; Wetzels, C.; Zondag, W.; Hazeleger, W.; Kemp, B. Effects of a second insemination after ovulation on fertilization rate and accessory sperm count in sows. J. Reprod. Fertil. 1995, 105, 135–140.
  44. Waberski, D.; Soares, J.; Bandeira, E.; Weitze, K. Effect of a transcervical infusion of seminal plasma prior to insemination on the fertilising competence of low numbers of boar spermatozoa at controlled AI-ovulation intervals. Anim. Reprod. Sci. 1996, 44, 165–173.
  45. De Rensis, F.; Kirkwood, R. Control of estrus and ovulation: Fertility to timed insemination of gilts and sows. Theriogenology 2016, 86, 1460–1466.
  46. Bai, J.H.; Qin, Y.S.; Zhang, S.L.; Xu, X.L.; Song, Y.Q.; Xiao, L.L.; Feng, T.; Tian, J.H.; Liu, Y. A comparison of the reproductive performance in primiparous sows following two timed artificial insemination protocols. Animal 2021, 15, 100410.
  47. Peltoniemi, O.; Easton, B.; Love, R.; Klupiec, C.; Evans, G. Effect of chronic treatment with a GnRH agonist (Goserelin) on LH secretion and early pregnancy in gilts. Anim. Reprod. Sci. 1995, 40, 121–133.
  48. Haen, S.M.; Heinonen, M.; Kauffold, J.; Heikinheimo, M.; Hoving, L.L.; Soede, N.M.; Peltoniemi, O.A. GnRH-agonist deslorelin implant alters the progesterone release pattern during early pregnancy in gilts. Reprod. Domest. Anim. 2019, 54, 464–472.
  49. Knox, R.; Willenburg, K.; Rodriguez-Zas, S.; Greger, D.; Swanson, M.; Hafs, H. Intravaginal GnRH agonist gel advances time of ovulation and facilitates timed AI in weaned sows. Am. Assoc. Swine Vet. 2003, 11, 495–498.
  50. Rodrigues, L.; Amezcua, R.; Cassar, G.; O’sullivan, T.; Friendship, R. Comparison of single, fixed-time artificial insemination in weaned sows using 2 different protocols to synchronize ovulation. Can. Vet. J. 2020, 61, 53.
  51. Dillard, D.; Flowers, W. Reproductive performance of sows associated with single, fixed-time insemination programs in commercial farms based on either average herd weaning-to-estrus intervals or postweaning estrous activity. Appl. Anim. Sci. 2020, 36, 100–107.
  52. Miller, A.T.; Picton, H.M.; Hunter, M.G. Suppression of ovarian activity in the gilt and reversal by exogenous gonadotrophin administration. Anim. Reprod. Sci. 1999, 54, 179–193.
  53. Knox, R.; Stewart, K.; Flowers, W.; Swanson, M.; Webel, S.; Kraeling, R. Design and biological effects of a vaginally administered gel containing the GnRH agonist, triptorelin, for synchronizing ovulation in swine. Theriogenology 2018, 112, 44–52.
  54. Knox, R.; Webel, S.; Swanson, M.; Johnston, M.; Kraeling, R. Effects of estrus synchronization using Matrix® followed by treatment with the GnRH agonist triptorelin to control ovulation in mature gilts. Anim. Reprod. Sci. 2017, 185, 66–74.
  55. Diskin, M.G.; Austin, E.J.; Roche, J.F. Exogenous hormonal manipulation of ovarian activity in cattle. Domest. Anim. Endocrinol. 2002, 23, 211–228.
  56. Bishop, B.; Thomas, J.; Abel, J.; Poock, S.; Ellersieck, M.; Smith, M.; Patterson, D. Split-time artificial insemination in beef cattle: I–Using estrous response to determine the optimal time (s) at which to administer GnRH in beef heifers and postpartum cows. Theriogenology 2016, 86, 1102–1110.
  57. Bó, G.A.; Baruselli, P.S. Synchronization of ovulation and fixed-time artificial insemination in beef cattle. Animal 2014, 8, 144–150.
  58. Yamada, K. Development of ovulation synchronization and fixed time artificial insemination in dairy cows. J. Reprod. Dev. 2005, 51, 177–186.
  59. Tummaruk, P.; Sang-Gassanee, K.; Audban, C.; Pichitpantapong, S.; Panyathong, R.; Lin, H.; De Rensis, F. Gilt reproductive performance in a tropical environment after oestrus synchronization and fixed-time artificial insemination. Theriogenology 2022, 192, 45–51.
  60. Zhao, Q.; Tao, C.; Pan, J.; Wei, Q.; Zhu, Z.; Wang, L.; Liu, M.; Huang, J.; Yu, F.; Chen, X. Equine chorionic gonadotropin pretreatment 15 days before fixed-time artificial insemination improves the reproductive performance of replacement gilts. Animal 2021, 15, 100406.
  61. Pursley, J.R.; Mee, M.O.; Wiltbank, M.C. Synchronization of ovulation in dairy cows using PGF2α and GnRH. Theriogenology 1995, 44, 915–923.
  62. Brusveen, D.J.; Cunha, A.P.; Silva, C.D.; Cunha, P.M.; Sterry, R.A.; Silva, E.P.; Guenther, J.N.; Wiltbank, M.C. Altering the time of the second gonadotropin-releasing hormone injection and artificial insemination (AI) during Ovsynch affects pregnancies per AI in lactating dairy cows. J. Dairy Sci. 2008, 91, 1044–1052.
  63. Geary, T.W.; Downing, E.R.; Bruemmer, J.E.; Whittier, J.C. Ovarian and estrous response of suckled beef cows to the select synch estrous synchronization protocol. Prof. Anim. Sci. 2000, 16, 1–5.
  64. Bello, N.M.; Steibel, J.P.; Pursley, J.R. Optimizing ovulation to first GnRH improved outcomes to each hormonal injection of Ovsynch in lactating dairy cows. J. Dairy Sci. 2006, 89, 3413–3424.
  65. Colazo, M.G.; Gordon, M.B.; Rajamahendran, R.; Mapletoft, R.J.; Ambrose, D.J. Pregnancy rates to timed artificial insemination in dairy cows treated with gonadotropin-releasing hormone or porcine luteinizing hormone. Theriogenology 2009, 72, 262–270.
  66. Martinez, M.F.; Kastelic, J.P.; Adams, G.P.; Mapletoft, R.J. The use of a progesterone-releasing device (CIDR-B) or melengestrol acetate with GnRH, LH, or estradiol benzoate for fixed-time AI in beef heifers. J. Anim. Sci. 2002, 80, 1746–1751.
  67. Bartolome, J.A.; Sheerin, P.; Luznar, S.; Melendez, P.; Kelbert, D.; Risco, C.A.; Thatcher, W.W.; Archbald, L.F. Conception rate of lactating dairy cows using ovsynch after presynchronization with prostaglandin F2α (PGF2α) or Gonadotropin Releasing Hormone (GnRH). Bov. Pract. 2002, 36, 35–39.
  68. Moreira, F.; Orlandi, C.; Risco, C.A.; Mattos, R.; Lopes, F.; Thatcher, W.W. Effects of presynchronization and bovine somatotropin on pregnancy rates to a timed artificial insemination protocol in lactating dairy cows. J. Dairy Sci. 2001, 84, 1646–1659.
  69. Kirkwood, R.; Kauffold, J. Advances in breeding management and use of ovulation induction for fixed-time AI. Reprod. Domest. Anim. 2015, 50, 85–89.
  70. Baer, C.; Bilkei, G. The effect of intravaginal applied GnRH-agonist on the time of ovulation and subsequent reproductive performance of weaned multiparous sows. Reprod. Domest. Anim. 2004, 39, 293–297.
  71. Knox, R.; Esparza-Harris, K.; Johnston, M.; Webel, S. Effect of numbers of sperm and timing of a single, post-cervical insemination on the fertility of weaned sows treated with OvuGel®. Theriogenology 2017, 92, 197–203.
  72. Knox, R. Artificial insemination in pigs today. Theriogenology 2016, 85, 83–93.
  73. Young, B.; Dewey, C.E.; Friendship, R.M. Management factors associated with farrowing rate in commercial sow herds in Ontario. Can. Vet. J. 2010, 51, 185–189.
  74. Bilodeau-Goeseels, S.; Kastelic, J. Factors affecting embryo survival and strategies to reduce embryonic mortality in cattle. Can. J. Anim. Sci. 2003, 83, 659–671.
  75. Garcia-Ispierto, I.; De Rensis, F.; Pérez-Salas, J.; Nunes, J.; Pradés, B.; Serrano-Pérez, B.; López-Gatius, F. The GnRH analogue dephereline given in a fixed-time AI protocol improves ovulation and embryo survival in dairy cows. Res. Vet. Sci. 2019, 122, 170–174.
  76. Couto, S.R.; Guerson, Y.B.; Caparelli, N.M.; Andrade, J.P.N.; Jacob, J.C.; Barbero, R.P.; Mello, M.R. Mitigation of low pregnancy rate in excitable Nellore cows by administration of GnRH or P4. Theriogenology 2022, 192, 14–21.
  77. Córdova-Izquierdo, A.; Villa-Mancera, A.; Olivares Pérez, J.; Sánchez-Aparicio, P. Environmental stress effect on animal reproduction. Open J. Anim. Sci. 2014, 4, 79–84.
  78. Mendonça, L.; Mantelo, F.; Stevenson, J. Fertility of lactating dairy cows treated with gonadotropin-releasing hormone at AI, 5 days after AI, or both, during summer heat stress. Theriogenology 2017, 91, 9–16.
  79. Kirkwood, R.; Baidoo, S.; Aherne, F.; Sather, A. The influence of feeding level during lactation on the occurrence and endocrinology of the postweaning estrus in sows. Can. J. Anim. Sci. 1987, 67, 405–415.
  80. Kirkwood, R.; Lapwood, K.; Smith, W.; Anderson, I. Plasma concentrations of LH, prolactin, oestradiol-17β and progesterone in sows weaned after lactation for 10 or 35 days. Reproduction 1984, 70, 95–102.
  81. Thatcher, W.W.; Macmillan, K.L.; Hansen, P.J.; Drost, M. Concepts for regulation of corpus luteum function by the conceptus and ovarian follicles to improve fertility. Theriogenology 1989, 31, 149–164.
  82. Mann, G.; Lamming, G.; Fray, M. Plasma oestradiol and progesterone during early pregnancy in the cow and the effects of treatment with buserelin. Anim. Reprod. Sci. 1995, 37, 121–131.
  83. Arshad, U.; Qayyum, A.; Hassan, M.; Husnain, A.; Sattar, A.; Ahmad, N. Effect of resynchronization with GnRH or progesterone (P4) intravaginal device (CIDR) on Day 23 after timed artificial insemination on cumulative pregnancy and embryonic losses in CIDR-GnRH synchronized Nili-Ravi buffaloes. Theriogenology 2017, 103, 104–109.
  84. Pacelli, C.; Barile, V.L.; Sabia, E.; Casano, A.B.; Braghieri, A.; Martina, V.; Barbato, O. Use of GnRH Treatment Based on Pregnancy-Associated Glyco-Proteins (PAGs) Levels as a Strategy for the Maintenance of Pregnancy in Buffalo Cows: A Field Study. Animals 2022, 12, 2822.
  85. Bartolome, J.; Kamimura, S.; Silvestre, F.; Arteche, A.; Trigg, T.; Thatcher, W. The use of a deslorelin implant (GnRH agonist) during the late embryonic period to reduce pregnancy loss. Theriogenology 2006, 65, 1443–1453.
  86. Cam, M.A.; Kuran, M.; Yildiz, S.; Selcuk, E. Fetal growth and reproductive performance in ewes administered GnRH agonist on day 12 post-mating. Anim. Reprod. Sci. 2002, 72, 73–82.
  87. Ataman, M.B.; Aköz, M.; Saribay, M.K.; Erdem, H.; Bucak, M.N. Prevention of embryonic death using different hormonal treatments in ewes. Turk. J. Vet. Anim. Sci. 2013, 37, 6–8.
  88. Bolzan, E.; Andronowska, A.; Bodek, G.; Morawska-Pucinska, E.; Krawczynski, K.; Dabrowski, A.; Ziecik, A.J. The novel effect of hCG administration on luteal function maintenance during the estrous cycle/pregnancy and early embryo development in the pig. Pol. J. Vet. Sci. 2013, 16, 323–332.
  89. Guthrie, H.; Bolt, D. Changes in plasma estrogen, luteinizing hormone, follicle-stimulating hormone and 13, 14-dihydro-15-keto-prostaglandin F2α during blockade of luteolysis in pigs after human chorionic gonadotropin treatment. J. Anim. Sci. 1983, 57, 993–1000.
  90. Macmillan, K.; Day, A.; Taufa, V.; Gibb, M.; Pearce, M. Effects of an agonist of gonadotrophin releasing hormone in cattle. I. Hormone concentrations and oestrous cycle length. Anim. Reprod. Sci. 1985, 8, 203–212.
  91. Macmillan, K.; Taufa, V.; Day, A. Effects of an agonist of gonadotrophin releasing hormone (Buserelin) in cattle. III. Pregnancy rates after a post-insemination injection during metoestrus or dioestrus. Anim. Reprod. Sci. 1986, 11, 1–10.
  92. Drew, S.; Peters, A. Effect of buserelin on pregnancy rates in dairy cows. Vet. Rec. 1994, 134, 267–269.
  93. Sheldon, I.; Dobson, H. Effects of gonadotrophin releasing hormone administered 11 days after insemination on the pregnancy rates of cattle to the first and later services. Vet. Rec. 1993, 133, 160–163.
  94. Ryan, D.; Snijders, S.; Condon, T.; Grealy, M.; Sreenan, J.; O’farrell, K. Endocrine and ovarian responses and pregnancy rates in dairy cows following the administration of a gonadotrophin releasing hormone analog at the time of artificial insemination or at mid-cycle post insemination. Anim. Reprod. Sci. 1994, 34, 179–191.
  95. Geisert, R.; Fazleabas, A.; Lucy, M.; Mathew, D. Interaction of the conceptus and endometrium to establish pregnancy in mammals: Role of interleukin 1β. Cell Tissue Res. 2012, 349, 825–838.
  96. Green, J.A.; Geisert, R.D.; Johnson, G.A.; Spencer, T.E. Implantation and placentation in ruminants. In Placentation in Mammals: Tribute to EC Amoroso’s Lifetime Contributions to Viviparity; Springer: Cham, Switzerland, 2021; pp. 129–154.
  97. Patel, O.V.; Sulon, J.; Beckers, J.F.; Takahashi, T.; Hirako, M.; Sasaki, N.; Domeki, I. Plasma bovine pregnancy-associated glycoprotein concentrations throughout gestation in relationship to fetal number in the cow. Eur. J. Endocrinol. 1997, 137, 423–428.
  98. Wallace, J.; Aitken, R.; Cheyne, M.; Humblot, P. Pregnancy-specific protein B and progesterone concentrations in relation to nutritional regimen, placental mass and pregnancy outcome in growing adolescent ewes carrying singleton fetuses. Reproduction 1997, 109, 53–58.
  99. Reimers, T.; Ullmann, M.; Hansel, W. Progesterone and prostanoid production by bovine binucleate trophoblastic cells. Biol. Reprod. 1985, 33, 1227–1236.
  100. Gross, T.; Williams, W. Bovine placental prostaglandin synthesis: Principal cell synthesis as modulated by the binucleate cell. Biol. Reprod. 1988, 38, 1027–1034.
  101. Bartolome, J.; Melendez, P.; Kelbert, D.; Swift, K.; Mchale, J.; Hernandez, J.; Silvestre, F.; Risco, C.; Arteche, A.; Thatcher, W. Strategic use of gonadotrophin-releasing hormone (GnRH) to increase pregnancy rate and reduce pregnancy loss in lactating dairy cows subjected to synchronization of ovulation and timed insemination. Theriogenology 2005, 63, 1026–1037.
  102. Szenci, O.; Takács, E.; Sulon, J.; De Sousa, N.M.; Beckers, J.F. Evaluation of GnRH treatment 12 days after AI in the reproductive performance of dairy cows. Theriogenology 2006, 66, 1811–1815.
  103. Campanile, G.; Vecchio, D.; Di Palo, R.; Neglia, G.; Gasparrini, B.; Prandi, A.; Zicarelli, L.; D’occhio, M. Delayed treatment with GnRH agonist, hCG and progesterone and reduced embryonic mortality in buffaloes. Theriogenology 2008, 70, 1544–1549.
  104. Vecchio, D.; Neglia, G.; Di Palo, R.; Prandi, A.; Gasparrini, B.; Balestrieri, A.; D’occhio, M.J.; Zicarelli, L.; Campanile, G. Is a delayed treatment with GnRH, hCG or progesterone beneficial for reducing embryonic mortality in buffaloes? Reprod. Domest. Anim. 2010, 45, 614–618.
  105. Bryan, M.A.; Bo, G.; Mapletoft, R.J.; Emslie, F.R. The use of equine chorionic gonadotropin in the treatment of anestrous dairy cows in gonadotropin-releasing hormone/progesterone protocols of 6 or 7 days. J. Dairy Sci. 2013, 96, 122–131.
  106. Kharche, S.D.; Srivastava, S.K. Dose dependent effect of GnRH analogue on pregnancy rate of repeat breeder crossbred cows. Anim. Reprod. Sci. 2007, 99, 196–201.
  107. Peter, A. An update on cystic ovarian degeneration in cattle. Reprod. Domest. Anim. 2004, 39, 1–7.
  108. Vanholder, T.; Opsomer, G.; De Kruif, A. Aetiology and pathogenesis of cystic ovarian follicles in dairy cattle: A review. Reprod. Nutr. Dev. 2006, 46, 105–119.
  109. Borș, S.I.; Ibănescu, I.; Creangă, Ș.; Borș, A. Reproductive performance in dairy cows with cystic ovarian disease after single treatment with buserelin acetate or dinoprost. J. Vet. Med. Sci. 2018, 80, 1190–1194.
  110. Walters, D.L.; Short, R.E.; Convey, E.M.; Staigmiller, R.B.; Dunn, T.G.; Kaltenbach, C.C. Pituitary and ovarian function in postpartum beef cows. II. Endocrine changes prior to ovulation in suckled and nonsuckled postpartum cows compared to cycling cows. Biol. Reprod. 1982, 26, 647–654.
  111. Edwards, S.; Roche, J.; Niswender, G. Response of suckling beef cows to multiple, low-dose injections of Gn-RH with or without progesterone pretreatment. Reproduction 1983, 69, 65–72.
  112. Phatak, A.P.; Whitmore, H.L.; Brown, M.D. Effect of gonadotrophin releasing hormone on conception rate in repeat-breeder dairy cows. Theriogenology 1986, 26, 605–609.
  113. López-Gatius, F.; Garcia-Ispierto, I. Treatment with an elevated dose of the GnRH analogue dephereline in the early luteal phase improves pregnancy rates in repeat-breeder dairy cows. Theriogenology 2020, 155, 12–16.
  114. Soumano, K.; Silversides, D.; Doizé, F.; Price, C. Follicular 3β-hydroxysteroid dehydrogenase and cytochromes P450 17α-hydroxylase and aromatase messenger ribonucleic acids in cattle undergoing superovulation. Biol. Reprod. 1996, 55, 1419–1426.
  115. Komar, C.; Berndtson, A.; Evans, A.; Fortune, J. Decline in circulating estradiol during the periovulatory period is correlated with decreases in estradiol and androgen, and in messenger RNA for P450 aromatase and P450 17α-hydroxylase, in bovine preovulatory follicles. Biol. Reprod. 2001, 64, 1797–1805.
  116. Tortorella, R.D.; Ferreira, R.; Dos Santos, J.T.; De Andrade Neto, O.S.; Barreta, M.H.; Oliveira, J.F.; Gonçalves, P.B.; Neves, J.P. The effect of equine chorionic gonadotropin on follicular size, luteal volume, circulating progesterone concentrations, and pregnancy rates in anestrous beef cows treated with a novel fixed-time artificial insemination protocol. Theriogenology 2013, 79, 1204–1209.
  117. Adams, G.P.; Singh, J. Ovarian follicular and luteal dynamics in cattle. In Bovine Reproduction; Hopper, R.M., Ed.; John Wiley & Sons: Hoboken, NJ, USA, 2021; pp. 292–323.
  118. Rocha, C.C.; Martins, T.; Cardoso, B.O.; Silva, L.A.; Binelli, M.; Pugliesi, G. Ultrasonography-accessed luteal size endpoint that most closely associates with circulating progesterone during the estrous cycle and early pregnancy in beef cows. Anim. Reprod. Sci. 2019, 201, 12–21.
  119. Jimenez-Krassel, F.; Folger, J.; Ireland, J.; Smith, G.; Hou, X.; Davis, J.; Lonergan, P.; Evans, A.; Ireland, J. Evidence that high variation in ovarian reserves of healthy young adults has a negative impact on the corpus luteum and endometrium during estrous cycles in cattle. Biol. Reprod. 2009, 80, 1272–1281.
  120. Hashem, N.M.; El-Hawy, A.S.; El-Bassiony, M.F.; El-Hamid, I.S.A.; Gonzalez-Bulnes, A.; Martinez-Ros, P. Use of GnRH-Encapsulated Chitosan Nanoparticles as an Alternative to eCG for Induction of Estrus and Ovulation during Non-Breeding Season in Sheep. Biology 2023, 12, 351.
  121. Hashem, N.M.; El-Sherbiny, H.R.; Fathi, M.; Abdelnaby, E.A. Nanodelivery system for ovsynch protocol improves ovarian response, ovarian blood flow Doppler velocities, and hormonal profile of goats. Animals 2022, 12, 1442.
  122. Hassanein, E.M.; Hashem, N.M.; El-Azrak, K.E.-D.M.; Gonzalez-Bulnes, A.; Hassan, G.A.; Salem, M.H. Efficiency of GnRH–loaded chitosan nanoparticles for inducing LH secretion and fertile ovulations in protocols for artificial insemination in rabbit does. Animals 2021, 11, 440.
  123. Samartzi, F.; Theodosiadou, E.K.; Rekkas, C.A.; Saratsi, A.; Lymberopoulos, A.G.; Vainas, E.; Tsiligianni, T. Effect of equine chorionic gonadotropin on glycosidase activity in the reproductive tract of ewes, in relation to ovarian response and embryo yield. Small Rumin. Res. 2020, 191, 106186.
  124. Luna-Palomera, C.; Macías-Cruz, U.; Sánchez-Dávila, F. Superovulatory response and embryo quality in Katahdin ewes treated with FSH or FSH plus eCG during non-breeding season. Trop. Anim. Health Prod. 2019, 51, 1283–1288.
  125. Lin, E.; Li, Z.; Huang, Y.; Ru, G.; He, P. High dosages of equine chorionic gonadotropin exert adverse effects on the developmental competence of IVF-derived mouse embryos and cause oxidative stress-induced aneuploidy. Front. Cell Dev. Biol. 2021, 8, 609290.
  126. Bor, S.S.; Bor, S.A. Ovarian cysts, an anovulatory condition in dairy cattle. J. Vet. Med. Sci. 2020, 82, 1515–1522.
  127. Cech, S.; Dolezel, R. Treatment of ovarian cysts in sows-a field trial. Vet. Med. 2007, 52, 413–418.
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