INTRODUCTION

In Mexico, most sheep farms are characterized by low reproductive and therefore productive indexes (^{Hernández-Marín et al., 2017}), being reproduction a determining factor of profitability and long-term sustainability, which generated, the need to develop different hormonal treatments aimed at regulating reproductive activity, making a reduced use of hormones, contributing to environmental safety and having an acceptable reproductive efficiency.

In this sense, estrus synchronization with progestogens by means of sponges or intravaginal devices are the most used in these protocols, which associated with artificial insemination or controlled mating schemes, turn out to be a tool that improves the reproductive and productive efficiency of herds (^{Marques et al., 2010}). However, most of these protocols are traditionally applied for periods of 12 to 14 days in order to imitate the normal luteal phase of a sheep (^{Viñoles et al., 2001}; ^{Olivera-Muzante et al., 2011}; ^{Swelum et al., 2015}). Long protocols with progesterone and its synthetic analogues have been reported to have several effects on circulating levels of progesterone, resulting in prolonged persistence and aging of ovulatory follicles (^{Menchaca & Rubianes, 2002}), due to a low concentration of progesterone (P4) towards the treatment end that can negatively affect fertility. Due to the suppressive effect of long-term progestogen treatment on subsequent fertility (^{Viñoles et al., 2001}), treatments that decrease progestogen use time and achieve estrus synchronization without decreasing reproductive efficiency are being sought.

It has been reported that short-term protocols with progesterone promote follicular turnover with the appearance of a new follicular wave in 3 or 4 days in most ewes (^{Vilariño et al., 2013)}, reducing the exposure of the preovulatory follicle to low concentrations of progesterone in serum during prolonged periods, which favors the ovulation of a younger follicle and an oocyte of better quality to be fertilized.

Currently, there are variations in drugs used and the exposure time of progestogens in short treatments, all with the purpose of improving reproductive variables and shortening the management time. Therefore, the aim of this study was to compare different residence times of an intravaginal progestogen (CIDR®) with a dose of prostaglandin F2 alpha (PGF2α) two days before withdrawal, in the synchronization of estrus of hair sheep in Campeche State, Mexico.

MATERIAL AND METHODS

The present study was conducted according to regulations for the use and care of animals destined for research at the Postgraduate College, Mexico (^{COLPOS, 2016}) and in accordance with the Mexican Official Standard ^{NOM-024-ZOO-1995} (SAGARPA, 2015).

RESULTS AND DISCUSSION

No difference (p > 0.05) was observed for the estrus incidence variable between treatments (Table 1). ^{Rubianes et al., (1998)}) mention that the insertion of an intravaginal device containing progesterone (CIDR®), causes an increase in serum progesterone concentrations (>5 ng/mL) for three or four days, which is higher than the concentrations observed physiologically during the middle of the luteal phase, which triggers a positive effect on follicular growth, increasing the number of large follicles with greater estradiol secretion and with potential to ovulate. This suggests that in this study, variations in P4 release by the device were similar between treatments and supports the idea that short- term progestogen-based protocols (CIDR^®, P4) are effective in inducing estrus in ewes. Similar studies report 96 and 100% incidence to estrus using 5 and 9 day protocols respectively with a PGF2α application in ewes (^{Fraire-Cordero et al., 2013}; ^{Jackson et al., 2014}) and 100% with 9 d protocols and an application of equine chorionic gonadotropin (eCG) (^{Nakafeero et al., 2020}).

The ewes that did not respond at T1 it is suggested may be related to the fact that some ewes are in the early luteal phase of the estrous cycle at the time of prostaglandin injection, so animals with corpus luteum development at this stage would not respond to prostaglandins (^{Fierro et al., 2013}) and would continue to have an estrous cycle of normal length after device removal. Whereas ewes with corpus luteum in a more advanced stage would respond to prostaglandin treatment by having a normal follicular phase and, subsequently estrus and ovulation, which possibly occurred for T2 and T3.

The average onset of estrus of ewes was 36.1 ± 3.5 h, T1 ewes initiated estrus in a shorter time (p ≤ 0.05) (Table 1) and better synchronized than the rest of treatments as observed in Figure 1, so it could influence the choice of a desirable protocol for estrus synchronization, when the objective is to use fixed-time insemination. Synchrony could be related to the dynamics of the preovulatory follicle. Authors such as ^{Viñoles et al. (2001)} and ^{Martínez-Ros et al. (2018)} mention that follicles that do not have as much contact with progesterone in short-term treatments, it is observed that the newly recruited follicle has larger size, linear growth and higher estradiol concentration thus promoting better estrous behavior.

In studies by ^{Martínez-Ros et al. (2018)} and ^{López et al. (2021)} using 6- and 7-day protocols with intravaginal sponge and a dose of PGF2α at withdrawal, they found an onset to estrus of 58.3 ± 10.0 h and 48.0 ± 16.9 h, respectively, being higher than those found in this study. As well as to the 43.6 and 40.3 h reported with similar protocols with 9 d in Pelibuey ewes (^{Fraire-Cordero et al., 2013}; ^{Sosa-Pérez et al., 2014)} and differing to that reported by ^{Nakafeero et al. (2020)} who report a shorter estrus interval in ewes with CIDR for 14 d (24.9 ± 1.6 h) compared to CIDR at 9 days (30.8 ± 2. 1 h) suggesting that in short protocols the onset of estrus is usually delayed since there is a growing follicle, so it still takes some time to trigger ovulation after progestogen withdrawal, while in long protocols progesterone levels are falling and when the device is removed, it ovulates and triggers signs of estrus more quickly. However, the conditions were different from those in this study.

Figure 1.  Survival curves of time to estrus onset in hair sheep synchronized with short protocols based on an intravaginal progestogen (CIDR®) and prostaglandin 

The average lambing was higher than 83%, being higher than that reported in ewes with protocols where P4 was used in combination with PGF2a at device removal for five and seven days (83 and 16%) (^{Martínez-Ros et al., 2019}) and for nine days with a similar protocol in ewes (50%) (^{Fraire-Cordero et al., 2013}). No significant differences (p > 0.05) were found between treatments for the percentage of ewes that lambed between treatments (Table 1), apparently, the different estrus synchronization protocols were able to generate corpus luteum with sufficient P4 production that allowed implantation and embryonic development. ^{Viñoles et al. (2001)} mention that a higher parturition can be attributed to the ovulation of high quality young follicles, where follicles that have not had much time in contact with progesterone, it is observed that the follicle earlier in the cycle has a larger size, grows in a more linear way, so it is a follicle that has high levels of estradiol and, at the moment it ovulates and insemination or mating is performed, it will have more possibilities of carrying a gestation to term.

For prolificacy, no differences were found between treatments (p > 0.05) as shown in Table 1, results were expected since no hormones were used to stimulate the ovarian response promoting multiple deliveries such as porcine follicle stimulating hormone or eCG. The average of this variable for the flock was 1.47 ± 0.2, higher than the 1.3 ewe lambs-1 reported with a short protocol with intravaginal sponges and eCG (^{López et al., 2021}). However, no differences in this variable were detected in this study.

CONCLUSION

Under the conditions of the study, it is feasible to maintain herd reproductive efficiency with the use of short synchronization protocols based on intravaginal progestogens with CIDR^® for five, seven or nine days associated with an injection of PGF2α two days prior to withdrawal.