INTRODUCTION

In different domestic species, for several decades a reproductive technology that is the synchronization of estrus has been as a routine practice in some production units used. Its use in sheep allows to improve reproductive efficiency; although factors such as the wide variety of synchronization protocols (^{Hashemi et al., 2006}) and the hormonal products used, including progestogens (^{Fleish et al., 2012}; ^{Hashemi et al., 2006}), can affect the response reproductive to these protocols and thereby increase production costs. A hormone included in a complementary way in the protocols is equine chorionic gonadotropin (eCG), favoring the conception rate and ovulatory rate. Although a risk associated with the use of this hormone is an increase in the ovulatory rate, which leads to multiple births and the possibility of having low-weight offspring; which compromises their survival (^{Lozano-González et al., 2012}). Various studies consider the use of eCG, in doses ranging from 0 to 500 U (^{Arroyo-Ledezma et al., 2013}; ^{Fleish et al., 2012}; ^{Garoussi et al., 2019}; ^{Iida et al., 2004}); however, there are practically no studies that justify the use of these different doses in Pelibuey sheep. A breed considered low seasonality (^{Arroyo et al., 2007}); so the use of high doses might not be justified. A reduction in the dose used would reduce the cost of the synchronization protocol, continue the transition to protocols with less use of hormones and reduce the risks associated with the presence of multiple births. Therefore, the objective of the present work was to analyze the effect of two different doses of eCG, included in estrous synchronization protocols, based on progestogens on the reproductive response of Pelibuey sheep.

MATERIAL AND METHODS

The study was carried out in the months of February and March (late winter) for two consecutive years, in Colón municipality, in Querétaro state, Mexico (20º42 ’North Latitude and 100º01’ West Longitude). In this region, a semi-dry temperate climate prevails, with an average annual temperature of 17.4 ºC. Pelibuey sheep (n=28 in year 1 and 24 in year 2) were used, which at the beginning of the study had an average weight of 50.1 ± 1.36 kg and an average body condition of 3.41 ± 0.07, measured on a scale from 1 to 5 ^{(Romero, 2015)}. Sheep were randomly assigned to two estrus synchronization protocols; the first group (LOW) received the insertion of an intravaginal device, containing progesterone (CIDR) on day 0; same that was withdrawn on day 6, in conjunction with the application of an i.m. of 300 U of eCG and a dose of 0.125 mg of a prostaglandin F2 alpha analog (Cloprostenol). The second group (HIGH) received the CIDR on day 0, withdrawing on day 6, in conjunction with the application of a higher dose of eCG (400 U) and a dose of 0.125 mg of Cloprostenol.

The sheep were subjected to estrus detection 24 h after the devices had been removed. The detection was carried out twice a day (09:00 and 18:00), using a stallion of the same breed, until reaching 72 hours. Natural service was using, ensuring a male-female ratio no greater than 1:10.

Subsequently, the ewes were subjected to pregnancy diagnoses 35 days after their service, to determine the conception rate. For this, an Aloka brand ultrasound was used, with a 5 MHz transrectal transducer (Mod. SSD500). At birth, the number of offspring born for each of the females was to determine their prolificacy recorded. For statistical analysis, Fisher's exact test was to establish possible differences in the percentages of sheep in estrus and pregnant sheep performed. Additionally, analysis of variance was performed to determine differences between groups for the variables live weight, body condition, the onset of estrus and prolificacy; considering a randomized complete block (year) design. Previously, the data of some variables were by their natural logarithm transformed; however, their untransformed values are for ease of reading shown. The onset of estrus was considered taking into account the time elapsed between the withdrawal of the CIDR and the first frank estrous behavior (the sheep allowed it to mount, without moving). Data were analyzed using the SAS statistical package (SAS Institute Inc., Cary, NC), considering in all analyzes a value of P <0.05, as statistical significance ^{(Chavalarias et al., 2016)}.

RESULTS AND DISCUSSION

In this study, body condition can affect reproductive indicators (^{De La Isla et al., 2010}) and the weight of the sheep. At the beginning of the study, it was similar (P> 0.05) between the synchronization groups (Table 1), which makes it possible to ensure that any possible, difference between the studied treatments. They would not be associated with these indicators, and that both groups entered the study with equal terms. Recorded body condition is within desirable for ewes entering reproductive service period. Only for this variable, the inclusion of the year as a block was significant (P <0.05).

Although no differences (P> 0.05) were observed between the treatments for the percentage of estrus (Figure 1), the response achieved for this indicator was good, taking into account what has been described for sheep with similar synchronization protocols ^{(Ali et al., 2009}), and better than those who have used intravaginal sponges (^{Alavez et al., 2014}). On the other hand, a perhaps more important indicator, such as the conception rate, was not affected (P> 0.05) by the dose of eCG used.

Figure 1 Percentage of sheep in estrus and conception rate of sheep with estrus synchronization protocol that included low or high dose eCG  

The results are slightly higher than studies with sheep using sponges (^{Garoussi et al., 2019}). This allows us to affirm that the use of a dose of only 300 U of eCG can be as efficient in promoting the expression of estrus and the conception of ewes, as a higher dose (400 U). In the estrous synchronization protocols used, eCG is the second-highest cost, so a decrease in its dose can represent considerable savings. However, it remains to be established whether this dose can be even lower for the breed used.

The percentage of sheep in estrus showed similar results between both groups from 24 to 72 h (Figure 2). The results obtained allow us to suggest that the control of sheep in estrus should be after 33 hours carried out, because at 24 hours, none of the sheep showed estrus.

Figure 2 Percentage of sheep in estrus, which were synchronized with CIDR and low or high dose of eCG 

The onset of estrus occurred at a similar average (P> 0.05), between synchronization groups (Table 1), with averages that were higher than that observed in other studies using ^{CIDR (Ali et al., 2009; Arroyo et al., 2013}), or intravaginal sponges (^{Alavez et al., 2014}). The discrepancy between studies could be due to race; since this onset of estrus was similar to that observed also with hair sheep, using intravaginal sponges (^{Alavez et al., 2014}). Unfortunately, the diversity of conditions used in these studies limits a more precise comparison. The eCG can increase the ovulatory rate (^{Lozano-González et al., 2012}); however, although less prolificacy was observed with the use of a high dose of eCG, it was not statistically different. Prolificity values are higher than these described for the Pelibuey breed by ^{Galina et al. (1996)} and ^{Magaña-Monforte et al. (2013)}. It is likely that the high prolificacy associated with this breed allows for more flexibility regarding the use of eCG, even to use even lower doses than those used in this study, without obviously affecting or impacting the reproductive indicators evaluated. Studies in the future should verify the above.

CONCLUSIONS

The use of a 300 U dose of equine chorionic gonadotropin can be used in protocols for synchronizing the estrus of Pelibuey sheep, based on the use of intravaginal devices for controlled progesterone release. This dose does not modify the reproductive response of the sheep, compared to the use of higher doses of this hormone. Future studies should establish whether it is possible to use even lower doses for this breed, in addition to considering the cost of the protocols used.