This study investigated the effect of three intramuscular injections of Prostaglandin F2α (PGF2α) administered seven days apart on estrus responses and some reproductive hormonal profiles in post-pubertal gilts.Sixteen gilts were divided into groups A, B, C, and D, with four animals in each group. Group A, B and C gilts were treated with PGF2α injections intramuscularly at doses 5mg,7.5mg and 10mg per gilt respectively on Days 0, 6 and 13 of the experiment. Group D (Control) were treated with intramuscular injections of 2ml phosphate-buffered saline (PBS) instead. Serum samples were collected every 24 hours for four days after each PGF2α injection and estrus detection was conducted daily. Results showed that 100% of Group A came into estrus 96 hours after the second PGF2α injection,while 50% of Group B came into estrus after the first injection; with the other 50% exhibiting estrus following the 2nd injection. Only 50% of Group C displayed estrus signs and this was observed following the second PGF2α injection. Control gilts did not express behavioral estrus during this study, suggesting that PGF2α may have induced estrus in the treated animals. LH and estradiol concentrations were significantly different in Group A and B when compared to the control following the 2nd PGF2α injection. In all, it was observed that serum concentrations of LH, FSH and oestradiol-17β reached maximal levels just prior to expression of estrus, thus suggesting that the pattern of reproductive hormonal changes in PGF2α-synchronized gilts may be quite similar to those of naturally cycling in-estrus gilts.

INTRODUCTION

Estrus synchronization is a well-established tool for the reproductive management of a variety of livestock species. Manipulating the estrous cycle of domestic animals is thus essential for improving livestock productivity, as it helps to generate more income for the farmer through the utilization of assisted reproductive techniques like artificial insemination, in vitro fertilization and embryo transfer technologies. In gilts, estrus manipulation is being increasingly used globally for achieving programmed piglet production through batch farrowing, especially in today’s modern intensive piggeries (1).

In spite of this widespread use, currently available methods for swine estrus synchronization face a number of challenges which include depression in reproductive performance, low conception rates, increased stress due to handling, risk of occurrence of cystic ovaries (as in allyl trenbolone feeding), occurrence of silent heats, anovulatory estrus, high cost, scarcity of pharmacological agents used, and the requirement of excessive handling (2). Consequently, options for estrus synchronization in pig production are still very limited, in comparison to options available in the husbandry of other domestic animals, like cattle, sheep and goat.

In a number of domestic livestock species, prostaglandin F_2α (PGF_2α) treatment is capable of inducing regression of the corpus luteum (luteolysis) during the greater part of the luteal phase and is therefore employed for estrus synchronization and timed artificial insemination in cattle (3), sheep (4), and buffaloes (5). Contrastingly, in female swine a single treatment with PGF_2α is not effective for inducing luteolysis until after Day 12 of the estrous cycle (6, 7), and therefore cannot be utilized for estrus synchronization. The reason for the species difference in luteolytic activity of exogenous PGF_2αis still unclear (8).

Notable investigators who have studied the luteolytic effect of PGF_2α in pigs include Diehl and Day (6). These authors were among the earliest researchers who reported that no luteolytic effect was observed when PGF_2α was administered to cycling gilts on day 10 and 12 of the estrous cycle. These same authors also observed that PGF_2α did show abortifacient effects when administered to pregnant gilts at 25 to 30 days post conception (6).

Subsequently, Hallford et al. (9) reported that PGF_2α given on days 12 and 13 of the estrous cycle was luteolytic in gilts, while PGF_2α administered on days 4 and 5 had minimal effects on the growth of the corpora lutea (CL). In furtherance to these works, Estill et al. (10) reported that although the pig corpora lutea had long been considered refractory to the luteolytic effect of PGF_2α before day 12 of the estrous cycle, luteolysis may well be possible if repeated injections are given from day 5 through day 10.

In view of the foregoing, it is important to continually study the potentials of PGF_2α as a possible agent for estrus synchronization in pigs by designing newer protocols until a very successful protocol is found. In addition to this, the need to look for viable alternative techniques for estrus synchronization in all domestic animals cannot be overemphasized particularly in today’s modern intensive global livestock industry.

The aim of this current study was therefore to investigate the possible outcome of single, double and triple intramuscular PGF_2α treatments on estrus responses and hormonal profiles in crossbred (Large White x Landrace) gilts.

MATERIAL AND METHODS

RESULTS

DISCUSSION

This study has shown that it is possible to induce estrus in post-pubertal cycling gilts by administration of PGF_2α intramuscularly seven days apart. The results of the mounting trials across the treatment groups showed that all four gilts (100%) in group A came into estrus at the same time; i.e. 4 days (96 hours) after the second injection, while in group B, two of the gilts (50%) came into estrus after the first injection; and the other two (50%) exhibited estrus following the 2nd injection. The result for group C further showed that only two gilts (50%) displayed estrus signs, and that occurred following the second injection of PGF_2α. Control gilts on the other hand, did not express behavioral estrus during this study, suggesting that PGF_2α may have induced estrus in the treated animals. Estrus responses of gilts to respective PGF_2α injections in the present investigation showed that with the exception of two gilts in Group B; the PGF_2α-treated animals generally did not exhibit estrus behavior after the first injection, but majority of them (66.6%) expressed signs of behavioral estrus after the second injection. This suggests that the second PGF_2α injection was more effective than the first and third injections in causing estrus in the test gilts. However, it could also be that there was a cumulative effect of repeated administration of PGF_2α which thus resulted in most of the gilts expressing estrus after the second injection. In addition, of all the three different doses used in this present investigation, it was demonstrated that the 5mg PGF_2α dose was the most effective in terms of estrus synchronization because it was in this group that the highest proportion of gilts came into estrus at the same time. It is also evident from this result that administration of the three different dosages of PGF_2α injected intramuscularly seven days apart was sufficient in bringing into estrus at least 50% of all the treated gilts in each group. However, the reason why the most successful synchronization was observed in the 5mg-treated group is still quite unclear. Since PGF_2α is not a specifically recommended drug for estrus synchronization in pigs, there is currently no recommended dose for synchronization in this species. It is therefore speculated here that the effective dose for inducing luteolysis and estrus in this species is not likely to be far from the 5 mg per gilt mark as shown by the result of the present study. These findings however conflict with an earlier report by Diehl and Day (6) who reported that 2mg and 5mg dose levels of PGF_2α was not luteolytic when administered during the luteal phase of the estrus cycle. With respect to this study, the reason for the disparity between the findings of the current study and the report of Diehl and Day (6) could have been because of the fact that the earlier authors administered PGF_2α once; i.e. only on Day 4 or 10 of the estrus cycle of pigs while the present study administered PGF_2α thrice, 7 days apart. The singular dosing regimen of Diehl and Day (6) was most likely responsible for the lack of luteolytic effect observed in their study. Furthermore, with regards to differences in estrus responses due to dose of PGF_2α in treated gilts, Puglisi et al. (11) had earlier reported that PGF_2α-induced luteolysis was not related to the dose used or to the weight of the animals, but was dependent upon the maturity of the animals; i.e. whether prepubertal or mature gilts. Their results showed significantly greater luteal regression was observed in pre-pubertal gilts than in mature gilts. In this present study, there was no comparison between pre-pubertal and post-pubertal gilts, so it was not possible to confirm this finding based on the investigation conducted.

The results of LH and estradiol-17β concentrations showed that there was a significant difference observed in the Group A and Group B gilts when compared to the control gilts following the 2nd PGF_2α injection. In all the treated groups, it was also observed that serum concentrations of LH, FSH and estradiol-17β reached maximal levels just prior to expression of estrus. These findings agree with the documented hormonal profile of naturally cycling estrus gilts which is reported to be characterized by a rise in estradiol and LH surge immediately prior to estrus expression in most domestic animals including pigs (12, 13).

Progesterone concentration, though not significantly different across the groups after the respective PGF_2α injections, showed a notable decrease in concentration across all the PGF_2α-treated groups corresponding with the days of serum collection after the second PGF_2α injection; which incidentally was the period when most of the test gilts in this study expressed estrus behavior. What this signifies is that during PGF_2α-induced estrus in pigs, it is expected that progesterone, the major hormone of the luteal phase, will decrease appreciably. This finding is also corroborated by the works of Noguchi et al. (12) who reported also that plasma progesterone concentrations dropped to less than 1 ng/ml 30-48 hours in pseudopregnant swine after treatment with cloprostenol (a PGF_2α analogue).

Changes in the concentrations of all the hormones in the animals that expressed behavioral estrus also showed that there was a significant difference in all the four hormones across all the PGF_2α-treated groups. While there was a marked increase in LH, FSH and estradiol during proestrus and estrus; progesterone was highest during the diestrus period in all the estrus induced animals. These findings agree with the findings of Noguchi et al. (12) who previously used PGF_2α to synchronize estrus in estradiol dipropionate (EDP) induced pseudopregnant sows.

CONCLUSION

From the results of this study it is hereby concluded as that it is possible to synchronize estrus in cycling post-pubertal gilts if PGF_2α is administered repeatedly seven days apart. In addition, treatment with 5 mg PGF_2α was the most effective dose of synchronizing gilts when three repeated intramuscular injections of PGF_2α were administered to cycling gilts. It is also advanced that if three injections of PGF_2α are administered to gilts 7 days apart, most gilts are likely to come to estrus after the second injection. Furthermore, hormonal profiles of gilts synchronized using 3 injections of PGF_2α are similar to hormonal profiles of naturally cycling gilts. Reproductive hormone sampling therefore is a good indicator of induced estrus in PGF_2α-treated gilts.

Based on the result of this present study, it is hereby recommended that future studies should investigate the fertility, conception, birth weight and litter size of gilts synchronized with intramuscular PGF_2α administered 7 days apart, as well as duration of PGF_2α-induced estrus and optimum time for artificial insemination