Optimal reproductive performance of breeding herd is important for profitable pig farming andalso for success of artificialinsemination programs. Schedule in piggery units are mainlydepended on the reproductive cycle of female breeding herd. If sufficient female breeding herd are not available for further breeding, subsequent pig production will suffer. Control of female reproduction forestrus induction and synchronization are thus considered as a criticalreproductive management tool for improving the reproductive performance in pigs.
It is ideal for pig farmers to have synchronized estrus occurring at an early period and at the same time for a group of animals as they can be bred at the same time and will deliver young ones together at the same period. The young ones will become a group of pigs that are about the same age and can be managed with the same feed housing and marketing. Reproductive performance can be improved with the use of suitableand cost-effective methods for estrus induction and synchronized breeding in replacement gilts and weaned sows that do not return to estrus when expected. Moreover, estrus synchronization can facilitate Fixed-time insemination which is a highly effective way of achieving pregnancy without reference to estrus.
Many methods based on hormonal agents are available for estrus induction and synchronization in pigs. Most of the methods of controlling ovarian activity are based on regulating events leading to follicular maturation and ovulation or altering the luteal phase. It is critical to clearly understand the estrous cycle of pigs before applying specific method. Estrous cycle, with an average duration of 21 days, is composed of comparatively longer luteal phase (approximately 16-days) and a shorter follicular phase (approximately 5 days). In the luteal phase, ovaries containing corpora lutea secrete systemic progesterone, which regulates follicular development to the medium-sized follicle stage and thusblocks the onset of estrus. At about 12–14 days of the luteal phase, uterine production of prostaglandin F2a (PGF2a) causes regression of corpora lutea and thus systemic progesterone level declines. Removing the progesterone block allows resumption of secretory patterns of the pituitary gonadotropins viz. luteinizing hormoneand follicle stimulating hormone which, subsequently allows follicular development to be completed with the estrogen production leading toonset of behavioral estrus.
Research efforts were made in past few decades for developing and validating various protocols for control of estrus and ovarian activity in pigs using exogenous hormonal preparations particularly to delay follicle development (progesterone analogues), to control follicle development (Pregnant Mare Serum Gonadotrophin, Follicle Stimulating Hormone-analogues) or to induce ovulation (Luteinizing Hormone or GnRH-analogues).
Specific applications and prospects of these hormonal agents are described below.
Progesterone and synthetic progestogens have been used to control estrus activity in pigs. Orally active synthetic progestins or progestogens like Altrenogest or Allyl trenbolone are used for effective control of estrus and ovulation in horses and pigs. These agents can be fed orally at a dose rate of 15-20 mg per day for 18 days interval in pigs and can be used to suppress estrus in female pigs in order to facilitate induction of normal estrous cycle activity. Onset of estrus can be expected around 3-8 days after the withdrawal of Altrenogest. Use of altrenogest has been shown to synchronize estrus in more than 80% of females within 5-8 days from last feeding of the synthetic progestogen. Since estrus suppression is required only during the time of luteal regression, if cycle dates are known, costs can be minimized by feeding Altrenogest from approximately 13 days after estrus detection until 5 days before scheduled breeding. In pigs, Altrenogest is used for synchronization of estrus in sows as well as sexually mature gilts that have had at least one estrous cycle. The preparation can also be used as an alternative treatment strategy for managing anestrus in pigs.
It is possible to induce follicle development, estrus and ovulation in the pre-pubertal gilt through other exogenous hormonal agents. If the cyclic status of the animal is unknown, agents like exogenous gonadotropins can be used successfully. PG 600 is one such hormonal agent commonly used in western countries, which is a combination of gonadotrophins viz. Pregnant Mare Serum Gonadotrophin (PMSG) and human Chorionic Gonadotrophin (hCG). PG600 is used to induce ovarian activity and puberty in gilts. PG 600 has also been effective in treating post-weaning anestrus in sows.
Pregnant Mare Serum Gonadotrophin (PMSG) or Equine Chorionic Gonadotrophin eCG is another commonly used and effective agent for induction of follicular growth, estrus, and ovulation in pigs. It can be therapeutically used to induce fertile estrus in anestrus sows as it has the effect of Follicle Stimulating Hormone (FSH) in stimulating ovarian follicular growth. When using PMSG alone, higher doses (900 vs. 600 IU) may improve the response of parity-one sows. Higher dose of PMSG (1200 IU) given on the day after weaning is also effective in inducing early return to estrus in weaned primiparous sows.
GnRH (gonadotrophin releasing hormone) and porcine luteinizing hormone (pLH) have luteinizing hormone effect and are sometimes used to stimulate or synchronize ovulation after induction of estrus or at the first sign of estrus. The timing for ovulation induction during the follicle development stage is very important as follicle maturity changes over time. Apart from female fertility, the success of the response is also determined by the stage of follicle development.
Besides GnRH analogues, human chorionic gonadotropin and porcine LH (pLH) can also induce ovulation. Compared to pLH, hCG injection does not provide adequate synchronization of ovulation.Also, combination of exogenous gonadotrophins can be administered on the day of withdrawal of oral progesterone feeding to better synchronize estrus.In case of longer and variable wean-to-estrus intervals in a herd, exogenous gonadotropins can also be used in sows at the time of weaning. Breeding of sows over short period of 3-4 days after weaning facilitates farrowing over fewer days and helps improve the uniformity of pigs at weaning.
Luteolytic agents like prostaglandin F2α can be used for synchronization but a major constraint is that the Corpus Luteum (CL) is resistant to prostaglandin-induced luteolysis prior to day 12 of the estrus cycle and CL is sensitive only after day 12 -13 of porcine estrus cycle. A single injection of PGF2α or its analogues in pigs is mostly ineffective as this intervention will not induceluteolysis before day 12 of the estrous cycle. However, the length of estrous cycle can be reduced by2–5 days by injecting PGF2α every 24 or 36 hours between days 6 and 10 of the cycle.
Recently, it has been reported that multiple injections of PGF2α analogues can induce luteolysis before day 12 of estrous cycle and may be utilized in the development of newer regimens for ovulation synchronization and timed insemination protocols.
Integration of suitable estrus induction and synchronized breeding schedules with management interventions hasbeen found to enhance the overall efficiency of the protocol. Management strategies include bio-stimulation through boar exposureand transport, relocation or mixing of breeding females. The visual and olfactory stimuli as well as physical presence of an active boar are external stimuli that the female hypothalamus translates into GnRH secretion, which eventually leads to estrus onset. Research suggests that shifting females to boar shedor vice versa improves estrus induction and detection rates. Sterile or vasectomized boar can be used for stimulation at a ratio of 1 boar per 12-15 females.
Mixing or relocation of female pigs is also a management tool that can be integrated with hormonal aids to induce puberty especially in pre-pubertal gilts. Exposure of gilts to a novel environment or a new group of pen mates is believed to have a mildly stimulatory effect on the hypothalamus. Moving gilts to new pens, increased exercise, and daily exposure to boars beginning between 5-6 months of age will help stimulate the ovarian activity and estrus onset. Group weaning is also a successful method for synchronization of estrus in sows for enabling production scheduling. However, post-weaning estrus is influenced by multiple factors like lactation length, parity, and season. Interestingly, partial weaning with or without gonadotropin treatment have also been found to induce estrus during lactation, but the results are inconsistent.
Commercially available exogenous hormones and their analogues have potential applications in the management of swine herd reproduction and have been used to control estrous cycle and ovarian activity in pigs. Hormonal strategies coupled with managerial interventions have allowed for improved reproductive performance in pigs and productivity of the farms. These tools should not be applied as an alternative togood management practices, and it is important to comply with therelevant protocols for satisfactoryoutcomes. A cost-benefit analysis should preferably be done to determine the real value of implementing the relevant strategies. Considering the productivity and farm requirement, more cost-effective and simplified protocols for estrus control can be of great benefit for pig farmers in enhancing farm profitability. Judicious use of hormonal agents through these protocols along with appropriately applied managerial tools can be of great benefit to improving productivity in swine breeding herds.