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Virginia Cooperative Extension - Knowledge for the CommonWealth

Uses of P.G. 600 in Swine Breeding Herd Management

Livestock Update, March 2001

Mark J. Estienne, Swine Research Physiologist and Allen F. Harper, Extension Animal Scientist-Swine, Tidewater Agricultural Research and Extension Center

Introduction

Non-productive days (NPDs) accumulate for females that are not pregnant or lactating and include the period between entry of a gilt into the "gilt pool" and her first service, and the interval between weaning and mating in sows. To enhance reproductive efficiency in the breeding herd, NPDs must be minimized.

P.G. 600, a non-prescription drug marketed by Intervet America Inc. (Millsboro, DE), is used to stimulate the onset of heat and ovulation in prepubertal gilts and weaned sows and thus can decrease NPDs. Each 5-ml dose of P.G. 600 contains 400 I.U. of pregnant mare serum gonadotropin (PMSG) and 200 I.U. of human chorionic gonadotropin (hCG). P.G. 600 can be purchased in single (approximately $5.00) or five dose (approximately $24.00) vials. The active ingredients in P.G. 600 are purchased as a freeze-dried powder that is mixed in a sterile diluent prior to use. P.G. 600 is labeled for intramuscular (i.m.) injection in the neck behind the ear. Knox et al. (2000), however, reported a higher proportion of gilts exhibited heat with subcutaneous (s.c.) P.G. 600 (76%) than with i.m. P.G. 600 (52%).

How P.G. 600 Works

In swine as well as in other mammals, luteinizing hormone (LH) and follicle stimulating hormone (FSH) are released into the blood stream from the pituitary gland, a garden pea-sized organ that is located near the base of the brain. Once LH and FSH are secreted from the pituitary gland, the hormones travel through the circulation and act upon the ovaries.

As gilts near puberty (typically 6 to 8 months of age), levels of LH and FSH in the bloodstream increase, causing ovarian follicles to grow. Growing follicles secrete estrogen, which is responsible for the behavioral changes associated with heat. Moreover, rising estrogen levels in the blood reach a threshold that causes a massive release of LH around the time of the onset of standing heat. The large amounts of LH in the circulation cause ovulation (release of eggs from the follicles) approximately 40 hours after the onset of standing heat. The eggs are fertilized by sperm cells in the oviducts, tubes connecting the ovaries to the uterus, and the process of pregnancy is initiated.

In lactating sows, the nursing of pigs causes physiological mechanisms that inhibit release of LH and FSH from the pituitary gland. As a consequence, sows do not display follicle growth, heat and ovulation during lactation. When pigs are weaned, however, the suckling stimulus is removed. Thus, LH and FSH levels are allowed to increase and ovarian follicles grow with the associated production of estrogen and eventually the behavioral characteristics of heat. Similar to the situation in prepubertal gilts, rising estrogen levels reach a threshold that triggers large amounts of LH to be released from the pituitary gland and ultimately, ovulation.

Sows normally exhibit heat 3 to 7 days after weaning. Sometimes, however, the return to heat is delayed. For example, the weaning-to-heat interval may be extended in sows with suppressed appetite during lactation due to being over conditioned during gestation or to being exposed to the high environmental temperatures that are characteristic of Virginia's summers.

The active ingredients of P.G. 600 are PMSG and hCG, two hormones that mimic the actions of FSH and LH, respectively. Thus, when P.G. 600 is administered to prepubertal gilts or weaned sows, follicular growth is advanced. Heat and ovulation follow. It is important to note that P.G. 600 will stimulate heat only in prepubertal gilts and weaned sows. The drug will not stimulate heat in females that are already displaying estrous cycles and cannot be used to synchronize heat in cycling sows and gilts.

Use of P.G. 600 in Prepubertal Gilts

Because swine producers replace up to 50% of their sows with gilts each year, the introduction of females into the breeding herd is an important component of the overall reproductive efficiency of the farm. Large gilt pools are often maintained so that pubertal gilts will be available for breeding at specified times. NPDs accumulate, however, during the period between a female's entry into the gilt pool and her first service. Moreover, gilts not mated are often sold for slaughter at a discounted price because they exceed desired market weights. If heat can be reliably induced, farmers can select fewer replacement gilts and sell excess hogs at desirable market weights.

P.G. 600 can be administered to prepubertal gilts that are at least 5.5 months of age and that weigh at least 185 pounds. In a trial conducted on ten commercial farms in North Carolina, Illinois, and Missouri, Britt et al. (1989) reported that P.G. 600 induced fertile heat in prepubertal gilts. Crossbred gilts that were 5.5 to 7.5 months of age and that weighed between 189 and 359 pounds were employed. Treatment with P.G. 600 increased the percentage of gilts in heat within 7 or 28 days and decreased the interval to heat compared to un-injected controls. Gilts were bred at first heat and farrowing rate and the number of pigs born alive were similar for the P.G. 600-treated and control groups (Table 1).

Having gilts exhibit heat at a predicted time allows more efficient scheduling of breeding and farrowing facilities and provides greater opportunity for introduction of females into farrowing groups when sows are bred following weaning. For example, prepubertal replacement gilts could receive an injection of P.G. 600 on the day a sow group is weaned. Gilts would then display heat and could be bred along with sows returning to heat approximately 5 days after weaning.

Work in Maryland (Estienne and Hartsock, 1992) demonstrated that following the induction of puberty with P.G. 600, unbred gilts continue to cycle. In that study, similar numbers of P.G. 600-treated gilts (86%) and saline-injected controls (88%) exhibited a second, and then third heat after the onset of puberty. Average length of the estrous cycle was similar between groups and averaged about 21 days.

In many herds, gilts are bred on the second or third post pubertal heat in order to maximize the number of ovulations and thus increase potential litter size. Thus, P.G. 600 could be used to synchronize a group of gilts for subsequent breeding at the second or third heat after treatment.

It has been suggested that the enhanced ovulation rate in gilts seen during the second or third post pubertal heat, compared to the pubertal heat, is due to previous exposure to progesterone. At the Tidewater Agricultural Research and Extension Center in Suffolk, VA, we recently conducted an experiment to determine the effects of an orally active progesterone-like compound (Regu-mate; Intervet America Inc.) on the response to P.G. 600 in prepubertal gilts (Horsley et al., 2001).

Prepubertal, crossbred gilts, 143 days of age and weighing 189 pounds were utilized. Twenty gilts received a daily ration containing Regu-mate (15 mg/day) for 18 days and the remaining 20 gilts served as controls and received no Regu-mate in their feed. Twenty-four hours after withdrawal of Regu-mate, all gilts received an i.m. injection of P.G. 600. Gilts were checked for heat twice daily in the presence of a mature boar. Animals were sacrificed 7 to 11 days after the onset of standing heat and ovulation rate was determined.

The percentage of gilts displaying heat and ovulating < 7 days after P.G. 600 injection and the P.G. 600-to-heat interval were similar (P > .1) for Regu-mate treatment (95% and 4.3 days, respectively) and controls (89% and 4.2 days, respectively). Ovulation rate was also similar (P > .1) for Regu-mate-treated gilts (16.6) and controls (14.4).

In our study, P.G. 600 effectively advanced the onset of puberty in gilts. Pre-treatment with Regu-mate, however, had no beneficial effects on the onset of heat and ovulation rate after P.G. 600 treatment. Our results are similar to those of Knox and Tudor (1999) that indicated that exposure of gilts to implants containing a progesterone-like compound (norgestomet) for 9 days before P.G. 600 administration did not enhance expression of heat or ovulation rates when compared to gilts that received P.G. 600 alone.

Use of P.G. 600 in Weaned Sows

In research utilizing nearly 600 commercial sows, scientists from the University of Missouri, North Carolina State University, and Purdue University demonstrated that administration of P.G. 600 to first and second litter females, weaned during the summer at day 21 to 28 of lactation, decreased days to heat (Bates et al., 1991; Table 2). P.G. 600 also lowered the percentage of first litter sows not exhibiting heat within 10 days after weaning. Sows conceiving within a few days after treatment with P.G. 600 at weaning experienced a slight, but statistically significant, reduction in number born alive per litter (10.10 versus 10.55 for P.G. 600-treated sows and control sows, respectively).

In an experiment conducted during the summer in Maryland (Estienne and Hartsock, 1998), treatment with P.G. 600 increased the percentage of sows in heat within 7 days after weaning (97% for P.G. 600 treated sows versus 83% for controls). Relative to controls, sows given P.G. 600 expressed estrus sooner (3.8 versus 4.5 days).

Thus, treatment with P.G. 600 has been shown to decrease NPDs by decreasing days to the first post weaning heat in sows that are weaned in conventional production systems (i.e., 3 to 4 week weaning). More research is needed to determine efficacy of P.G. 600 in early-weaned sows (less than 3 weeks of lactation).

An alternative scheme to injecting all weaned sows with P.G. 600 is the following: Wean sows and check for heat daily, breeding sows that return to heat on their own. Administer P.G. 600 to sows that have not displayed heat by day 7 post-weaning. These animals should return to heat within 5 days after treatment and can be bred. With this system, only sows experiencing a delayed return to heat are treated with P.G. 600.

Summary

P.G. 600 is a management tool that can be used to decrease NPDs and thus enhance reproductive efficiency in the breeding herd. Research studies have shown that P.G. 600 is effective in inducing first heat and ovulation in prepubertal gilts and in decreasing the weaning-to-heat interval in sows. When considering the possible use of P.G. 600, farmers should perform a cost-benefit analysis, comparing the cost of the drug against the beneficial effects of its use on decreasing NPDs and allowing more effective scheduling of breeding and gestation facilities.

References

Bates, R.O., B.N. Day, J.H. Britt, L.K. Clark, and M.A. Brauer. 1991. Reproductive performance of sows treated with a combination of pregnant mare's serum gonadotropin and human chorionic gonadotropin at weaning in the summer. J. Anim. Sci. 69:894.

Britt, J.H., B.N. Day, S.K. Webel, and M.A. Brauer. 1989. Induction of fertile estrus in prepubertal gilts by treatment with a combination of pregnant marešs serum gonadotropin and human chorionic gonadotropin. J. Anim. Sci. 67:1148.

Estienne, M.J., and T.G. Hartsock. 1992. Induction of puberty with a combination of pregnant mare serum gonadotropin (PMSG) and human chorionic gonadotropin (HCG) in gilts. Proceedings of the Association of Research Directors Ninth Biennial Research Symposium. p. 47.

Estienne, M.J., and T.G. Hartsock. 1998. Effect of exogenous gonadotropins on the weaning-to-estrus interval in sows. Theriogenology 49:823.

Horsley, B.R., M.J. Estienne, A.F. Harper, C.E. Estienne, and J.W. Knight. 2001. Effects of P.G. 600 on estrus and ovulation in prepubertal gilts treated with Regu-mate. J. Anim. Sci. 79(Suppl. 1):24.

Knox, R.V., and K.W. Tudor. 1999. Influence of norgestomet in combination with gonadotropins on induction of estrus and ovulation in prepubertal gilts. J. Anim. Sci. 77:1348.

Knox, R.V., K.W. Tudor, S.L. Rodriguez-Zas, and J.A. Robb. 2000. Effect of subcutaneous vs intramuscular administration of P.G. 600 on estrual and ovulatory responses of prepubertal gilts. J. Anim. Sci. 78:1732.

Table 1. Reproduction in prepubertal gilts treated with P.G. 600 and control gilts (Britt et al., 1989).
    Control P.G. 600 SEc
Trait No: 341 337 ---
Heat < 7 days after treatment (%)  40.9 57.5a NRd
Heat < 28 days after treatment (%)  59.5 72.9a 2.7
Days from treatment to heat   10.47.5a 0.4
Farrowing rate (%)   78.9 78.1b 3.1
Pigs born alive per litter   8.68.6b 0.2
a P.G. 600 group differs from control group (P < .05).
b No effect of treatment.
cPooled standard error.
dNot reported.

Table 2. Reproduction in sows treated at weaning with P.G. 600 and control sows (Bates et al., 1991).
Parity and Trait Control P.G. 600 SEd
Parity 1   
Anestrusa, % 29.2 (85)b 15.6c (87) 3.9
Days from treatment to heat7.8 (72) 6.0c (77) .6
Parity 2   
Anestrus, % 14.5 (63) 9.6 (55) 4.8
Days from treatment to heat 6.4 (60) 4.8c (52) .7
Parity 3   
Anestrus, %8.2 (135) 10.4 (152) 2.9
Days from treatment to heat 5.5 (142) 5.6 (160) .4
aAnestrus defined as no heat within 10 days of treatment.
bNumber of observations per subclass.
cP.G. 600 group differs from control group (P < .06).
dPooled standard error.



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