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TIMING OF ESTRADIOL INJECTIONS REQUIRED TO INDUCE
SHORT AND LONG PSEUDOPREGNANCY IN GILTS

A.E. Pusateri and M.A. Diekman
Department of Animal Sciences

Cycling gilts may have extended estrous cycles following exposure to estrogenic compounds, such a zearalenone in feed, or after experimental injection of estrogens. Properly timed injections of estrogen induce responses which are normally associated with pregnancy, including the production and release of endometrial secretions, increased uterine blood flow, decreased uterine release of prostaglandin F2 alpha, and extended luteal function. Therefore, this estrogen-induced phenomenon has been termed pseudopregnancy. Pseudopregnancy is frequently used in research examining various aspects of pregnancy in swine. Pseudopregnancies of varying lengths have been reported. Recent evidence suggests that there may be two types of pseudopregnancy, short and long, and that these two types may be triggered by estrogen at different times after estrus.

The precise timing of estrogen injections required to induce either short or long pseudopregnancy have not yet been determined. Varying treatment regimens have been used. These have often included high doses of estrogenic compounds which remain active for long periods because they are more slowly metabolized than the pig's natural estrogens, which are estradiol and estrone. Therefore, this study was conducted to determine the timing of estradiol (E2) injections required to induce short and long pseudopregnancy. An additional objective was to more fully characterize short and long pseudopregnancy in gilts.

MATERIALS AND METHODS

General - Crossbred gilts (Yorkshire x Hampshire x Duroc) exhibiting estrous cycles of normal duration (18-23 days) were used. Gilts were checked twice daily for estrus with fenceline exposure to a mature boar. Animals were housed inside in groups of five to ten on concrete flooring. A complete corn-soybean meal ration was fed at 1.5 - 2.0 kg/hd/d and animals had ad libitum access to water.

Pseudopregnancy - Gilts were assigned randomly to treatments. Treatments consisted of intramuscular injections of soybean oil or E2 dissolved in soybean oil (0.5 mg E2/ml) as described below. Dosages of E2 were either 4 ug/kg BW, 8 ug/kg BW, or 17 ug/kg BW. These dosages are equivalent to 0.5, 1.0, and 2.0 mg/122kg BW. During a series of four experiments, gilts received injections of E2 on various combinations of days between 12 and 25 following estrus and subsequent estrous cycle lengths were determined.

Definitions of pseudopregnancy - Short pseudopregnancy (SPP) required a cycle length of greater than 23 days with a minimum cycle extension of 3 days over the pretreatment cycle. Long pseudopregnancy (LPP) required a cycle length of greater than 50 days.

RESULTS

The proportions of gilts exhibiting SPP and LPP in response to E2 injections are shown in Table 1. The 0.5 mg E2/kg BW dose was not as effective as the higher doses. Pretreatment cycle length was 19.7 + 1.2 days. Gilts exhibiting SPP had a mean cycle length of 27.0 + 2.7 days. There was no effect of treatment on the duration of the SPP. The range of cycle lengths among the gilts exhibiting SPP was 23.5 to 35.0 days. There was no evidence of an effect of pretreatment cycle length on the incidence of either SPP or LPP.

DISCUSSION

Swine embryos signal their presence to the sow by synthesizing and releasing E2 and possibly other substances which interact with the sow's reproductive system to allow maintenance of the corpus luteum. This signal for maternal recognition of pregnancy occurs on approximately day 12 of pregnancy. If embryos are removed on day 11, estrous cycle length is not altered; however, if the embryos are removed on day 13, the interestrus interval is extended to about 26 days. Since the embryonic signals initiated on approximately day 12 of pregnancy are sufficient to extend luteal function only for a few days, it appears that a further signal is required for continued luteal function during pregnancy. The SPP results presented here are consistent with results obtained when pregnancy is terminated shortly after the initial signal for the maternal recognition of pregnancy. The maintenance of E2 administration to day 19 consistently resulted in the extension of the interestrus interval beyond 50 days in the current study. To the extent that the induction of pseudopregnancy represents the events leading to the extension of luteal function during pregnancy, these data are an indication that the second signal for pregnancy may be the continued presence of embryonic estrogen to day 19.

TABLE 1. Effect Of E2 Injection On Incidence Of Pseudopregnancy In Gilts.

CONCLUSIONS

1. Optimal induction of SPP is by injection of E2 daily from day 12 through day 19. While some gilts exhibit LPP in response to achieved by injection of E2 on days 12 and 13 of estrous cycle.
2. Administration of E2 beyond this 2 day period has no effect on the incidence of pseudopregnancy.
3. Optimal induction of LPP is achieved intermittent E2 administration, continuous exposure to E2 is required for a consistent response.
4. There is a clear demarcation between the SPP and LPP responses.
5. Intramuscular injection of E2 at dosages from 0.5 to 2.0 mg/118 kg BW results in elevated serum E2 concentrations which return to baseline within 16 to 18 hours.

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