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Treatment of long-term anestrous sows with estradiol benzoate and GnRH: Response of serum LH and occurrence of estrus
THERIOCENOLOGY TREATMENT OF LONG-TERM ANESTROUS SOWS WITH ESTRADIOL BENZOATE AND GnRH:
RESPONSE OF SERUM LH AND OCCURRENCE OF ESTRUSI'*
N. M. Cox3 , K. L. Esbenshade and J. H. Britt Department of Animal Science North Carolina State University Raleigh, North Carolina 27650 USA Received for publication: Accepted:
March 2, 1983 September 15, 1983
Abstract Seventeen primiparous sows, anestrous for 41 i 4 days after weaning, received i.m. injections of 500 pg estradiol benzoate (EB) or corn oil. At 48 hr after treatment, LH averaged 12.1 i 2.6 ng/ml in EB-treated sows and 0.7 ? 0.1 ng/ml in corn oil-treated sows. At 55 hr after EB or corn oil, each sow was given 50 pg gonadotropin releasing hormone (GnRH). Average LH 1 hr after GnRH was 5.7 + 1.1 and 5.1 + 0.9 ng/ml in EB- and corn oil-treated sows, respectively. All EB-treated sows exhibited estrus 2.3 f 0.2 days after treatment and were mated. None of the corn oil-treated sows exhibited estrus and all were slaughtered two weeks after treatment. Examination of reproductive tracts revealed that the ovaries of corn oil-treated sows were small and did not contain corpora lutea. In mated sows, progesterone concentrations in blood two weeks after mating indicated luteal function in eight of the nine animals. Positive pregnancy diagnoses were made in all eight animals; however, only three sows farrowed, with litter sizes of four, five and seven, respectively. Results of the present experiment indicate that the hypothalamus and anterior pituitary of long-term anestrous sows are capable of responding to endocrine stimuli (i.e. estradiol and GnRH). Moreover estradiol induced estrus and ovulation, but subsequent farrowing rate was only 33 percent and size of litters was small.
1 Paper No. 8760 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh. The use of trade names in this publication does not imply endorsement of the products named, nor criticism of similar ones not mentioned. 2
Purified porcine LH (LER 786-3) was kindly supplied by Dr. L. E. Reichart, Jr., Albany Medical College of Union University, Albany, New York; porcine LH antiserum by Dr. G. D. Niswender, Colorado State University; and synthetic GnRH by M. D. Brown, Ceva Laboratories.
'Present address: Department of Animal Science, P.O. Drawer 5228, Mississippi State University, Mississippi State, Mississippi 39762.
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THERIOGENOLOGY Introduction Delayed returns to estrus in sows occur when litters are weaned during summer,and return intervals are longer year-round in primiparous sows than in multiparous (1,2,3,4). During July to October, the proportion of sows in estrus within seven to ten days after weaning decreased by 10 to 35% (1,2,3,4), and rebreeding performance of primiparous sows was 20 to 30% below that of multiparous sows (2,3). Ovaries of anestrous sows slaughtered one or more months after weaning were usually small and did not contain corpora lutea (5,6,7). Endocrine changes between weaning and estrus in sows are characterized by increases in serum luteinizing hormone (LH) and follicle stimulating hormone (FSH) during two to three days after weaning (8,9,10). Concentrations of GnRH in the hypothalamus and stalk median eminence increase concomitantly with increased gonadotropins during this period (9). The increased gonadotropins stimulate follicular growth, which results in increased estrogen levels two to four days after weaning (8,lO). Normally, these endocrine changes result in estrus and ovulation within four to ten days after weaning (11,12). The importance of estrogens as a stimulus for preovulatory gonadotropin surges has been demonstrated by administration of estradiol to ovariectomized sows (8). In that study, ovariectomized weaned sows given estradiol exhibited estrous responses and preovulatory gonadotropin surges similar to intact weaned sows. Therefore, once estrogen secretion is stimulated by increased gonadotropins after weaning, this steroid may be the primary stimulus for further events that lead to onset of estrus and ovuiation. Some malfunction in endocrine signalling at weaning apparently occurs in sows which remain anestrous. The objective of the present experiment was to determine the responsiveness of long-term anestrous sows to hormonal stimulation similar to that which precedes a normal postweaning estrus. To this end, weaned anestrous sows were given estradiol benzoate and GnRH, and serum LH concentrations and incidence of estrus were assessed. Materials and Methods Animals utilized in this study were from a group of 64 primiparous sows which farrowed from May to July 1981 on a 600-sow commercial farm. These 64 sows had lactated 26 to 31 days before pigs were weaned and had not exhibited estrus by 55 Jo8 days (X + SEM; range 29 to 134) after weaning. The sows were all crossbreds and the crosses included Duroc, Hampshire, Yorkshire and Landrace breeds. After weaning, groups of six to eight sows were placed in pens and fed 1.8 to 2.3 kg daily of a corn-soybean diet formulated to meet or exceed NRC (13) guidelines. Beginning four days after weaning, individual sows and boars were moved to pens daily for detection of estrus.
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THERIOGENOLOGY To verify that these 64 sows were acyclic, blood fur progesterone analysis (8) was obtained from 29 sows selected at random. Progesterone concentrations in these 29 sows were less than 1 ng/ml, indicating that Sows were acyclic. A further subsample of 17 sows anestrous for 41 f 4 days (range 34 to 55) after weaning was chosen to receive i.m. injections of 500 pg EB in corn oil (n = 9) or corn oil alone (n = 8). Blood was obtained by jugular venipuncture from restrained sows at 0, 48, 52, 55 and 56 hr after injection. This sampling scheme was chosen to determine if estradiol would induce a LH surge (8, 14). Immediately after the 55-hr sample, 50 vg GnRH was given i.m. to all sows to determine whether the pituitary would respond by releasing a surge of LH. Sows which exhibited estrus after treatment were mated at the onset of estrus and 24 hr later. Concentrations of LH at 0 hr and 48 to 56 hr after EB treatment were quantitated by radioimnunoassay (8). Formation of corpora lutea was monitored by measurement of progesterone in blood collected 14 days after treatment. Pregnancy was assessed by measurement of progesterone in blood collected 22 days after breeding, by nonreturn to estrus and by ultrasonic pregnancy diagnosis 35 days after breeding. Sows not observed in estrus were slaughtered 14 days after treatment. Treatment effects on progesterone concentrations two weeks after breeding and LH response one hr after GnRH treatment were analyzed by Student's t-test (15). Effects of EB on LH were analyzed by split plot analysis of variance (16). The final model used included effects of treatment, sow within treatment, hour, and treatment by hour interaction. Results Concentration of LH in blood serum was 12.1 + 2.6 ng/ml at 48 hr after EB treatment and was higher than in corn oil-treated sows (P
NOVEMBER 1983 VOL. 20 NO. 5
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THERIOGENOLOGY diagnosed pregnant by nonreturn to estrus and by ultrasonic analysis at 35 days after mating. In the ninth EB-treated sow, progesterone concentration was 0.3 ng/ml two weeks after mating,which was similar to progesterone concentrations (0.3 f 0.04) in acyclic corn oil-treated sows two weeks after treatment.
+CORN
0 46 HOURS AFTER
Fig. 1.
TABLE I.
OIL
52 5556 TREATMENT
Serum concentrations of luteinizing hormone (LER 786-3) after treatment of anestrous sows with estradiol benzoate or corn oil at 0 hr and gonadotropin releasing hormone at 55 hr.
Follicle size and ovarian and uterine weight in anestrous sows at slaughter. Anestrous Sows
Normal Proestrousa
Mean
SEM
Range
Mean
5.3
d.2
0 to 11
17.4
Ovarian weight (g)b
7.5
0.8
6 to 11
9.5
Uterine weight (g)
212
33
145 to 306
594
Item No. ovarian follicles 5 to 10 mm diameterb
aValues for sows killed 96 hr after weaning (9). b Total for both ovaries.
502
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THERIOGENOLOGY Of the eight sows diagnosed pregnant, three farrowed litters of four, five and seven pigs, respectively. We did not observe whether the remaining mated sows returned to estrus and (or) aborted. Discussion Results of this experiment indicate that the weaned anestrous sow is capable of responding to estradiol by secreting a preovulatory LH surge, exhibiting estrus and .~ ovulating. Level of LH and timing of the LH surge after EB were similar to those in weaned intact sows after endogenous surges of estrogen (8,lO) and in weaned ovariectomized sows after exogenous estradiol (8). These results illustrate the importance of estradiol in inducing estrus and eliciting a LH surge to cause ovulation. The mode of action of estradiol on the hypothalamohypophyseal unit may involve effects on the synthesis and secretion of GnRH and on the pituitary's response to GnRH, as has been demonstrated with ovariectomized gilts (17). The ability of long-term anestrous sows to respond to EB, yet their failure to exhibit estrus after weaning, indicates that one or more endocrine mechanisms necessary for estrus did not function properly at weaning. However, efforts to relate hormonal changes in sows that return to estrus after weaning to those that fail to return have produced mixed results. NO differences in serum LH and estradiol patterns during three to five days after weaning were detected between sows which exhibited normal (within ten days) or delayed returns to estrus (18,19). However, comparison of anestrous sows with sows that exhibited estrus within two weeks after weaning revealed in the latter higher urinary estrone two days after weaning and higher serum FSH one day after weaning (6,20). It is unlikely that lack of ovarian responsiveness to gonadotropins is a primary cause of postweaning anestrus, since injections of pregnant mare's serum gonadotropin (PMSG) and human chorionic gonadotropin (HCG) produce estrus in lactating sows (21,22,23,24). Ovaries and uteri of anestrous sows were morphologically similar to those of lactating sows (9,25) and presumably were equally capable of responding to gonadotropic stimuli. The innate ability of the ovary of the anestrous sows to respond to proper gonadotropic stimulation is also supported by experiments in which PMSG given after weaning reduced the incidence of anestrus (I). Less evidence is available about possible malfunctions of the anterior pituitary, which could cause anestrus by failing to stimulate the ovary sufficiently. However, in the present study, serum LH after exogenous GnRH was similar in EB- and corn oil-treated sows. When lactating sows were given hourly injections of GnRH for three to five days, they exhibited estrus and had serum gonadotropin patterns similar to weaned estrous sows (10). Moreover, hourly injections of GnRH (1.5 pg/hr) into four weaned primiparous SOWS that had been anestrous for 30 days resulted in estrus in all four sows between 72 and 96 hours
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THERIOGENOLOGY after onset of GnRH treatment (J. D. Armstrong and J. H. Britt, unpublished observation). The fact that proper stimulation with GnRH can overcome anestrus also argues against pituitary insufficiency as a primary cause of postweaning anestrus. Endocrine abnormalities in the hypothalamus or higher centers may be the primary factor in the etiology of postweaning anestrus, but this area has not been investigated directly. The farrowing rate (three of eight) of EB-treated sows was low despite positive pregnancy diagnoses. The total litter sizes of four, five and seven in the three successful pregnancies indicate that ovulation rate may have been low and (or) pregnancy may have failed in the majority of animals. A previous study (26) suggested that sows bred during mid to late summer experienced a delayed return to estrus. In that study, 26 sows had low, but detectable, progesterone levels (~5 ng/ml) at 18 and 30 days after mating. These sows were slaughtered if they had not returned to estrus by day 35. At slaughter, these 26 sows were found to have small inactive ovaries,and no embryonic residues could be detected. The authors (26) suggested that low progesterone on day 18 could have been from small ovarian cysts that may have subsequently regressed to form small inactive ovaries. Evidence for ovulation rates being lower than normal is provided by the luteal phase progesterone concentrations of 3 to 6 ng/ml, which are approximately 25 to 50% of normal luteal phase sows in our laboratory. Administration of estradiol valerate (EV) to prepubertal gilts induced ovulation (27) but the ovulation rate was lower than in gilts treated with PMSG/HCG alone. Moreover, when EV was administered for four days in combination with PMSG/HCG given on day two, the ovulation rate was lower than in gilts treated with PMSG/HCG alone. Comparison of the number of follicles greater than 5 to 10 mm in diameter in control anestrous sows two weeks after weaning with those of proestrous sows (Table I) also indicates that fewer potentially ovulatory follicles were present at the time of treatment with EB. Maturation of additional follicles during the interval between EB treatment and estrus is unlikely, since serum FSH and LH are expected to be suppressed prior to the estradiol-induced preovulatory surge (8,14,17). Treatment of anestrous sows with PMSG and HCG usually results in normal rates of ovulation and litter sizes (28,29). Presumably the PMSG stimulates adequate follicular growth prior to ovulation. Overall, the results of this study show that long-term anestrous sows will respond to EB and GnRH by secreting a surge of LH. Estrus occurs after treatment with EB, but subsequent luteal function is not normal -- either because ovulation rate is low or because progesterone secretion arises from luteinized follicles rather than corpora lutea. Thus, treatment of long-term anestrous sows with EB should not be recommended because the farrowing rate after treatment is unacceptably low.
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THERIOGENOLOGY
References 1.
Hurtgen, J. P. and Leman, A. D. Use of PMSG in the prevention of seasonal post-weaning anestrus in sows. Theriogenology -12:207-214 (1979).
2.
Hurtgen, J. P., Leman, A. D., and Crabo, B. on estrous activity in sows and gilts. 176:119-123 (1980).
3.
Britt, 3. H.,Szarek, V. E., and Levis, D. G. Characterization of summer infertility of sows in large confinement units. Theriogenology -20:133-140 (1983).
4.
Cox, N. M., Britt, J. H., Armstrong, W. D., and Alhusen, H. D. Effect of feeding fat and altering weaning schedule on rebreeding in primiparous sows. J. Anim. Sci. -56:21-29 (1983).
5.
Dyck, G. W. Puberty, postweaning estrus and estrouscycle length in Yorkshire and Lacombe swine. Can. J. Anim. Sci. -51:135-140 (1971).
6.
Dyck, G. W., Palmer, W. M., and Simaraks, S. Postweaning plasma concentrations of luteinizing hormone and estrogens in sows: effect of treatment with pregnant mare's serum gonadotropin or estradiol-17B plus progesterone. Can. J. Anim. Sci. -59:159-166 (1979).
7.
Svajgr, A. J., Hamrnell,D. L., Degeeter, M. J., Hays, V. W., Cromwell, G. L., and Dutt, R. H. Reproductive performance of sows on a protein restricted diet. J. Reprod. Fert. -30:455-458 (1972).
8.
Stevenson. J. S.. Cox. N. M.. and Britt. J. H. Role of the ovary in controlling luteinizing hormone; follicle stimulating hormone, and prolactin secretion during and after lactation in pigs. Biol. Reprod . !4:341-353 (1981).
9.
cox
N. M. and Britt, J. H . Relationships between endogenous gonadotropin-releasing hormone, gonadotropins and follicular development after wean ing in sows. Biol. Reprod. -27:70-78 (1982).
10.
cox,
N. M. and Britt, J. H. Pulsatile administration of gonadotropin releasing hormone to lactating sows: endocrine changes associated with induction of fertile estrus. Biol . Reprod. -27:1126-1137 (1982).
11.
Svajgr, A. J., Hays, V. W., Cromwell, G. L., and Dutt, R. H. Effect of lactation duration on reproductive performance of sows. J. Anim. Sci. -38:100-105 (1974).
)
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1983 VOL. 20 NO. 5
Seasonal influences J. Am. Vet. Med. Assoc.
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THERIOGENOLOGY 12.
Cole, D. J. A., Varley, M. A., and Hughes, P. E. Studies in sow reproduction. 2. The effect of lactation length on the subsequent reproductive performance of the sow. Anim. Prod. -20:401-406 (1975).
13.
National Research Council. Nutrient Requirements of Domestic Animals, No. 2. Nutrient Requirements of Pigs. National Academy of Science--National Research Council, Washington, D.C. (1979).
14.
Lantz, W. B. and Zimmerman, D. R. treated ovariectomized gilts. (1972).
15.
Snedecor, G. W. and Cochran, W. G. Statistical Methods (6th Ed.). Iowa State Univ. Press, Ames. (1967).
16.
Gill, J. L. and Hafs, H. D. Analysis of repeated measurements of animals. J. Anim. Sci. -33:331-336 (1971).
17.
Cox, N. M. and Britt, J. H. Effect of estradiol on hypothalamic GnRH and pituitary and serum LH and FSH in ovariectomized pigs.
18.
Plasma LH levels in estradiolJ. Anim. Sci. -35:1120 (Abstr.)
J. Anim. Sci. -55:901-908 (1982).
Van de Wiel, D. F. M., Van Landeghem, A. A. J., Willemse, A. H., and Bevers, M. M. Endocrine control of ovarian function after weaning in the domestic sow. J. Endocrinol. -80:69 (Abstr.)
(1978).
19.
te Brake, J. A. A,, Osinga, A., and Jansen,
20.
Aherne, F. X., Christopherson, R. J., Thompson, J. R., and Hardin, R. T. Factors affecting the onset of puberty, post-weaning estrus and blood hormone levels of Lacombe gilts. Can. J. Anim. Sci. -56:681-692 (1976).
21.
Crighton, D. B. Induction of pregnancy during lactation in the sow. J. Reprod. Fert. -22:223-228 (1970).
22.
Martinat-Botte, F. Induction of gestation during lactation in the sow. Ann. Biol. Anim. Bioch. Biophys. %:369-374 (1975).
23.
Guthrie, H. D., Pursel, V. G., and Frobish, L. T. Attempts to induce conception in lactating sows. J. Anim. Sci. 47:1145-1156 (1978). -
24.
Hausler, C. L., Hodson, Jr., H. H., KUO, D. C., Kinney, T. J., Rauwolf, V. A., and Strack, L. E. Induced ovulation and conception in lactating sows. J. Anim. Sci. -50:773-778 (1980).
H. J.
Steroid
levels
in weaned sows and the occurrence of post-weaning oestrus. VIII Inter. Cong. on Anim. Reprod. and AI. -111:439-441 (1976).
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THERIOGENOLOGY 25.
Palmer, W. M., Teague, H. S., and Venzke, W. G. Macroscopic observations of the reproductive tract of the sow during lactation and early postweaning. J. Anim. Sci. -24:541-545 (1965).
26.
Johnston, N. E., Hennessy, D. P., and Stelmasiak, T. Stress and infertility in the sow. Proc. Int. Pig. Vet. Sot. Cong. p. 40 (1980).
27.
Schilling, E. and Minar, M. Die hormonale stimulation des schiveineovars zur zeit der geschlechtsreife. Zbl. Vet. Med. A -18:277-288 (1971).
28.
Rasbech, N. 0. A review of the causes of reproductive swine. Dr. Vet. J. 125:599-616 (1969).
29.
Schilling, E. and Cerne, F. Induction and synchronization of oestrus in prepubertal gilts and anestrous sows by a PMS/HCGcompound. Vet. Rec. _91:471-474 (1972).
NOVEMBER
1983VOL.20NO.S
failure
in
507
RESPONSE OF SERUM LH AND OCCURRENCE OF ESTRUSI'*
N. M. Cox3 , K. L. Esbenshade and J. H. Britt Department of Animal Science North Carolina State University Raleigh, North Carolina 27650 USA Received for publication: Accepted:
March 2, 1983 September 15, 1983
Abstract Seventeen primiparous sows, anestrous for 41 i 4 days after weaning, received i.m. injections of 500 pg estradiol benzoate (EB) or corn oil. At 48 hr after treatment, LH averaged 12.1 i 2.6 ng/ml in EB-treated sows and 0.7 ? 0.1 ng/ml in corn oil-treated sows. At 55 hr after EB or corn oil, each sow was given 50 pg gonadotropin releasing hormone (GnRH). Average LH 1 hr after GnRH was 5.7 + 1.1 and 5.1 + 0.9 ng/ml in EB- and corn oil-treated sows, respectively. All EB-treated sows exhibited estrus 2.3 f 0.2 days after treatment and were mated. None of the corn oil-treated sows exhibited estrus and all were slaughtered two weeks after treatment. Examination of reproductive tracts revealed that the ovaries of corn oil-treated sows were small and did not contain corpora lutea. In mated sows, progesterone concentrations in blood two weeks after mating indicated luteal function in eight of the nine animals. Positive pregnancy diagnoses were made in all eight animals; however, only three sows farrowed, with litter sizes of four, five and seven, respectively. Results of the present experiment indicate that the hypothalamus and anterior pituitary of long-term anestrous sows are capable of responding to endocrine stimuli (i.e. estradiol and GnRH). Moreover estradiol induced estrus and ovulation, but subsequent farrowing rate was only 33 percent and size of litters was small.
1 Paper No. 8760 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh. The use of trade names in this publication does not imply endorsement of the products named, nor criticism of similar ones not mentioned. 2
Purified porcine LH (LER 786-3) was kindly supplied by Dr. L. E. Reichart, Jr., Albany Medical College of Union University, Albany, New York; porcine LH antiserum by Dr. G. D. Niswender, Colorado State University; and synthetic GnRH by M. D. Brown, Ceva Laboratories.
'Present address: Department of Animal Science, P.O. Drawer 5228, Mississippi State University, Mississippi State, Mississippi 39762.
NOVEMBER
1983 VOL. 20 NO. 5
499
THERIOGENOLOGY Introduction Delayed returns to estrus in sows occur when litters are weaned during summer,and return intervals are longer year-round in primiparous sows than in multiparous (1,2,3,4). During July to October, the proportion of sows in estrus within seven to ten days after weaning decreased by 10 to 35% (1,2,3,4), and rebreeding performance of primiparous sows was 20 to 30% below that of multiparous sows (2,3). Ovaries of anestrous sows slaughtered one or more months after weaning were usually small and did not contain corpora lutea (5,6,7). Endocrine changes between weaning and estrus in sows are characterized by increases in serum luteinizing hormone (LH) and follicle stimulating hormone (FSH) during two to three days after weaning (8,9,10). Concentrations of GnRH in the hypothalamus and stalk median eminence increase concomitantly with increased gonadotropins during this period (9). The increased gonadotropins stimulate follicular growth, which results in increased estrogen levels two to four days after weaning (8,lO). Normally, these endocrine changes result in estrus and ovulation within four to ten days after weaning (11,12). The importance of estrogens as a stimulus for preovulatory gonadotropin surges has been demonstrated by administration of estradiol to ovariectomized sows (8). In that study, ovariectomized weaned sows given estradiol exhibited estrous responses and preovulatory gonadotropin surges similar to intact weaned sows. Therefore, once estrogen secretion is stimulated by increased gonadotropins after weaning, this steroid may be the primary stimulus for further events that lead to onset of estrus and ovuiation. Some malfunction in endocrine signalling at weaning apparently occurs in sows which remain anestrous. The objective of the present experiment was to determine the responsiveness of long-term anestrous sows to hormonal stimulation similar to that which precedes a normal postweaning estrus. To this end, weaned anestrous sows were given estradiol benzoate and GnRH, and serum LH concentrations and incidence of estrus were assessed. Materials and Methods Animals utilized in this study were from a group of 64 primiparous sows which farrowed from May to July 1981 on a 600-sow commercial farm. These 64 sows had lactated 26 to 31 days before pigs were weaned and had not exhibited estrus by 55 Jo8 days (X + SEM; range 29 to 134) after weaning. The sows were all crossbreds and the crosses included Duroc, Hampshire, Yorkshire and Landrace breeds. After weaning, groups of six to eight sows were placed in pens and fed 1.8 to 2.3 kg daily of a corn-soybean diet formulated to meet or exceed NRC (13) guidelines. Beginning four days after weaning, individual sows and boars were moved to pens daily for detection of estrus.
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NOVEMBER
1983 VOL. 20 NO. 5
THERIOGENOLOGY To verify that these 64 sows were acyclic, blood fur progesterone analysis (8) was obtained from 29 sows selected at random. Progesterone concentrations in these 29 sows were less than 1 ng/ml, indicating that Sows were acyclic. A further subsample of 17 sows anestrous for 41 f 4 days (range 34 to 55) after weaning was chosen to receive i.m. injections of 500 pg EB in corn oil (n = 9) or corn oil alone (n = 8). Blood was obtained by jugular venipuncture from restrained sows at 0, 48, 52, 55 and 56 hr after injection. This sampling scheme was chosen to determine if estradiol would induce a LH surge (8, 14). Immediately after the 55-hr sample, 50 vg GnRH was given i.m. to all sows to determine whether the pituitary would respond by releasing a surge of LH. Sows which exhibited estrus after treatment were mated at the onset of estrus and 24 hr later. Concentrations of LH at 0 hr and 48 to 56 hr after EB treatment were quantitated by radioimnunoassay (8). Formation of corpora lutea was monitored by measurement of progesterone in blood collected 14 days after treatment. Pregnancy was assessed by measurement of progesterone in blood collected 22 days after breeding, by nonreturn to estrus and by ultrasonic pregnancy diagnosis 35 days after breeding. Sows not observed in estrus were slaughtered 14 days after treatment. Treatment effects on progesterone concentrations two weeks after breeding and LH response one hr after GnRH treatment were analyzed by Student's t-test (15). Effects of EB on LH were analyzed by split plot analysis of variance (16). The final model used included effects of treatment, sow within treatment, hour, and treatment by hour interaction. Results Concentration of LH in blood serum was 12.1 + 2.6 ng/ml at 48 hr after EB treatment and was higher than in corn oil-treated sows (P
NOVEMBER 1983 VOL. 20 NO. 5
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THERIOGENOLOGY diagnosed pregnant by nonreturn to estrus and by ultrasonic analysis at 35 days after mating. In the ninth EB-treated sow, progesterone concentration was 0.3 ng/ml two weeks after mating,which was similar to progesterone concentrations (0.3 f 0.04) in acyclic corn oil-treated sows two weeks after treatment.
+CORN
0 46 HOURS AFTER
Fig. 1.
TABLE I.
OIL
52 5556 TREATMENT
Serum concentrations of luteinizing hormone (LER 786-3) after treatment of anestrous sows with estradiol benzoate or corn oil at 0 hr and gonadotropin releasing hormone at 55 hr.
Follicle size and ovarian and uterine weight in anestrous sows at slaughter. Anestrous Sows
Normal Proestrousa
Mean
SEM
Range
Mean
5.3
d.2
0 to 11
17.4
Ovarian weight (g)b
7.5
0.8
6 to 11
9.5
Uterine weight (g)
212
33
145 to 306
594
Item No. ovarian follicles 5 to 10 mm diameterb
aValues for sows killed 96 hr after weaning (9). b Total for both ovaries.
502
NOVEMBER
1983 VOL. 20 NO. 5
THERIOGENOLOGY Of the eight sows diagnosed pregnant, three farrowed litters of four, five and seven pigs, respectively. We did not observe whether the remaining mated sows returned to estrus and (or) aborted. Discussion Results of this experiment indicate that the weaned anestrous sow is capable of responding to estradiol by secreting a preovulatory LH surge, exhibiting estrus and .~ ovulating. Level of LH and timing of the LH surge after EB were similar to those in weaned intact sows after endogenous surges of estrogen (8,lO) and in weaned ovariectomized sows after exogenous estradiol (8). These results illustrate the importance of estradiol in inducing estrus and eliciting a LH surge to cause ovulation. The mode of action of estradiol on the hypothalamohypophyseal unit may involve effects on the synthesis and secretion of GnRH and on the pituitary's response to GnRH, as has been demonstrated with ovariectomized gilts (17). The ability of long-term anestrous sows to respond to EB, yet their failure to exhibit estrus after weaning, indicates that one or more endocrine mechanisms necessary for estrus did not function properly at weaning. However, efforts to relate hormonal changes in sows that return to estrus after weaning to those that fail to return have produced mixed results. NO differences in serum LH and estradiol patterns during three to five days after weaning were detected between sows which exhibited normal (within ten days) or delayed returns to estrus (18,19). However, comparison of anestrous sows with sows that exhibited estrus within two weeks after weaning revealed in the latter higher urinary estrone two days after weaning and higher serum FSH one day after weaning (6,20). It is unlikely that lack of ovarian responsiveness to gonadotropins is a primary cause of postweaning anestrus, since injections of pregnant mare's serum gonadotropin (PMSG) and human chorionic gonadotropin (HCG) produce estrus in lactating sows (21,22,23,24). Ovaries and uteri of anestrous sows were morphologically similar to those of lactating sows (9,25) and presumably were equally capable of responding to gonadotropic stimuli. The innate ability of the ovary of the anestrous sows to respond to proper gonadotropic stimulation is also supported by experiments in which PMSG given after weaning reduced the incidence of anestrus (I). Less evidence is available about possible malfunctions of the anterior pituitary, which could cause anestrus by failing to stimulate the ovary sufficiently. However, in the present study, serum LH after exogenous GnRH was similar in EB- and corn oil-treated sows. When lactating sows were given hourly injections of GnRH for three to five days, they exhibited estrus and had serum gonadotropin patterns similar to weaned estrous sows (10). Moreover, hourly injections of GnRH (1.5 pg/hr) into four weaned primiparous SOWS that had been anestrous for 30 days resulted in estrus in all four sows between 72 and 96 hours
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THERIOGENOLOGY after onset of GnRH treatment (J. D. Armstrong and J. H. Britt, unpublished observation). The fact that proper stimulation with GnRH can overcome anestrus also argues against pituitary insufficiency as a primary cause of postweaning anestrus. Endocrine abnormalities in the hypothalamus or higher centers may be the primary factor in the etiology of postweaning anestrus, but this area has not been investigated directly. The farrowing rate (three of eight) of EB-treated sows was low despite positive pregnancy diagnoses. The total litter sizes of four, five and seven in the three successful pregnancies indicate that ovulation rate may have been low and (or) pregnancy may have failed in the majority of animals. A previous study (26) suggested that sows bred during mid to late summer experienced a delayed return to estrus. In that study, 26 sows had low, but detectable, progesterone levels (~5 ng/ml) at 18 and 30 days after mating. These sows were slaughtered if they had not returned to estrus by day 35. At slaughter, these 26 sows were found to have small inactive ovaries,and no embryonic residues could be detected. The authors (26) suggested that low progesterone on day 18 could have been from small ovarian cysts that may have subsequently regressed to form small inactive ovaries. Evidence for ovulation rates being lower than normal is provided by the luteal phase progesterone concentrations of 3 to 6 ng/ml, which are approximately 25 to 50% of normal luteal phase sows in our laboratory. Administration of estradiol valerate (EV) to prepubertal gilts induced ovulation (27) but the ovulation rate was lower than in gilts treated with PMSG/HCG alone. Moreover, when EV was administered for four days in combination with PMSG/HCG given on day two, the ovulation rate was lower than in gilts treated with PMSG/HCG alone. Comparison of the number of follicles greater than 5 to 10 mm in diameter in control anestrous sows two weeks after weaning with those of proestrous sows (Table I) also indicates that fewer potentially ovulatory follicles were present at the time of treatment with EB. Maturation of additional follicles during the interval between EB treatment and estrus is unlikely, since serum FSH and LH are expected to be suppressed prior to the estradiol-induced preovulatory surge (8,14,17). Treatment of anestrous sows with PMSG and HCG usually results in normal rates of ovulation and litter sizes (28,29). Presumably the PMSG stimulates adequate follicular growth prior to ovulation. Overall, the results of this study show that long-term anestrous sows will respond to EB and GnRH by secreting a surge of LH. Estrus occurs after treatment with EB, but subsequent luteal function is not normal -- either because ovulation rate is low or because progesterone secretion arises from luteinized follicles rather than corpora lutea. Thus, treatment of long-term anestrous sows with EB should not be recommended because the farrowing rate after treatment is unacceptably low.
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