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All Graduate Theses and Dissertations Graduate Studies
5-1990
Endocrine Interrelationships During Early Postpartum In St. Croix Sheep
Richard Michael Anderson Utah State University
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Recommended Citation Anderson, Richard Michael, "Endocrine Interrelationships During Early Postpartum In St. Croix Sheep" (1990). All Graduate Theses and Dissertations. 4140. https://digitalcommons.usu.edu/etd/4140
This Thesis is brought to you for free and open access by the Graduate Studies at DigitalCommons@USU. It has been accepted for inclusion in All Graduate Theses and Dissertations by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. ENDOCRINE INTERRELATIONSHIPS DURING EARLY
POSTPARTUM IN ST. CROIX SHEEP
by
Richard Michael Anderson
A thesis submitted in partial fulfillment of the requirements for the degree
of
MASTER OF SCIENCE
in
Animal Science (Reproductive Biology) Approved by:
UTAH STATE UNIVERSITY Logan, Utah 1990 ii
TABLE OF CONTENTS
Page
LIST OF TABLES ...... i ii
LIST OF FIGURES ...... i v
ABSTRACT ...... vi
I NTRODUCTION ...... 1
OBJECTIVES ...... 3
LITERATURE REVIEW ...... 4
First Postpartum Ov u lation a nd Estrus ...... 4 Progesterone in the Postpartum Ewe ...... 8 Estradiol-17-beta i n the Postpartum Ewe ...... 10 Luteinizing Hormone in the Postpartum Ewe ...... 1 3
MATERIALS AND METHODS ...... 18
Group 1 ...... • ...... 18 Group 2 ...... 19 Prelambing Management ...... •• ...... 19 Mana gement at Lambi ng ...... 19 Estrus Detection ...... 2 0 Blood Sampling . • ...... 20 Hormone Analysis ...... 2 2 Determination of Ov ulation ...... 23 Luteinizing Hormone ...... 2 4 Statistical Analys is ...... 24
RESULTS AND DISCUSSION ...... 2 6
Postpartum Ovulation and Estrus ...... 26 Progesterone Patterns .. • ...... 33 Luteinizing Hormone Patterns ...... 35 Estradiol-17-beta Patterns ...... • .. .• ...... 38
CONCLUSIONS ....• ...... •• ...... 51
LI TERATURE CITED ...... • . •...... 53 iii
LIST OF TABLES
Table Page
1. Serum LH patterns in White-Face mastectomized and intact ewes before lambing and during the postpartum period ...... 17
2 . Experimental groups ...... 18
3. The occurrence of ovulation and estrus and the length of the luteal phase in i ntact, postpartum ewes ...... 27
4. Association of ovulation and estrus ...... 27
5 . Mean progesterone level in intact (pre-ovulation) ewes from day 2-14 postpartum and the mean p rogesterone level in ovariectomized ewes ...... 34
6 . Basal LH patterns for group 1 (intact, postpartum) .... 35
7. Estradiol associated with acute release of LH ...... 38
8 . Length of luteal phase in relation to maximum estradiol levels and estrus behavior ...... 39
9. Estradiol concentrations in relation to estrus detection ...... 4 o iv
LIST OF FIGURES
Figure Page
1. Concentration of progesterone in relation to bas e LH level and LH pulse height for ewe 481 ...... 29
2 . Concentration of progesterone in relation to base LH lev el and LH pulse height for ewe 591 ...... 29
3 . Concentration of progesterone in relation to base LH level and LH pulse height for ewe 597 ...... 30
4 . Concentration of progesterone in relation to base LH level and LH pulse height for ewe 600 ...... 30
5 . Concentration of progesterone in relation to b ase LH level and LH pulse height for ewe 625 ..•...... 31
6 . Concentration of progesterone in relation to base LH level and LH pulse height for ewe 626 ...... 31
7. Concentration of progesterone in relation to ba s e LH level and LH pulse height for ewe 645 ...... 32
8 . Concentration of progesterone in relation to ba s e LH level and LH pulse height for ewe 3495 ...... 32
9 . Concentration of progesterone in relation to base LH level and LH pulse height for ewe 3514 ...... 33
10. Av erage LH base level and pulse height for group 1 (intact, postpartum) ...... 3 6
11. Concentration of LH and estradiol during the a c ute release of LH in ewe 481 ...... 42
12. Concentration of LH postpartum in ewe 481...... 42
13. Concentration of LH and estradiol during the acute release of LH in ewe 591 ...... •...•...... 43
14 . Concentration of LH postpartum in ewe 591 .•...... 43
1 5 . Concentration of LH and estradiol during the acute release of LH in ewe 597 .•...... ••...... 4 4
16. Concentration of LH postpartum in ewe 597 ...... 44 v
17. Concentration of LH and estradiol during the acute release of LH in ewe 600 •....•...... ••...... • . 45
18. Concentration of LH postpartum in ewe 600 ...... •. .. 45
19. Concentration of LH and estradiol during the acute release of LH in ewe 625 . .•••...... •... •. . • 46
20 . Concentration of LH postpartum in ewe 625 •. •...• ...... 4 6
21. Concentration of LH postpartum in ewe 626 ..• ...... 4 7
22. Concentration of LH a nd estradiol during the acute release of LH in ewe 64 5 ...... •. . . •...... •...... 48
23 . Concentration of LH postpartum in ewe 645 •• ...... 48
24. Concentration of LH and estradiol during the acute release of LH in ewe 3495 ...... •••••••.•.•...... 49
25. Concentration of LH postpartum in ewe 3495 •.•...... 49
26. Concentration of LH and estradiol during the acute release of LH in ewe 3514 . . • .• •• •.••••• ..•.• ...... 50
27 . Concentration of LH postpartum in ewe 3514 ...... 50 vi
ABSTRACT
Endocrine Interrelationships During Early
Postpartum in St. Croix Sheep
by
Richard Michael Anderson, Master of science
utah state university, 1990
Major Professor: Dr. Warren C. Foote Depa rtment: Animal, Da iry and Veterinary Sciences
The relationships of estradiol-17 -beta, progesterone , and
LH in the early postpartum St. Croix ewe were monitored during
the breeding season in 1988. A second group of non-
postpartum, ovariectomized St . croix ewes were used to
determine non-ovarian levels of progesterone for comparison.
Results of this study indicate that:
1. The short duration rise in progesterone exhibited by some ewes is indicative of an ovulation.
2 . Ewes that do exhibit a short luteal phase prior to the
first "normal" luteal phase have a longer period from parturition to the first "normal" luteal phase.
3. Serum levels of LH increase beginning 3 days postpartum.
4. There is a strong relationship between the concentration of estradiol and the exhibition of behavioral estrus in the postpartum ewe. vii 5. There appears to be a definite but somewhat irregular pulsatile release pattern of estradiol from the follicle of the postpartum ewe. The abilit y of the endocrine system of the St. Croix to return to functional levels of production and release during the first 15 days postpartum is likely related to their relatively short postpartum intervals and subsequent pregnancies.
(65 pages) INTRODUCTION
The duration of the postpartum interval is of e xtreme importance to the sheep industry. Current management techniques do not permit two lamb crops per year, which does not take advantage of the ewes, potential. The biological limit for sheep is estimated at a 6-month lambing interval, with 5 lambs per pregnancy (Wilson, 1968). Unfortuna tely, t he lengthy postpartum interval in combination with the seaso na l breeding habits of sheep have prevented the worl d 's s heep producers from achieving this goal. Intensive studie s of the postpartum interval have answered many questions. Howev er, many critical questions remain unanswered.
The first postpartum ovulation in the St. Croix and s ome other breeds occurs at about 16 days postpartum (Simoe s , 1988). This initial ovulation is often not accompanied by behavioral estrus and may result in a luteal phase of shorter duration than normal (Gonzales et al., 1987) . This short luteal phase may be caused by a reduction in the level of estradiol or progesterone from the ovary (Foote, 1971; Sha rpe et al., 1986). The interval between the first ovulation and the first ovulation accompanied by behavioral estrus in the
St. Croix ewe is 11.5 days (Simoes, 1988), or between 27.3 and
35.8 days postpartum (Evans and Foote, 1987; Simoes, 1988).
The postpartum interval is directly influenced by the number of ovulations prior to an ovulation accompanied by estrus. 2
The importance of the short luteal phase has been suggested to be that the short rise in progesterone may ensure that the
following LH surge will result in ovulation accompanied by
estrus and followed by a luteal phase of normal duration
(Legan et al., 1985). Studies indicate that for normal luteal
function to occur, there needs to be an adequate period of LH
stimulation of the ovary before the LH surge (Wright et al.,
1983, Wright et al., 1984). An inadequate period of LH stimulation may in part be due to insufficient release of GnRH
from the hypothalamus (Wright et al., 1984) or to a decreased
ability of the pituitary to respond to GnRH stimulation.
Decreased sensitivity of the pituitary may be caused by a reduction in estradiol receptors in the pituita r y (Wise e t al., 1986). Wise et al. (1986) reported that the estradiol receptor number decreases at about day 50 of gestation and remains low until day 13 postpartum, at which time the receptor number increases through day 35. Since LH secretion is influenced by estradiol, there may be an important link between estradiol and the initiation of postpartum ovarian cycles. 3
OBJECTIVES
The objectives of this study were
1. to determine the hormone patterns (LH, beta-estradiol,
and progesterone) during early postpartum,
2. to monitor the resumption of ovulation and behavioral
estrus in the postpartum ewe, and
3 . to correlate the information on hormone patterns with
ovulation and behavioral estrus. 4
LITERATURE REVIEW
This literature review will focus mainly on the
relationship of luteinizing hormone, estradiol, and
progesterone in the resumption of ovarian cyclicity.
First Postpartum ovulation and Estrus
The postpartum interval is defined as the period from parturition to the first ovulation accompanied by behavioral estrus. During this time uterine involution must occur, the ovaries must resume ovarian cyclic function including ovulation, and there must be a resumption of estrus behavior.
In many cases, the first postpartum ovulation in the ewe is not accompanied by behavioral estrus (Foote et al., 1967;
Land, 1971; Knight et al., 1981; Sharpe et al., 1986; and
Simoes, 1988) . These ovulations often result in a corpus luteum that is maintained for short periods of time (2-6 days)
(Gonzales et al., 1987) with levels of progesterone production that remain lower than those produced by corpora lutea maintained for normal periods. Sharpe et al. (1986) found that the progesterone level produced by these short-duration corpora lutea does not exceed 0.5 ngjml. In most cases, these corpora lutea were not found to be accompanied by behavioral estrus, perhaps due to insufficient estradiol production by 5
the follicle (Foote et al., 1967; Sharpe et al., 1986).
Large variations in the length of time from parturition
to the first ovulation are found within and between breeds
(Land, 1971; Evans and Robinson, 1980). The time from
parturition to first ovulation can be influenced by many
factors, such as nutritional status of the ewe and the season
in which parturition occurs (Jainudeen and Hafez, 1980; Land,
1982). There are mixed reports on the effect of lactation on
the postpartum interval. While some have reported slight
delays in postpartum ovulation due to lactation (Call et al.,
1976; Gonzalez et al., 1987), most researchers have failed to
show a statistical difference between lactating and non lactating ewes (Call et al., 1976; Restall and Starr, 1977;
Moss et al., 1980; Lewis and Bolt, 1983; Simoes, 1988). Ewes
lambing in the middle of the breeding season have been shown
to have a shorter period from parturition to resumption of
ovarian cyclicity than ewes lambing at the beginning or end of
the breeding season (Novoa, 1984). The extreme length of the
postpartum interval in ewes lambing at the end of the breeding
season is explained by the influence of seasonal anestrus overlapping postpartum anestrus.
As mentioned earlier, the first postpartum ovulation is not always accompanied by behavioral estrus. Although estrus behavior has been reported to occur as early as 15 to 27 hours postpartum, this estrus is not accompanied by ovulation
(Barker and Wiggins, 1964a; Simoes, 1988). As with the first 6
postpartum ovulation, the first postpartum ovulation accompanied by behavioral estrus occurs at widely varied times
both within and between breeds (Barker and Wiggins, 1964b).
For ewes lambing during the breeding season, the period from
parturition to first estrus can be from 2 to 60 days (Gardner et al., 1955; Amir et al., 1984; Sharpe et al., 1986; Dewulf and Lahlou-Kassi, 1986). The St. Croix breed of sheep is reported to show behavioral estrus at 27 to 40 days postpartum
(Evans and Foote, 1987; Simoes, 1988). For ewes tha t lamb out of the normal breeding season, the effect of seasonal anestrus overlaps postpartum anestrus, resulting in a greatly lengthened postpartum interval (Fitzgerald and Cunningham,
1981). In addition, the level of nutrition can have a major effect on the postpartum interval (Jainudeen and Hafez, 1980;
Ashmawi et al., 1984) .
Reports on the effect of lactation on the postpartum interval are inconsistent. However, most research seems to indicate that lactation is not a major factor in the time period required for the resumption of ovarian cyclicity in the postpartum ewe. While many studies show a tendency towards a longer postpartum interval in lactating than in non-lactating ewes, most do not show a statistically significant difference
(Roux et al., 1975; Call et al., 1976; Restall and Starr,
1977; Moss et al., 1980; Lewis and Bolt, 1983; Simoes, 1988) .
Since early removal of the lambs can result in a slightly shortened postpartum interval in some breeds of sheep (Timariu 7
et al., 1974; Ford, 1979; Hulet et al., 1983; Martemucc i et
al., 1983), it has been proposed that any lactational effect
may in fact be the result of suckling stimulus from the l amb
(Cognie et al., 1981). This hypothesis is further supported
by studies conducted by Kann and Martinet (1975), in wh i ch
they showed that by severing the nerves from the mammary
glands, thereby interrupting the suckling stimulus response,
the interval from parturition to resumption of ovula tion and
estrus is shortened. However, Moss et al. (1980) found t hat
contrary to the findings of Kann and Martinet (1977), suckled
and non-suckled ewes resumed estrous cycles at similar times
postpartum. In addition, Moss et al. (1980) reported tha t
there is no difference in hypothalamic concentrations of
gonadotropin releasing hormone (GnRH) and pituitary
responsiveness to GnRH between suckled and non-suckled ewes.
At this time, information is not available to adequately define the endocrine mechanisms involved in the initiatio n of
ovulation and the establishment of ovarian cycles accompanied by estrus. With further study, a method may be found that will allow the ewe to exhibit behavioral estrus at the time of the first ovulation, thereby allowing breeding to occur after a shorter postpartum interval than naturally occurs.
Shortening the period to first postpartum estrus is a key element in the reduction of the postpartum interval and as such justifies further study. 8
Progesterone in the Postpartum Ewe
At approximately the time of parturition, progesterone
levels drop to concentrations ranging from 0.1 to 0.2 ngjml
(Sarda et al., 1973; Gonzales, 1977; Boulfekhar and Brudieux,
1980) . The postpartum concentrations of progesterone remain
low until the first ovulation, which may or may not result in
development of a fully functional corpus luteum of normal
duration. Small peaks of progesterone have been detected in
ewes before the first "normal" luteal phase (Sharpe et al., 1986; Knight et al., 1981; Fitzgerald and Cunningham, 198 1;
Simoes, 1988). In most cases, these small progesterone peaks
last only 2 to 6 days (Sharpe et al., 1986; Fitzgerald and
Cunningham, 1981; Simoes, 1988) and reach a maximum
concentration of 0.5 ngj ml or less. The presumed source of
these small progesterone peaks is a corpus luteum of reduced
functional ability and duration (Manns et al., 1983) or a
luteinized unovulated follicle. Sharpe et al. (1986) reported
observing large follicles 2 to 3 days prior to these small
peaks as well as fresh corpora hemorrhagica just before and pale corpora lutea just after the small peaks in progesterone concentration. These observations indicate that ovulations occurred, resulting in the rise in progesterone concentration.
Corpus luteum weight, serum progesterone, and luteal concentration of receptors for luteinizing hormone are a ll 9
highly correlated in cycling ewes (Diekman et al., 1978). In a ddition, it has been reported that there is a high
correlation between progesterone secretion and the number of
receptors occupied by luteinizing hormone (Suter et al.,
1980). McLeod and Haresign (1984) have, therefore, suggested
that progesterone may have a direct effect on the ovulating
follicle, perhaps by altering the follicle's ability t o respond to changes in tonic luteinizing hormone secretion.
They reported that when the anestrous ewe was primed with progesterone for 2 days, a normal luteal phase followed GnRH treatment. However, other researchers have failed to show a progesterone-priming effect on the release of luteinizing hormone or follicle stimulating hormone in response to GnRH challenge (Lewis and Bolt, 1987). Another proposed explanation of the effect of progesterone priming on the development of a normal corpus luteum in anestrous ewes is that the progesterone treatment lengthens the period of time that the developing follicles are exposed to gonadotrophins
(Pearce et al., 1985). The ovary is not the only source of progesterone in the postpartum ewe. In addition to the ovary, the adrenal gland produces small amounts of progesterone.
Although many studies have been conducted to determine the role of progesterone in the postpartum ewe and many new facts have come to light, there is not yet a full understanding of how it relates in the overall scheme. 10
Estradiol-17-beta in the Postpartum Ewe
The role of estradiol in the postpartum ewe is not
c learly understood. However , evidence indicates tha t there i s
an increased inhibitory (negative-feedback) effec t of estradiol on luteinizing hormone release and a lower intrinsic
frequency of pulsatile r e lease of LH (Wright et al., 198 1;
Novoa, 1984; Clarke and Cummins, 1985; Leakakos et al., 1987) .
Kann and Martinet (1977 ) reported that the positive
feedback of estradiol on LH and FSH secretion is suppresse d or absent during the first 21 days postpartum. This was c onfirmed by Wright et al . (198 0), who found that while 100% of non-parturient anestrous ewes showed a positive feedba c k t o estradiol challenge, only 46% of postpartum ewes responded t o a n estradiol challenge prior to day 30 postpartum. The reasons for the failure of estradiol-positive feedback are not known. However, the failure of estradiol-positive feedback i s not due to an inhibitory effect of progesterone at the time o f estradiol challenge (Scaramuzzi et al., 1971; Cumming et al.,
1971) because plasma progesterone concentrations were less than 0.1 ngj ml in those ewes failing to show positive feedback. This defect in the feedback mechanism of estradiol may be the result of an inability of estradiol to i nduce ovulatory LH surges during the early postpartum period in the ewe (Wright et al., 1980) . 11
Evidence for an increased inhibition of tonic LH release
by estradiol was reported by Wright et al. (1981), who found that the negative-feedback effect of estradiol on LH release
is greater in ovariectomized postpartum ewes than in ovariectomized cyclic ewes. In addition, the increase in LH secretion after ovariectomy is more rapid during the breeding season than in the non-breeding season, suggesting that during the breeding season the hypothalamo-hypophysial axis is more responsive to removal of negative feedback control by ovaria n steroids. Seasonal variation in the negative feedback control of LH secretion has been established. During the non-breeding season, less estrogen is required to suppress LH secretion than is required during the breeding season (Legan et al.,
1977) . Evidence points to the pituitary as the point at which the estradiol exerts its negative feedback since GnRH pulses continue throughout the negative feedback phase (Clarke and
Cummins, 1985).
Wise et al . (1986) suggested that the increase in the negative feedback effect of estradiol in the postpartum ewe may be due to a decrease in concentration of receptors for estradiol in the anterior pituitary gland. Pituitary responsiveness to GnRH is directly influenced by the amount of r~ceptors and the distribution of receptors for estradiol
(Kamel and Krey, 1982) . Wise et al. (1986) also reported that receptors for estradiol in the hypothalamus and anterior pituitary gland are decreased during gestation and remain low 12
during the early postpartum period. The basis for the
decrease in estradiol receptors during gestation may be the
elevated concentration of progesterone found during gestation
(Stabenfeldt et al., 1972), because in the ewe, progesterone
has been shown to cause a decrease in concentrations of
estradiol receptors in uterine tissues (Koligan and Stormshak,
1977; Stone et al., 1979).
As previously stated, the first postpartum ovulation is
often not accompanied by behavioral estrus. Occurrence of
postpartum ovulations not accompanied by behavioral estrus and
followed by corpora lutea of short duration rather than of
normal duration indicates that follicles andjor resulting
corpora lutea involved in these ovulations may be different
from normal ovulatory follicles and corpora lutea in cycling
ewes. The absence of behavioral estrus may be due to
insufficient production of estradiol by these follicles, which
is required for manifestation of behavioral estrus (Sharpe et al., 1986).
The role of estradiol in the postpartum ewe is not fully understood, but evidence indicates that it has a major
influence on the endocrine events of the postpartum interval.
Further study of estradiol as it relates to the establishment of cycling in the postpartum ewe may lead to discoveries that will allow producers to artificially shorten the postpartum interval. 13
Luteinizing Hormone in the Postpartum Ewe
In addition to other factors, postpartum anestrus in the
ewe may involve an aberration of the neuroendocrine mechanisms
controlling gonadotrophin release (Wright et al., 1981).
Changes in pituitary responsiveness to GnRH and the positive
feedback effect of estrogen do not appear to be factors
limiting normal ovarian cyclicity in the postpartum ewe
(Wright et al. , 1980).
Preovulatory growth and development of ovarian follicles
may require an increased tonic secretion of LH (Hauger, Karsch
and Foster, 1977; Baird, 1978), and therefore ovaria n
acyclicity postpartum could be due to an alteration in the
response of the hypothalamo-hypophyseal axis to the negative feedback of estradiol, similar to that shown for
ovariectomized ewes during the anestrous season (Legan, Karsch and Foster, 1977).
Elevated concentrations of progesterone (Stabenfeldt et al., 1972) during gestation in the ewe were shown to reduce secretion of LH (Diekman and Malven, 1973). Pituitary concentrations of LH are at their lowest at days 120 to 135 of pregnancy and begin to rise soon after (Charnley et al., 1976) . No significant differences in GnRH concentration were found in the preoptical nucleus, or median eminence regions of the hypothalamus (Crowder et al., 1982) at different periods 14
after parturition (Moss et al., 1980), indicating that if Gn RH
is implicated in the release of LH, it would be a result o f a
reduction of its release, rather than reduced synthesis.
Since pituitary content of LH returns to prepregnancy leve l s
soon after parturition (Moss et al., 1980; Wright et a l.,
1980; Knipe, 1981; Crowder et al., 1982), and GnRH levels in
the hypothalamus are at adequate levels, low LH release is
probably explained by a factor that suppresses the release of
LH f rom the pituitary . Hwan and Freeman ( 1987) suggested that
the inhibitory effect of progesterone could be due to a direct
influence on the pituitary or acting at the hypothalamus t o
stimulate the release of a glycoprotein, recently identified,
that suppresses GnRH-stimulated LH release.
Direct evidence indicating that in postpartum ewes the r e
is an increased negative feedback effect of estradiol on LH
release and a lower frequency of pulsatile release of LH was
reported by Wright et al. (1981, 1983). As in seasona l
anestrus, the role of estradiol seems to be related to its
ability to maintain tonic LH secretions below the levels
required to promote the final phase of follicle growth and development (Haresign et al., 1985). The decrease in estradiol negative feedback allows an increase in tonic LH secretion, which stimulates a rise in serum estradiol causing a LH surge and resultant ovulation (Legan and Karsch, 1979 ) .
Once the decrease in response to estradiol negative feedback is sustained for more than a few days, full length estrous 15
cycles will occur (I' Anson and Legan, 1988) . It has been
suggested that the negative feedback mechanism is the result of reduced concentration of estradiol receptors in the
anterior pituitary and hypothalamus which increase about the
time cyclicity is resumed (Wise et al., 1986). It could therefore be theorized that the negative feedback effect
during early postpartum is in fact, a lack of positive feedback due to the low concentration of estradiol receptors.
The reason for the reduced number of estradiol receptors during gestation may be the increased concentration of progesterone found during gestation (Stabenfeldt et al.,
1972), because progesterone has been shown to cause a decrease in concentrations of estradiol receptors in uterine tissues in the ewe (Koligan and Stormshak, 1977; Stone et al., 1979). However, neural and hypophyseal receptors for estradiol may not be regulated in a similar manner in postpartum ewes, because concentrations of estradiol receptors in these tissues were highest at day 35 postpartum, when serum concentrations of progesterone were highest (Wise et al., 1986).
Plasma LH concentration, pulse frequency, and pulse amplitude seem to vary widely between and within breeds and individual experiments. Wright et al. (1981) reported that in mature Merino ewes at 29 to 31 days postpartum, the pulse frequency was 1.8±0.42 per 6 hours. LH pulse amplitude was 6.0±0.97 ngjml, and mean LH concentration was 1.16±0.32 ngjml.
Novoa (1984) cited studies in ewes lambing in June (Southern 16 Hemisphere) with mean basal levels of 1.85±0.25 and 2.65±0.27 ngj ml from days 1 to 5 postpartum, which increased steadily to
3.05±0.45 and 3.45±0.35 ngj ml on days 21 to 28 postpartum in lactating and non-lactating animals, respectively. LH surges were detected in 4 out of 8 lactating and 4 out of 6 non lactating animals, respectively . Although, only 5 of the 8 surges detected were associated with an ovulation (Restall and Starr, 1977) . Similarly, ewes lambing in May (Southern
Hemisphere) had LH pulse values of 3.17±0.18, 3.14±0. 30, and
3.45±0.28 ngj ml for days 5, 10, and 20 postpartum, respectively (Restall et al., 1977). LH pulses of 1-4 per 6 hours in ewes that lambed in October (Northern Hemisphere) were observed in 100% of 21 ewes at 21 days postpartum
(Poindron et al., 1980). The number of pulses per 6 hours in 12 ewes rearing single lambs (2.3±0.2 ngj ml) was higher than in 11 ewes rearing twins (1.9±0.2 ngj ml, P< .05). The basal levels ranged from 1. 0 to 2. 4 ngj ml, whereas the maximum values of pulses ranged from 3.4 to 9.6 ngjml.
Simoes (1988) reported that basal levels of LH did not increase significantly before 17 to 19 days postpartum
(P> .05), but significant rises occurred on days 24 to 26 postpartum, compared to 5 to 8 days prepartum and 8 to 11 days postpartum (P<.05), but not on days 17 to 19 postpartum (P> .05). This suggests a gradual increase in basal LH levels from parturition to day 17-19 postpartum. The average amplitude of LH pulses on days 5 to 8 prepartum was 17
significantly lower (P< .05) than on days 8 to 11 and 24 to 26 , but not on days 17 to 19 postpartum. During the postpartum
period, the average LH amplitude did not differ between days
8 to 11 and 17 to 19 postpartum, but increased significantly
(P<.05) on days 24 to 26. Table 1 shows the serum LH patterns
observed in white-face Targhee-type ewes.
Table 1. Serum LH patterns in White-Face mastectomized and intact ewes before lambing and during the postpartum period (mean and standard error of the mean in ngj ml) ; Simoes (1988) .
Basal Mean Pulse Pulse Bleedings Treatment n Cone. Cone . Height Int. 1
5-8 days Intact 8 0.12±.2 0.18±.5 0.40±. 6 8 .0 prepartum Mastec. 8 0.13± .2 0.21±.5 0. 58±. 6 8 .0
8-11 days Intact 6 0.13±.2 0.76±.5 2 . 19± .6 3.2 postpartum Mastec. 8 0.52±.2 1.21±.5 2.31±.6 4.0
17-19 days Intact 8 0.58±.2 0 . 93±.5 2.12± . 6 3 .7 postpartum Mastec. 8 0.37±.2 1.25±.5 1.47±.6 3.1 24-26 days Intact 8 0.95±.2 3.42±.5 5.65±.6 4 . 5 postpartum Mastec. 8 o. 72±. 2 2.42±.5 5.00± . 6 4.0 1) LH pulse interval in hours
While much has been learned about the role of LH in the postpartum ewe, there are many questions that remain unanswered. Further research in this topic is required before the hormone relationships in the postpartum ewe are understood well enough to allow effective manipulation of this period. 18
MATERIALS AND METHODS
This study was conducted at the Utah State University,
Sheep and Goat Research Facilities, Logan, Utah. Two groups
of st. Croix ewes were used in the experiment. Group l
consisted of 9 intact postpartum ewes, and group 2 consisted
of 7 ovariectomized non-postpartum ewes, as shown in Table 2.
Table 2. Experimental groups.
Group Genotype n Treatment
l St. Croix 9 Intact, postpartum
2 St. Croix 7 Ovariectomized, non-postpartum
All animals were identified by numbered ear tags, and all were
2 years of age or older.
Group 1
15 ewes were bred in May to lamb in mid October, thereby
avoiding the confounding effect of seasonal anestrus . Estrus
and ovulation for breeding in May were induced by intravaginal
sponges containing 40 mg of fluorogestone acetate (FGA,
Chrono-gest) for 14 days and 750 I.U. of PMSG IM, 36 hours before sponge removal. A second injection of PMSG was administered 16 days after the first injection to induce a 19
second ovulation and estrus, to maximize the percentage of
ewes lambing. The ewes were placed with fertile males for one
week after each treatment. Of the 15 ewes bred, 9 gave birth
in the fall.
Group 2
The seven ewes in group 2 were ovariectomized in
September, 1988. All ovariectomies were performed under
general anesthesia induced by thiobarbital (5 percent)
(Biotal), and maintained with halothane. The purpose of the
ovariectomized ewes was to determine the non-ovarian levels of
progesterone, to provide comparisons to progesterone levels in the postpartum ewes.
Pre1ambing Management
Two weeks prior to expected initiation of parturition for ewes in group 1, both groups were removed from the pasture and kept in sheltered pens. All animals received alfalfa pellets and grass hay to meet nutritional requirements as recommended by NRC.
Management at Lambing
Lambing began October 10, and ended October 15, 1988. At 20 the time of birth, the lambs were weighed, their navels disinfected with iodine solution, and each was individually identified by a numbered eartag. Ewes and their lambs were placed individually in confinement stalls for two days to allow maternal bonding to occur. The ewes were kept together with their lambs throughout the experiment. All ewes in the study lambed without assistance, and no ewe was allowed to raise more than two lambs.
Estrus Detection
After the 2 day confinement period, the ewes and their lambs were placed in a pen containing a vasectomized ram. The brisket of the ram was painted daily to allow detection of mounting as an indication of estrus. The color of paint was changed weekly to minimize mistakes in estrus detection. The ewes were checked twice daily for paint marks indicating the ewe was in estrus. However, the accuracy of estrus detection is difficult to determine due to the constant presence of the ram, allowing possible mounting during feeding, or the ram showing particular attention to specific ewes, while possibly ignoring others.
Blood Sampling
Blood samples (10 mljsample) were collected daily from 21 group 1 (intact, postpartum) ewes, beginning approximately 7
days prior to the expected initiation of parturition. Until
day 21 postpartum, the ewes were bled according to the
following 3 day rotation schedule:
day 1: 14 samples taken at 1 hour intervals (8:00am - 9:00pm)
day 2: 2 samples taken at 1 hour intervals (8:00am- 9:00am)
day 3 : 2 samples taken at 1 hour intervals (8:00am- 9:00am)
The first day of sampling (14 hour schedule) was
initiated on the day of parturition. The date of parturition
was recorded as day 0 postpartum. This 3 day rotation was
continued until each ewe reached 21 days postpartum, at which time the bleeding schedule was changed to the following 6 day
rotation, and the sampling schedules of all ewes were shifted to the same day as follows:
day 1: 14 samples taken at 1 hour intervals (8:00am- 9 :00pm)
day 2: 2 samples taken at 1 hour intervals (8:ooam - 9:00am)
day 3: 2 samples taken at 1 hour intervals (8:00am- 9:0oam)
day 4: 2 samples taken at 1 hour intervals (8:00am- 9:00am)
day 5: 2 samples taken at 1 hour intervals (8:00am - 9:00am)
day 6: 2 samples taken at 1 hour intervals (8:00am- 9:00am)
Blood sampling continued following the 6 day rotation schedule until 42 days postpartum. The reasoning behind this bleeding 22
schedule was to maximize the length of the bleeding period on
the 14 sample day, without causing excessive stress to the
ewe. The daily bleedings allowed for the even monitoring of
the progesterone levels to assure that any short duration
rises in progesterone levels were observed.
Blood samples from group 2 (ovariectomized, non
postpartum), were taken at 8:00am and 9:00am on September 20 ,
October 15, October 29, November 12, and November 26. Serum
levels of progesterone were determined to quantify the level
of non-ovarian progesterone produced by the ewe. Thereby, the
occurrence and the source of small rises in progesterone
during the early postpartum period might be more accurately
assessed as ovarian or adrenal. Although all ewes were accustomed to frequent handling,
care was taken to minimize stress. Blood samples were taken
from the jugular vein, using a 21 gauge needle, and
transferred to a 10 ml glass tube which was then immediately
refrigerated at 4° C. The blood was allowed to clot for a
minimum of 2 hours after which it was centrifuged. The serum
was removed by pipet, and placed in labeled vials which were
kept at -20° C until assayed.
Hormone Analysis
Concentrations of serum progesterone were determined by radioimmunoassay at the Animal Physiology Laboratory at Utah 23
State University. Concentrations of serum LH, and estradiol were determined by radioimmunoassay at the Pathology
Laboratory at the University of Michigan.
Serum LH levels were determined following the
methodology described by Migdley Jr. ( 1966). Serum progesterone levels were determined using the test-tube kit
"Coat-A-Count" manufactured by Diagnostic Products Corporation, Los Angeles, California.
Determination ot ovulation
The time of ovulation was estimated by progesterone levels. Concentrations of 0.1 to 0.2 ngjml have been reported during anestrus, follicular phase, and in ovariectomized ewes
(Sarda et al., 1973). Although there is no definitive level of progesterone concentration at which ovulation occurs, a level greater than 0.4 ngjml is interpreted as being
indicative of luteal activity (Fitzgerald and Cunningham, 1981; Sharpe et al., 1986). In addition, progesterone levels
in this study were compared to available LH levels to
determine if an acute release of LH had preceded the rise in
progesterone levels, providing additional evidence that
ovulation had occurred. The time of ovulation was therefore determined to be the point at which an acute release of LH was
observed. 24
Luteinizing Hormone
Data on serum LH was expressed as 1) mean serum concentration; 2) mean basal serum concentration; 3) mean
pulse height (measured as the mean of LH peaks); and 4) mean
pulse interval. Poultron and Robinson (1987) concluded that hourly
bleedings are suitable for LH determinations, based on evidence presented by Pelletier et al. (1982) , who concluded that 90 percent of LH pulses are detected at this frequency of
blood sampling. Mean plasma concentrations are defined as the average of all 14 hourly blood samples per day. Basal serum concentration is defined as the average of the 7 lowest
concentrations for each day. A pulse is defined as a n increase in concentration that exceeds the basal level by 4
times the coefficient of variation, followed by a decrease (Pelletier et al. 1982). An increased LH concentration of this magnitude at the beginning or ending of the sampling
period was considered a pulse.
statistical Analysis
Results are expressed in table form as means, and
standard errors of the mean, and in figure form as means, unless otherwise stated. Statistical difference was 25 determined by analysis using the Student's T test. 26
RESULTS AND DISCUSSION
Postpartum ovulation and Estrus
Of the 9 ewes in group 1 (intact, postpartum) , 3 exhibited
a luteal phase of reduced length prior to the first full
length luteal phase. Data relating to ovulation and estrus are
shown in Tables 3 and 4. The period from parturition to
initiation of the first short luteal phase was 20.7 days with
the subsequent rise in progesterone lasting 3.7 days.
For the 3 ewes exhibiting one or more shortened luteal
phases prior to the first normal luteal phase, the interval
from parturition to the first normal luteal phase was 29.0
days . The number of days between the initial ovulation and
the first ovulation resulting in a normal length luteal phase
was 8.3 days for these 3 ewes.
All of the remaining ewes in group 1 exhibited a normal
luteal phase following the first postpartum ovulation. The
interval in days from parturition to first ovulation in these
6 ewes was 22.7, which is significantly shorter than the 29.0
(P<.05) for the three that exhibited one or more shortened lqteal phases prior to the first normal duration luteal phase.
These results in the St. Croix do not confirm the postulate that short luteal phases are necessary to prime the 27
Table 3. The occurrence of ovulation and estrus, and the length of the luteal phase in intact, postpartum ewes.
Occurrence of s hort Ovulation followed CL phase by normal CL pha se Estrus
Days Nl.ntler Cl pha se Days from Days Days pp per duration 1st ovulation PP PP (days) followed by normal Cl phase
Ewes with short CL phase 20. 7t.8 1.3!.3 3. 7t.3 8.3%1.3 29.0t1.3 24.0:t4. 0
Ewe s 'W/0 short Cl phase 22.7t1.3 31.8t5.4
Combinded 24.8t1.3 29.2!3 .9 n - number of ewes PP = postpartum
Tabl e 4 . Association of ovulation and estrus.
Days First ovulation Second ovulation None pp accOO"panied by acc~nied by first estrus (n) first estrus (n)
Ewes with short Cl phase 23.4t4.2 1 of 3 2 of 3
Ewes \J/0 short CL phase 19.5t4.5 2 of 2
CorTtlined 21.8t2.9 3 of 5 2 of 5
Ewes that fai'led to show estrus 4 of 9 28 system with progesterone, thereby ensuring that the next
ovulation will result in a functional corpora lutea, in the
St. Croix breed. The mean interval in days for all 9 ewes in
group 1 from parturition to the first ovulation resulting in
a normal luteal phase was 24.8. The occurrence of one or more ovulations prior to the
first postpartum estrus was observed in 5 of 9 ewes, which is
consistent with published reports (Foote et al., 1967; Sharpe
et al., 1986; Evans and Foote, 1987; Simoes, 1988). First ovulation accompanied by behavioral estrus occurred at 29.2 days, which is consistent with the interval of 27.2
days reported by Simoes (1988), but somewhat less than the 40.2 days reported by Evans and Foote (1987). Corpora lutea of variable lifespan are responsible for the difference in cycle length of the first 1 to 3 postpartum
ovulations. This is consistent with earlier reports (Sharpe et al., 1986; Gonzalez et al., 1987; Simoes, 1988) that
suggest that first postpartum ovulations may result from follicles that differ from those formed in later cycles, or
that they may result in corpora lutea that differ from those
formed in later cycles. As seen in Figures 1 through 9,
several of the ewes exhibited a luteal phase that is much
longer than the normal 12-14 day duration. This may suggest that longer as well as shorter than normal periods of CL
function may occur during the postpartum period. Further studies are needed to address this issue. EWE 481 29
14.00 u.oo l - =Progesi HI.OO c ·-· 9.00 _J w a.oo > * w 7.00 -' w 6.00 z 5.00 0 2 4.00 a::: 0 J.OO I 2.00 l.OO
0.00 -· -2 0 8 10 12 14 16 IS 20 22 24 26 28 30 .32 34 J6 38 +0 4 ~
EW E 591
14.00
l J.OO - -Progf!~tero ne CJ - Bose t..ri 12.00 - = LH Pul
1.00
0.00 - 6 - 4 -2 0 10 12 ,.. 16 18 20 22 2+ 26 28 JO Jl J4 J6 lEI 40 DAYS POSTPARTUM EWE 597 30
14.00 - =Proqesterone IJ.OO D =8Clse lJ-i - =LH Pulse Height 12.00 .. =lii Acute Release ) 7.50 r c:-, 11.00 CJ1 10.00 c '-' 9,00 _J * w 8.00 >w 7-00 _J w 6.00 ;v~ z 5.00 0 __; 2 4.00 a: 0 J ,OO :r: 2.00 1.00 \ l /I 0.00 10 12 14 Hi 18 20 22 2+ 26 2B ..30 J2 3~ J6 J8 40 42 44 -~ EWE 600 14 .00 ~ =Prag eshHone u.oo c:J =Bo$e LH - =LH Pul~e Height 12.00 '\ " ""'LH At:ute Rele-ase) 7.50 --...E __ 1\.C.;) CJ> c 10.00 9,00 _.J w 8.00 * C;j 7,00 _J 6,00 w z 5.00 0 4 ,00 2 n::: J.OO ~\~J,J 0 2,00 I 1.00 I J 0.00 -6 -4 -2 D e 10 12 H 16 18 20 22 2+ 26 2B ..30 J2 34 36 3>9 4ll DAYS POSTPARTUM EWE 625 31 14.00 - =Proqasteron& u .oo c:J =Base L.H • ==LH Pv !se Height 12.00 E • -LH Acute Rete(]st: ) 7.50 '-, 11.00 ry c 10.00 '•../ 9.00 _j * w B.OO w> 7.00 _j .\ r\ 5.00 u z 0.00 0 4.00 2 cr:: J.OO \ JVJ'VvF\J 0 2.00 I 1.00 j ~ fl ~ I ~ 0.00 -2 0 a 10 12 14 15 18 2.0 22 2.4 26 28 30 ~2 34 ~s 3Ei 4 EWE 626 1'\.00 - =ProgasteroniQ 1J. 0 =Bose LH - =LH Pulse Height 12.00 E • =LH Acute Release ) 7.50 '-,, , 1.00 0'> c 10.00 '-./ 9.00 _j w B.OO w> 7.00 _j 6.00 w ~ z 0.00 0 4.00 2 . a:: J.OO 0 2.00 I 1.00 ~- 0.00 L - 2 0 ~ 6 6 10 12 14 16 18 20 22 2 '*" 26 26 ..50 32 34 J6 JS 40 ~ 2 44 DAYS POSTPA RTUM EWE 645 32 ,._.00 - =Pro9esterone u.oo c::J =Bose LH - = LJ-i Pulse He1ght 1'2.00 E • =LH Acute Rel ~ osl!} 7.50 "-,_ 11,00 CJl c 10.00 '-"' 9.00 _J .. w B.OO > w 7.00 _J 6.00 w 0.00 \ z 0 • .00 2. \ 0::: .1.00 0 2.00 I 1.00 ~ 0.00 ~ J rJ - 4 - 2 0 a 10 12 14 16 16 20 22 24 26 2a JC> J2 J .. :16 Je 4 EWE 3495 1'1 .00 - =Progesteron• lJ.OO c::J =Bose LH - = LH PulM HeiqM 12.00 • ""'LH Acut~ Re~ose ) 7 eoO E 11.00 01 10.00 "c '-/ 9.00 _J .. w B.OO * w> 7.00 _J w 6.00 z "·00 0 2 •.oo 0::: 0 .1.00 :r: 2.00 1.00 0.00 -2 0 6 10 12 H 16 18 20 22 2 4- 26 26 30 32 J~ 3& J8 .. 0 4- Z H DAYS POSTPARTUM EWE 3514 33 14 .00 - =Progestilrone l J .OO c:J =Bose LH - =-LH Pulse Hetqht 12.00 \ • -u-t Acute Release > 7.50 E 11 .00 '- 0' 10.00 c 9.00 w....J 8.00 • w> 7.00 ....J w 6.00 z 0.00 0 2 o& .C 1.00 ~ h vv 0.00 r• .. - 4 - 2 o 2 4 s a 10 12 14 16 1e zo 22 24 2o 2a Jo 32 .3-4 J6 Je 4-0 4 .r.. DAYS POSTPARTU M Progesterone Patterns Average progesterone concentrations declined rapidly at the time of parturition, and remained low for at least 15 days (Figures 1-9). Mean concentration of progesterone for all ewes in group 1, from day 2 to 14 postpartum was 0 . 16 ngj ml, which is similar to that of the ovariectomized ewes i n group 2 , which had a mean level of 0.14 ngj ml, as shown in Table 5 , indicating that the ovaries were producing little or no progesterone . 34 Table 5. Mean progesterone level in intact (pre-ovulation) ewes from day 2 to day 14 postpartum, and the mean progesterone level in ovariectomized ewes. Group n Progesterone level (ngjml) 1 (intact) 9 0.16±0.02 2 (ovx) 7 0 .14±0. 02 Small peaks of progesterone were detected in 3 out of 9 ewes in group 1, which is consistent with published literature (Sharpe et al., 1986; Simoes, 1988). These small peaks were interpreted to indicate that ovulation had occurred if preceded by an acute release of LH. These periods of short luteal function were of variable duration, lasting up to 4 days. Maximum progesterone concentrations reached during these short luteal phases for the 3 ewes that exhibited them were, 1.02 ngjml, 1.14 ngjml, and 0.81 ngjml. Simoes (1988) reported that small peaks of progesterone were considered to represent ovulation or luteinization of follicles if the progesterone levels reached a minimum concentration of 0. 4 ngjml (Fitzgerald and cunningham, 1981: Sharpe et al., 1986). Of the 4 short luteal phases observed, all were preceded by an acute release of LH, further indicating that ovulations did occur. Progesterone levels in relation to LH levels for group 1 (intact, postpartum) are shown for each ewe in Figures 1-9. 35 Luteinizing Hormone Patterns As shown in Table 6, mean basal LH level on the day of parturition (day 0) was 0.54 ngjml, however, the accuracy is limited because the lower limit of sensitivity for the assay was o. 43 ngjml. A gradual increase occurred in the basal level of LH, reaching 1.54 ng/ml by day 21 postpartum. Figure 10 shows the basal LH patterns during the postpartum interval. Table 6. Basal LH patterns for group 1 (intact, postpartum). (reported as the mean and standard error of the mean) Days Basal Pulse Pulse PP n3 level n height n interval (ngjm1) (ngjm1) (hrs) 0 9 0.54±.07 9 0.98±.23 31 6.7±1.33 3 9 0.60±.05 9 2.34±.37 81 3. 1±.25 6 9 0.83±.13 9 3.73±.63 9 2.9±.22 9 9 0.86±.10 9 4.87±.49 9 3.1±.32 12 9 0.86±.11 9 4.55±.51 9 3.5±.53 15 72 0.80±.10 72 4.50±.31 72 3.3±.31 18 9 1.43±.23 9 5.70±.42 9 3.0±.36 21 62 1.54±.27 62 5.40±.56 62 3.5±.45 1) Variation in (n) on days 0 and 3 is due to no more than one pulse of LH occurring during the 14 hour bleeding schedule in some ewes. 2) Variation in (n) on days 15 and 21 is due to an acute release of LH during the 14 hour bleeding schedule. Ewes in which these patterns of LH release occurred were excluded from calculations of basal LH release. 1) n indicates the number of ewes. 36 GROUP --- 7.~ = Basal le,,el 7.CoJ - Pulse height ·-- (; . ~ 6.00 E 5.!::<1 en' ~-C<.l c 4 .!:(1 · ~- 4.00 (/) 3.50 Q) > 3.00 Q) 2.:1<1 _j 2.00 I l.:xl _j 1.00 0.50 ~ ~ r 0.00 nl ~ ~ ~ -1 0 2 3 ._ 5 6 1 e g 10 11 12 n 1+ 15 1& 11 16 19 20 21 n. DAYS POSTPl\RTUNI Five of nine ewes showed pulsatile release of LH on day 0. Of the five ewes showing pulsatile release of LH, the mean pulse height was 0.98±0.23 ngj ml. LH p ulse height gradually increased until it reached 5.70 ngj ml on day 18 postpartum, as shown in Figure 10. LH pulse interval was difficult to measure on day 0 because only 3 of the 9 ewes showed more than one pulse during the bleeding schedule on day 0. For the 3 that did have more than one pulse, the interval was 6. 7 hours. ' On day 3 37 postpartum, 8 of the 9 ewes demonstrated 2 or more pulses during the 14 hour bleeding period, and the pulse interval had decreased to 3.1 hours, and continued at this interval throughout the experiment. It would appear that the LH interval in the St. Croix returns to normal, in a shorter period of time postpartum than some other breeds. For example, the pulse interval at day 3 postpartum in the St. Croix was 3.1 hours, as compared to 8.0 hours at days 5 to 8 in White-face (Targhee-type) sheep (Simoes, 1988). In addition the basal concentration and pulse height apparently increases earlier in the St. Croix than other breeds. At day 17 postpartum, the mean basal level of LH was 1.43 ngjml and the mean pulse height was 5.70 ngjml in the St.Croix, compared to 0.58 ngjml and 2.12 ngjml in the White-face sheep (Simoes, 1988). An acute release of LH was observed in 8 of the 9 ewes in group 1 (intact, postpartum), between day 15 and 27 postpartum. The interval from parturition to the first postpartum LH surge was 20.0±.43 days. Mean LH concentration in group 2 (ovariectomized) ewes was 15.58±1.56 ngjml, as would be expected when the negative feedback effects of the ovarian hormones are removed. Pulse interval, and pulse height could not be determined due to the hourly bleeding schedule. In the ovariectomized ewe, LH pulse interval is less than one hour. 38 Estradiol-17-beta Patterns Estradiol concentration was determined in ewes tha t exhibited an acute release of LH that coincided with an intensive bleeding day. Acute release of LH was interprete d to have occurred when the concentration of LH remained at the maximum measurable level for several consecutive bleeding s . Of the 9 ewes, 6 showed an acute release of LH on an intensi ve bleeding day, thereby allowing a maximum of a 14-hour monitoring of estradiol concentrations. The results are shown in Table 7. Table 7. Estradiol associated with acute release of LH (reported as the mean and standard error of the mean) Days Mean Maximum Estrous Ewe # gostgartum cone. (gg[mll cone. (gg[mll behavio r 481 23 13.10± 2.83 40.59 No 591 15 63 . 24±27.82 319.81 Yes 5971 22 45.99±18.43 249.44 No 625 15 10.84± 2.30 39.68 No 645 24 48.09±14.30 180.20 Yes 3514 21 12.52± 2.26 31.95 No 1} This ewe seems to vary from the trend for ewes with estrus behavior to have an increased estradiol level, possibly suggesting an error in estrus detection . The luteal phase in relation to maximum estradiol lev els 39 and estrus behavior is shown in Table 8. Table 8. Length of luteal phase in relation to maximum estradiol levels and estrus behavior. Followed by Days Max. estradiol Estrous luteal phase of postpartum cone. lpg/mll behavior normal duration 481 23 40.59 No Yes 591 15 319.81 Yes No (short) 597 22 249.44 No Yes 625 15 39.68 No Yes 645 24 180.20 Yes Yes 3514 21 31.95 No No (short) Of those ewes that demonstrated behavioral estrus within 1 day of acute LH release, the mean estradiol concentration was 55.67±7.58 pgj ml, with a maximum concentration of 250.01±69.81 pgj ml. If the assumption is made that there was an error in estrus detection and that ewe number 597 did exhibit estrus behavior, then mean concentration of estradiol was 52.44±5. 43 pgj ml, with a maximum concentration of 249.82±40.30 pgjml as shown in Table 9. Of those ewes that did not exhibit behavioral estrus within 1 day of acute LH release, mean estradiol concentration was 20.61±8.47 pg/ml, with a maximum concentration of 90.42±53.04 pgjml. If the assumption is made that ewe number 597 did exhibit estrus and should therefore be removed from these calculations, the mean concentration was 12 . 15±0.68 pgjml, with a maximum 40 concentration of 37.41±2.74 pgjml. Table 9. Estradiol concentrations in relation to estrus detection (reported as the mean and standard error of the mean in pgjml). Assuming error in n Observed n detection of 597 Estrus Behavior: Mean concentration 2 55.67± 7.58 3 52.44± 5.43 Max. concentration 2 250.01±69.81 3 249.82±40.30 No Estrus Behavior: Mean concentration 4 20.61± 8.47 3 12.15± 0.68 Max. concentration 4 90.42±53.04 3 37 0 41± 2.74 From these results, it appears that there is a relationship between proportionately high estradiol levels released from the postpartum ovary and oestrus behavior. These data seem to confirm the theory that the absence of behavioral estrus in the early postpartum period may be due to insufficient production of steroids by the postpartum follicle needed for manifestation of oestrus (Sharpe et al., 1986). The reasons for the lack of estradiol production are unknown at this time but could lie in the possibility that the postpartum follicle is somehow deficient or that the follicle ovulates prior to maturation due to an early acute release of LH from the pituitary, brought about by the increased sensitivity of the postpartum pituitary to the feedback 41 effects of estradiol. For those ewes that were analyzed for estradiol concentration, it is interesting to note the apparent pulsatile release of estradiol and the long pulse interval. The reason for this is not known, nor is it known if this is the normal pattern for other breeds of sheep. However, it may explain the apparent trailing pattern of release of estradiol to the preovulatory surge in several ewes as seen in Figures 11 through 27. As would be expected, there is a strong correlation between the release of estradiol, the LH surge, and the subsequent production of progesterone in the postpartum ewe. Further research is needed to more fully understand the patterns of release in other breeds of sheep to determine if the data reported in this study apply only to the St. Croix or to other breeds as well. EWE 481 42 so lH NGIMLX •o E2 PGIML X ~ ~ 3 ~ 10 520 530 5<0 550 560 570 580 •o 35 30 ~ 25 az"' 3 20 " 10 HOUR EWE 591 43 "' NG.IML LH X PG/Ml E2 X " N w ~ "a .. 30 3 ~ '!' "z "' 10 330 34 0 350 360 370 3 8 0 390 •o 35 30 3 25 :;! az 20 " 10 10 0 2 0 0 300 •oo 6 00 700 HOUR EWE 597 44 5(J NG/ML LH X PG.IM L E2 X " N w ~ "'~ 30 3 ~ az 20 ,, 500 "0 520 530 5< 0 550 560 " 35 30 3 25 ~ a"z 20 " " 200 300 600 HOUR EWE 600 45 50 NG/M L LH X •o PGIML E2 X N w g :;: JO 5 ~ a"'z 20 10 \ •5o <60 <70 • e o <90 500 5 10 " 35 JO 3 25 ~ a"'z 20 15 HOUR EWE 625 46 50 NG!ML LH X PG/ML E2 X ' 0 :;: ~ a "'0. 30 3 i az 20 •o 33 0 3•o 3 50 360 3 70 380 390 " 35 30 3 25 ~ ~z 20 " " 500 HOUR 47 EWE 626 •o 35 30 3 25 ~ az"' 20 " 10 100 200 300 •oo 500 600 HOUR EWE 645 48 50 NGfML LH X •o PGIML E2 X N w i ~ 30 3 ~ az"' 20 10 550 560 570 580 590 600 610 •o 35 30 3 2S ~ az"' 20 15 10 100 200 300 • oo 500 600 700 HOUR EWE 3495 49 50 NGIML LH X PGIML E2 X " N w :i ~ 30 3 ~ a"z 20 >0 620 630 640 6 5 0 660 670 6 6 0 •o 35 30 3 25 ~ a"z 20 " >0 >00 200 300 •oo 5 0 0 600 700 HOUR EWE 3514 50 50 NGrML LH X " PGIML E2 X N '"~ :> ~ 30 3 ~ :> az 20 10 470 480 490 500 5 10 520 530 40 35 30 3 25 ~ "z 20 15 10 100 200 300 400 5 00 6 0 0 700 HOUR 51 CONCLUSIONS The duration of the postpartum interval is of extreme importance to the sheep industry. Current management techniques do not make full use of the ewes, potential to produce two lamb crops per year. The biological ceiling for sheep is estimated at a 6-month lambing interval. However, the lengthy postpartum interval in combination with the seasonal breeding habits of sheep have prevented the world's sheep producers from achieving this potential. This research was undertaken to study the endocrinologic events surrounding the postpartum interval. The data from this study indicate that the small rise in progesterone prior to a "normal" luteal phase is not needed to ensure that the following ovulation results in a fully functional CL in the St. Croix. This would seem to contradic t the idea that "progesterone priming" as an important factor in the postpartum St. Croix ewe. In fact, those ewes that did exhibit a short luteal phase prior to the first "normal" luteal phase had a longer period from parturition to the first "normal" luteal phase than those that did not. The release of increasing amounts of LH in the st. Croix seems to resume very rapidly postpartum, with pulse interval apparently stabilizing by day 3 postpartum and increasing basal level appearing to level off by day 18-21 postpartum. 52 This may account for the St. Croix's ability to rebreed within a relatively short period. This study indicates that a very important relationship exists between estradiol levels and estrous behavior in the postpartum ewe. Additionally, the low levels of estradiol present in those ewes that did not show behavioral estrus may indicate that the growing follicle ovulated prior to reaching levels of estradiol production and release to stimulate estrus behavior. However, it is not clear from the results if there is any correlation between low levels of estradiol and the subsequent development of a corpus luteum of shorter than normal duration (Table 9). Further study of these and other endocrine relationships in the postpartum ewe is needed. Perhaps by increasing the concentration of estradiol receptors in the pituitary or delaying ovulation to allow a longer period of follicle stimulation, the postpartum interval can be manipulated to improve the efficiency of the ewe. 53 LITERATURE CITED Amir, D., H. Schindler, M. Rosenberg, H. Gasitua and I. Dimmerman, 1984. The reproductive performance of Finn cross ewes to natural and artificial photoperiod, hormone treatments and introduction of rams . In The Promotion of Prolific Strains of Sheep by Nutritional and Managerial Means. Final Report Submitted to United States-Israel Binational Agricultural Research and Development Fund. Bet Dagan, Israel, Volcani Center pp., 201-212 Ashmawi, G.M., M. El-Fouly, Y. El-Talty, 0. Salma, 1984. Rahmani sheep: lambing performance and postpartum estrus manifestation as affected by season of mating and type of feed. Egyptian J. Anim. Prod. 24 :119-125 Baird, D.T. 1978. 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