FACTORS APPECTING REPRODUCTIVE EFPICIENCY

IN GILTS AND SOV/S

by

ANDREA KOMKOV, B.S.

A THESIS

IN

ANIMAL BREEDING

Submitted to the Graduate Paculty of Texas Tech University in

Partial Pulfillment of the Requirenents for

the Degree of

MASTER OP SCIENCE

December, 1983

ACKNOWLEDGEMENTS

I would like to express my gratitude to Larry Wiginton,

Joe Hancock, Jorge Navar and Steve Brazier for the help they

provided during the data collection. My special thanks to

Sharon Pooshee for her companionship and company during all

those trips to the farm (day and night). Sincere gratitude

goes to my committee for their time and effort in reviewing

this thesis: Drs. L.P. Tribble, D.E. Orr, and M.K. Rylander.

Dr. J.R. Clark deserves a medal more than thanks. His pa-

tience and invaluable assistance not only made this research

possible, but worthwhile. I thank hin for hanging on for

all these years and not giving up on me. Lastly, I would

like to recognize my seminar colleagues Dr. R.D. Galyean,

Jim Stalder, Ron Waters and Nancy Ryan for putting it all in

the proper perspective.

ii

ACKNOWLEDGEMENTS

I would like to express my gratitude to Larry Wiginton,

Joe Hancock, Jorge Navar and Steve Brazier for the help they

provided during the data collection. My special thanks to

Sharon Pooshee for her companionship and company during all

those trips to the farm (day and night). Sincere gratitude

goes to my committee for their time and effort in reviewing

this thesis: Drs. L.P. Tribble, D.E. Orr, and M.K. Rylander.

Dr. J.R. Clark deserves a medal more than thanks. His pa-

tience and invaluable assistance not only made this research

possible, but worthwhile. I thank hin for hanging on for

all these years and not giving up on me. Lastly, I would

like to recognize my semlnar colleagues Dr. R.D. Galyean,

Jim Stalder, Ron Waters and Nancy Ryan for putting it all in

the proper perspective.

ii

CONTENTS

ACKNOWLEDGEMENTS ii

LIST OF TABLES iv

LIST OF FIGURES v

CHAPTER page

I. INTRODUCTION 1

II. LITERATURE REVIEW 3

Course of Follicular Development 3 Endocrine Regulation of the Ovary 6 The Ovulatory Follicle 10 Cautery of Follicles 12 Lactation and the Lactational Anestrus . . . 15

III. EFPECTS OF FOLLICLE-CAUTERY ON THE DAY OF ESTRUS ON SUBSEQUENT ESTROUS CYCLE LENGTH AND OVULATION RATE IN GILTS 21

Summary 21 Introduction 2 2 Materials and Methods 2 3 Results and Discussion 27

IV. EPFECTS OF ALTERED SUCKLING INTENSITY AND GONADOTROPIN RELEASING HORMONE ON POSTPARTUM INTERVAL IN SOWS ' . . . 3 3

Summary 3 3 Introduction 34 Materials and Methods 3 5 Results and Discussion 37

V. , GENERAL DISCUSSION 41

LITERATURE CITED 4 3

APPENDIX 51

111

Table page

3-1. MEANS FOR REPRODUCTIVE ENDPOINTS BEFORE AND AFTER FOLLICLE-CAUTERY ON THE DAY OF ESTRUS IN GILTS 2 8

4-1. EXPERIMENTAL DESIGN . . . . . 36

4-2. EPFECTS OF GnRH, ALTERED SUCKLING INTENSITY AND PARITY ON POST-PARTUM REPRODUCTIVE AGTIVITY IN SOWS o . . 3 8

A - 1 . ANALYSIS OP VARIANCE TABLES FOR REPRODUCTIVE ENDPOINTS IN SOWS . o . 52

A - 2 . ANALYSIS OF COVARIANCE TABLE FOR INTERVAL FROM TREATMENT TO ESTRUS IN SOWS o . 5 3

IV

LI5T OP FIG.URES

Figure page

3-1. EXPERIMENTAL DESIGN 25

3-2. MEAN SERUM CONCENTRATIONS OF PROGESTERONE IN GILTS SUBJECTED TO SHAM FOLLICLE-CAUTERY . . 3 0

3-3. MEAN SERUM CONCENTRATIONS OF PROGESTERONE IN GILTS SUBJECTED TO FOLLICLE-CAUTERY . . . . 30

v

CHAPTER I

INTRODUCTION

An important goal in swine production is to farrow more

pigs per sow per year (Guthrie, 1975). There are several

approaches that can be taken to achieve this goal. The

first would be to lower the age at which a gilt reaches pu-

berty, thereby increasing her reproductive life. A second

approach would be to increase the ovulation rate of the gilt

or sow via manipulation of the endocrine environment and(or)

genetic selection for increased number of ovulations fZim-

merman and Cunningham, 1975). Thirdly, increased reproduc-

tive efficiency could be obtained if the interval from par-

turition to rebreeding could be shortened in the sow.

Many approaches to increasing the efficiency of swine

reproduction have been tried and some have proven success-

ful. Researchers have tackled these problems in a number of

ways. The use of exogenous hormones is not new, and has

been a useful tpol in altering or intensifying the biologi-

cal events that normally occur during the estrous cycle.

Surgical manipulation has also been used to study the

functions of various organs, and how these functions affect

the animal as a whole. Therefore, by destroying, removing,

isolating, or transplanting various organs or their

components, one can begin to assess the importance of that

organ in relation to other organs, or to events that normal-

ly occur within the animal.

One must bear in mind, however, that the reproductive

process is also affected by other factors. Environmental

conditions such as temperature, light, humidity, and shelter

can greatly affect the reproductive performance of swine.

Peed intake CDailey et al., 1975) and nutrient content of

feed are also important contributors to sound reproduction.

The objective of this thesis is to examlne tv/o projeots

that were designed to assess follicular development in the

gilt, and the effects of altered suckling intensity and ex-

ogenous hormones on the postpartum interval in the sow.

These studies will hopefully contribute information that

will one day be utllized to increase the efficiency of swine

production, and add to the understandlng of reproductive

processes in domestic farm animals.

CHAPTER II

LITERATURE REVIEW

Course of Pollicular Development

Ovarlan follicular development is a continuous process.

Once growth has been initiated, it can be terminated only by

ovulation and luteinization or by atresia. (Pederson,

1970). Pollicular development begins with the organization

of the pool of non-proliferatlng small follicles. The small

follicle which resides in the ovary throughout the fertile

period forms an independent unit consisting of an oocyte

surrounded by one or more cells closely connected with its

surface and a basement membrane (Peters, 1978).

Little is known about the physiology of the pool of

small follicles. It is not yet understood what stimulus

causes a follicle or follicles to begin development. . This

selection of follicles from the pool could be due either to

a stimulation or to the removal of an inhibition. It is

known that the total number of small follicles has an influ-

ence on the number of follicles that begin to grow. The

larger their number, the larger the number of follicles that

begin to develop (Krohn, 1967; Krarup et al., 196Q). Peters

(196Q) reported that artificial reduction of the number of

small follicles in infant mice caused a subsequent reduction

in the number of developing follicles.

Once the follicle begins to grow, its stage of develop-

ment can be classified according to size, the number of lay-

ers of granulosa cells, development of the theca, and the

presence of an antrum. Primordial follicles consist of an

oocyte surrounded by a single layer of granulosa cells.

These cells increase in number and the follicle is then con-

sidered to be a secondary follicle. The cell layers contin-

ue to increase, and fluid begins to-accumulate between the

granulosa cells, forming the antrum which is characteristic

of a tertiary follicle. The preovulatory or Graafian folli-

cle eventually consists of an oocyte surrounded by granulosa

cells, wlthin an antrum, that is enclosed by a well-defined

granulosa and thecal layer.

After a follicle reaches this point in development,

ovulation occurs, the oocyte is released, and the follicle

begins to undergo a series of changes to become a functional

corpus luteum. However, the majority of growing follicles

never reach this stage of maturity. The term "atresia" is

used to denote those processes through which the ovarian

follicle loses its integrity and the egg is expelled by

means other than ovulation (Byskov, 197Q). Allen et al.

(1Q2S) estimated that there was ?=»n ^^'^- attrition of

follicles during the estrous cycle of the mouse. Dailey et

al. (1972) reported a 50'̂- decline in the number of vesicular

follicles between day 15 and estrus in the pig. Byskov

(1974) defined three stages of atresia in large follicles:

Stage I was characterized by mitosis and pyknotic nuclei

within the granulosa layer, Stage II in which few cells were

proliferating with many pyknotic granulosa cells, and Stage

III as atresia which signified a collapsed, nongrowing fol-

licle.

The mechanisms that initiate and control the process of

atresia are not known. It is not possible to determine by

examination which fo.llicles will be destined to ovulate or

undergo atresia. •It is possible, however, to manipulate the

attrition rate with the use of exogenous hormones. It was

shown by Peters et al. (1975) that pregnant mares serum go-

nadotropin (PMSG) given to 21-day-old mice could "rescue"

follicles in the early stages of atresia and cause them to

continue development and proceed through ovulation. The

findings of Dailey et al. (1Q75) suggested that full-feeding

could increase the number of preovulatory follicles through

a sllghtly lessened amount of atresia. Their study also

showed that attrition rate of follicles could be affected by

genetic differences in pigs.

Endocrine Regulation of the Ovary

The actions of the pituitary gonadotropins on the ovary

promote the growth and maturation of ovarian follicles, ovu-

lation of matured oocytes contained within, and formation

and maintenance of corpora lutea (Richards, 1978). These

gonadotropins, luteinizing hormone (LH) and follicle stimu-

lating hormone (PSH), are largely under the control of a hy-

pothalamic releasing hormone known as gonadotropin releasing

hormone (GnRH). It is still questionahle as to whether GnRH

is responsible for stimulating the synthesis and release of

both LH and PSH, or if there are two separate releasing fac-

tors, luteinizing hormone releasing hormone (LHRH) and fol-

licle stimulatlng hormone releasing hormone (FSHRH). Wlse

et al. (1Q78) reported that synthetic LHRH could produce a

marked increase in plasma PSH without causing a simultaneous

or subsequent increase in plasma LH in rats. Due to this

effect, they hypothesized that LHRH is also PSHRH, or that

the two gonadotropins are indeed controlled by the same re-

leasing factor.

Secretion of GnRH, LH and PSH begins during the prena-

tal period. Studies on female lambs during the first nine

weeks of postnatal life showed that although the secretion

of PSH remained relatively constant, concentrations of

circulating LH were often higher than those common to an

7

adult ewe (Poster et al., 1^75). Data such as these

indicate that the hormonal mechanisms and feedback systems

are operable before puberty. This would suggest that puber-

ty is initiated by one of several means: (1) that the hypo-

thalamus develops an increased capacity to secrete GnRH

and(or) a decreased sensitivity to the inhibitory influences

of gonadal steroids, (2) that the pituitary becomes more

responsive to the effects of GnRH and secretes increasing

amounts of gonadotropins, or (̂ ) that areas of the central

nervous system outside the hypothalamus mediate the changes

listed in the first possibility above (Steinberger et al.,

1977).

Although the mechanisms governing the onset of puberty

are not yet understood, it is known that the normal rhythm

of the adult reproductive cycle in the female is initiated

hy a peak or surge of gonadotropins that causes ovulation to

occur. Once the cycle has been established in an adult ani-

mal, it is thereafter governed by an intricate system of

feedback regulation.

The hypothalamo-hypophyseal axis concerned with secre-

tion of gonadotropins consists of a hypothalamic neuron

which elaborates and secretes GnRH, an adenohypophyseal cell

which secretes LH and PSH, and a system of vessels which

provides a link between the two (Taleisnik, 1974). This

8

system of vessels, known as the hypothalamo-hypophyseal

portal system, carries GnRH from the hypothalamus to the an-

terior pituitary where it stimulates the synthesis and(or)

release of LH and PSH. The gonadotropins are then carried

via the circulatory system to the target organ--the ovary.

Upon reaching the ovary, PSH works to stimulate growth and

development of ovarian follicles and acts synergistically

with LH to promote estrogen secretion by the follicles. The

estrogen produced, predominantly estradiol-17 beta in pigs,

initially exerts a negative feedback effect upon the hypo-

thalamus and anterior pituitary to reduce secretion of LH.

However, after estrogen concentrations rise above a thresh-

hold level, the feedback becomes positive, and results in a

surge of LH Just before ovulation. The precise role of the

LH surge at the time of ovulation has not been determined,

but it has been hypothesized that LH may stimulate proteo-

lytic enzymes within the follicle that degrade the connec-

tive tissues of the folllcle wall, resulting in rupture of

the wall. Another theory has proposed that LK stimulates

follicular synthesis of prostaglandins which cause follicu-

lar rupture (Kaltenbach and Dunn, 1980).

After ovulation, granulosa and thecal cells undergo

luteinization and are reorganized to form the corpus luteum,

an ephemeral gland that secretes the horTnone nost

responsible for estrous cycle regulation, progesterone.

Q

Astwood (1941) reported that prolactin was required for

corpus luteum maintenance in rats. After this data was pub-

lished, it was generally accepted that prolactin was luteo-

tropic in most mammals. Then, Bradbury et al. (1Q50) found

that human chorionic gonadotropin (HCG) was able to prolong

the lifespan of the corpus luteum in women. This finding

led researchers to suspect that the mechanism for luteal

maintenance in rats may not apply to all animals, and that

there could exist other luteotropic substances. Kaltenbach

et al. (1968) reported that hypophysectomizing sheep immed-

iately after ovulation caused the corpus luteum to regress,

or blocked its formation completely. This suggested that

there was a luteotropic substance, possibly LH, that was

arising from the pitultary gland. The pig, on the other

hand, does not seem to require the support of a hypophyseal

luteotropin. Brinkley et al. (lQ64a,b) formulated the con-

cept that in the pig, LH-induced ovulation was sufficient to

cause corpus luteum formation and progesterone synthesis,

and that the corpus luteum of the 21 day cycle does not de-

pend on further hypophyseal support for formation or func-

tion. However, they did flnd that to maintain corpora lutea

beyond the end of the cycle, as in pregnancy, a hypophyseal

luteotropic substance was required.

10

The Ovulatory Follicle

In recent years, much interest has been focused upon

identification of the ovulatory follicle. Research in this

area has primarily attempted to identify those follicles

destined to ovulate, and to determ.ine at what point in the

cycle follicles are chosen.

Using laparoscopy, Dierschke and Clark (1976) reported

that in rhesus monkeys, the follicle which eventually ovu-

lated could usually be distingulshed from the others by day

6 of the menstrual cycle and that it increases by .7 mm in

diameter per day until ovulation. Smeaton and Robertson

(1971) marked follicles on different days of the estrous cy-

cle in ewes and found that only large follicles marked on

day 14 of the cycle ovulated. When follicles were marked

earlier in the cycle, they regressed. In their study, it

was found that follicles can attain a diameter of 5 mm or

more at three different stages of the cycle, but only those

follicles that achieve this size 36 to 48 h before the onset

of estrus will ovulate. Their data prompted the hypothesis

that there are three waves of folllcular growth in ewes.

Previous studies had also reported similar findings in mice

(Peters and Levy, 1Q66) and in rabbits (Hlll and White,

1Q33). Cows have two discrete waves of follicular growth.

Rajakoski (1960) found that one wave of follicular growth

11

occurred from estrus until day 11, at which time the largest

follicle regressed. A second wave then began around day 13

which resulted in an ovulating follicle. Dufour et al.

(1Q72) found that the follicles marked on day 18 of the bo-

vine estrous cycle were those that were destined to ovulate.

A drastic reduction in the number of developing folli-

cles was observed during the estrous cycle of pigs (Kirkpa-

trick et al.,lQ67). They observed that the largest number

of growing follicles could be seen on day 15 of the estrous

cycle, but that this number was reduced by 40*̂- by day 19.

Presumably, a second growth wave begins on day 15 or 16 that

results in ovulatory follicles. This agrees with research

done by Kelly (1979). After destroying follicles on diffe-

rent days of the cycle in the pig, she observed that folli-

cles destined to ovulate are seemingly selected between days

14 and 16 of the estrous cycle. According to Anderson

(1Q80), this is the point in the cycle that corresponds to

the beginning of the follicular phase. Using unilateral

ovariectomy (ULO) on different days of the cycle, Clark et

al. (1982) again arrived at the conclusion that follicular

selection occurs before day 17 in the porcine estrous cycle.

Their work showed that ovarian compensation after ULO ceased

around day 15 of the cycle.

12

Cautery of Follicles

Disruption of the normal ovarian cycle can be extremely

informative in that it can be used to elucidate the step-

wise progress of folliculogenesis. Some of the methods that

have been used to intervene in the natural progression of

the cycle are: removal or destruction of the ovarian folli-

cle, removal or destruction of the corpus luteum, or removal

of one or both of the ovaries themselves.

Destruction of ovarian follicles by the process of

electrocautery has been utilized by some researchers as a

convenient means for observing follicular growth patterns

and physiological phenomena associated with the absence of

the follicle and the steroids it produces. Dailey et al.

(1976) reported that follicle-cautery on day l^ or 16 of the

estrous cycle delayed the onset of the ensuing estrus in the

pig. They also observed a decrease in the number of normal

and atretic follicles, but found that there was an increase

in the frequency of follicles greater'than 11 mm in diame-

ter. These data were supported by the flndings of Kelly

(1979) who observed that cauterization of follicles on day

16 of the cycle in gilts caused the estrous cycle to be

lengthened.

The results from studies. such as these indicate that

the period between days 14 and 16 of the swine estrous nycle

13

is a crucial time for the development of ovulatory

follicles. When these preovulatory follicles are destroyed,

the estrogens that they were produclng are removed from the

system resulting in a decrease in the feedback upon the go-

nadotropins. Therefore, until the ovary can recover and re-

cruit a new set of ovulatory follicles, ovulation is de-

layed.

When the largest follicle is marked on day 14 in the

ewe, it is the only follicle that is observed to ovulate

with the second largest follicle undergoing atresia. How-

ever, when the largest follicle is destroyed on day 14, the

next largest follicle is observed to ovulate (Bherer et al.,

1975). Cauterization of all follicles on days 2, 8 or 14 of

the swine estrous cycle followed by necropsy 6 days later

showed no differences in follicular development between days

2 and 8 or days 8 and 14; however, there were fewer small (1

to 2 mm) and medium (3 to 6 mm) follicles, but more large

follicles observed between days 14 and 20 (Clark et al.,

1975). When all of the surface follicles on the left ovary

..ere destroyed on day 2, new follicles were observed to de-

velop, however, the total number of follicles that developed

was smaller, as was the number of small follicles.

Nevertheless, the number of newly developed large follicles

did not differ from the number observed on the left ovaries

of the surgical control group (Brinkley and Young, 1Q6Q).

l^

Destruction of follicles followed by normai numbers of

large follicles and resultant decreases in the number of

small follicles suggests that the ovary may recruit from the

pool of small follicles to replace the destr'oyed follicles.

The studies conducted by Clark et al. (1Q75) and Brinkley

and Young (IQ69) support this hypothesis.

Ablation of the largest visible follicle on day ^, Q or

10 of the menstrual cycle in monkeys abolished the mid-cycle

surges of gonadotropins and delayed the surges until two

weeks after ablation (Goodman and Hodgen, 1Q7Q). These

findings indicated that 1) in monkeys, the dominant follicle

has been selected by the mid-follicular phase and no other

follicle is available as a surrogate for mid-cycle ovula-

tion; 2) the next follicle destined to ovulate is not se-

lected until after the removal of the dominant follicle and

3) in the presence of the dominant follicle, the contrala-

teral ovary contributes little, if at all, to the regulation

of gonadotropin secretion, contralateral follicle growth, or

subsequent contralateral corpus luteum fornation.

15

Lactation and the Lactational Anestrus

Estrous cycles in the pig are normally interrupted only

by the establishment of pregnancy, with the inhibition of

ovulatory estrus continuing throughout the period of lacta-

tion unless the lactation is prolonged (Burger, 1Q52).

Just before parturition, the corpora lutea of pregnancy

regress and cause a subsequent rapid decline in the circu-

lating concentratlons of progesterone (Short, 1Q60). Estro-

gen secretion, on the other hand, begins to increase about

the tenth week of gestation, and in the last two weeks be-

fore parturition, rises to massive proportions (Catchpole,

1977.). Since the ovary contains only small or atretic fol-

licles Just before parturition, Melampy et al. (1Q66) have

theorized that the estrogen must be feto-placental in ori-

gin. The dramatic increase in circulating concentrations of

estrogen Just before parturition is believed to be the cause

of the postpartum estrus in sows. This estrus usually oc-

curs 3 to 7 days after parturition in sows (Rurger, 1Q52),

however, the general concensus is that the postpartum estrus

is anovulatory (Warnick et al., 1^50; Baker et el., 1953)

and is merely a response to rising estrogen concentrations.

Van Landeghem and van de Wiel a<^78) measured

concentrations of prolactin during parturition, lactation

16

and suckling in sows and found the highest concentrations of

prolactin occurred during parturitlon. Throughout the pro-

cess of parturition, prolactin concentrations varied from QO

to 150 ng/ml, then dropped to concentrations ranging between

35 and 80 ng/ml during the lactation period. After the pigs

were weaned, prolactin fell to around 10 ng/ml.

Prolactin secretion during the postpartum period is of

concern to researchers since its secretion is believed to

exert a negative feedback on secretion and release of the

gonadotropins, and thereby block the occurrence of estrus

and ovulation during the suckling period. Studies using

breast-feeding women have directly attributed the occurrence

of prolonged lactational amenorrhea to elevated concentra-

tions of prolactin in those women (Gross and Eastman, 1979;

Gross et al., 1979).

Prolactin is normally secreted in a cyclical fashion

during lactation. Increases in the secretion of prolactin

are elicited by a number of exteroceptive stimuli including

suckling, presence of the young and stressful situations

(Goodman et al., 1979).

The inhibitory effects of prolactin on the hypophyseal-

hypothalamo system have prompted some researchers to

investigate the effects of reduced suckling stimulus upon

the reproductive perform.ance of the postpartum sow. Self

17

and Grummer (1958) completely weaned litters of pigs from

their dams at 10, 21 or 56 days of age. They observed a

significantly longer interval from weaning to post-weaning

estrus in the day 10 group. Two of the sows in the same

group were also observed to have developed follicular cysts.

There were no differences in percent fertility between the

sows in any of the groups; however, the litters from the 10

day weaning group did not survive the conditions of the ex-

periment.

In a similar study, Peters et al. (1969) reported that

sows from which pigs were removed early in lactation bad a

longer interval to fertile estrus and a higher incidence of

abnormal estrus than did sows from which pigs were rem.oved

later in lactation. In addition, they showed that suckled

sows, as opposed to non-suckled sows, tended to have a

greater number of ovulations and a higher percentage of ova-

rian structures that were corpora lutea. Pinally, they re-

ported that sows from which pigs were removed at birth tend-

ed to exhibit cystic ovaries and persistent estrus. Svajgr

et al. (1974) reported that there was a direct relationship

between length of lactation and fertilization rate, embryo

survival and litter size. As lactation length increased

from 2 to 24 days, significant increases were observed in

the other factors. Even a two-day suckling period was found

18

to be more beneficial than no suckling at all (Baker et al.,

1953). Sows from which pigs were removed at birth tended to

have a higher incidence of ovarian cysts and a longer inter-

val to a fertile postpartum estrus. These findings agree

with those of Warnick et al. (1950) who reported a shorter

interval to estrus for suckled sows than for sows whose pigs

were removed at birth.

The problems associated with early weaning such as re-

duced reproductive performance of the sows as well as re-

duced performance of the pigs prompted researchers to turn

to alternate methods to reduce the suckling stimulus. A

technique that has shown promise is altered suckling inten-

sity (ASI). This involvfes separating the litter from the

sow for a period of time each day, after which the litter is

returned for a similar period of time (Smith, lQ6l). In many

cases, ASI has been combined with hormonal therapy to maxim-

ize the effect on resumption of estrus activity. V/hen PMSG

was injected on day 23 postpartum preceded by three 12 h in-

tervals of ASI, the incidence of estrus in lactating sows

was 80«̂^ conpared to 33*̂ . for the PMSG treatment alone or Q**̂

for the ASI alone. When PMSG was followed by an injection

of HCG 96 h later, and sows were inseminated without regard

to estrus, P>5% of the sows were diagnosed pregnant ^^ to 44

days after HCG (Kuo et al., 1Q76). Crighton (lQ70a,b) was

IQ

able to induce estrus, ovulation and pregnancy consistently

during the fourth week of lactation by separating the sow

and litter for 12 h per day for three consecutive days fol-

lowed by an injection of PMSG. Britt et al. (1978) reported

that altering the suckling schedu e alone resulted in a re-

duction in the interval from weaning to rebreeding, in-

creased the conception rate and reduced the number of sows

culled open as compared to control sows that suckled through

a normal lactation period.

The use of exogenous hormones alone to stimulate estrus

and ovulation during lactation has produced variable re-

sults. PMSG given in early lactation (1 to 38 days) did not

manifest estrus regularly, but 26 of 27 sows injected bet-

ween day 39 and 68 of lactation showed estrus wlthin 7 days

after treatment (Cole and Hughes, 1946). Hausler et al.

(1980) achieved pregnancy In 17 of 20 sows that were treated

on day 25 postpartum with prostaglandin P 2-alpha followed

by PMSG and HCG. The sows were artificially inseminated 24

to 42 h post-HCG. Guthrie et al. (1Q78) reported an induc-

tion of ovulation in 6 of 7 sows after a treatnent of PMSG

and HCG given between days 14 and 17 of lactation after an

injection of GnRH.

Although other methods have been attempted to induce

ovarian activity during or after lactation such as altered

20

feeding levels (Reese et al., 198O) and exposure of

lactating sows to a boar (Petchey and English, I98O), the

most viable means of achieving fertility during lactation

still appears to be a combination treatment using hormone

therapy and ASI.

CHAPTER III

EPFECTS OP FOLLICLE-CAUTERY ON THE DAY OP ESTRUS ON SUBSEOUENT ESTROUS CYCLE LENGTH AND OVULATION RATE IN

GILTS

Summary

The objective of this study was to deternine the inter-

val of time required for follicular regrowth after destruc-

tion of follicles on the day of estrus in the porcine est-

rous cycle. Crossbred gilts (7 to Q mo of age) were

randomly assigned to one of two treatm.ents: a) control--sham

follicle-cautery or b) follicle-cautery on the day of estrus

(day 0). Blood samples were collected daily beginning on

day 16 before the treatment estrus, and continuing beyond

the post-treatment estrus. At surgery the large follicles

(7 to 10 mm in diameter) were either destroyed by electro-

cautery or counted in the control animals. After treatment,

gilts were checked daily for estrus, then necropsied within

16 days after estrus was detected. Serum progesterone con-

centrations were determined by radioimmunoassay. The means

for estrous cycle length (days) and ovulation rate as

affected by treatment were a) 18.4, 12.Q and b) 12.Û, 11.7,

respectively. A shorter (P<.ni) cycle length was observed

in the cautery gilts when compared to the control gilts. No

21

22

difference (P>.05) was observed in ovulation rate between

the groups. In conclusion, follicle-cautery on the day of

estrus resulted in a shorter subsequent estrous cycle, but

did not affect the subsequent ovulation rate.

Introduction

The length of time required for follicular growth to

proceed to ovulation is not clearly understood in domestic

animals. Dailey et al. (1976) proposed that in the gilt,

follicles destined to ovulate began developing around day 5

of the cycle. He further estimated that those follicles re-

quire about 15.6 days to complete development. Clark et al.

(1982) reported that unilateral ovariectomy after day 13 or

15 of the swiné estrous cycle resulted in decreased compen-

sation by the remaining ovary.

Electrocautery of follicles has been utilized as a con-

venient means of obliterating ovarian components. Clark et

al. (1975) destroyed visible follicles on different days of

the estrous cycle and found a significant difference in fol-

licular growth after cautery, with the period between days

14 and 20 showing the greatest amount of growth. A delay in

the onset of estrus after follicle-cautery between days 14

and 20 (Dailey et al.,lQ76) is substantiated by the findings

of Clark et al. (1979, 1Q82) who reported that the follicles

23

destined to ovulate are apparently selected between days 14

and 17 of the cycle.

The present study was designed to further clarify the

process of follicular growth, and to more closely pinpoint

the amount of time required for growth of ovulatory folli-

cles during the porcine estrous cycle.

Materials and Methods

Yorkshire X Hampshire X Landrace X Duroc gilts were

moved from a total confinement facility to an outslde dirt

lot at approximately 6 mo of age. Beginning the day after

the move, all gilts were checked daily for evidence of Ini-

tiation of estrous activity using an intact boar. A gilt

was considered to be in estrus (day 0 of the estrous cycle)

when it adopted an immobile stance and allowed the boar to

mount (Signoret, 1970). The boar was then removed to pre-

vent impregnation. Before assignment to a treatment group,

the gilts were followed through two complete cycles to es-

tablish their normal cycle length.

On day 16 of the second estrous cycle, daily blood sam-

pling was initiated. This entailed immobilization of the

animal with the least stress possible, then sampling fron

the anterior vena cava as described by Carle and Dewhirst

(1942). Twenty milliliters of blood were collected each

24

morning between O8OO and 1000 h. The blood was allowed to

clot at room temperature, then refrigerated for 24 h before

centrifugation at 2000 rpm for 20 min. Serum was stored at

-20 C until assays were performed.

On their thlrd estrus, gilts were randomly assigned to

one of two treatment groups: a) control—subjected to sham

follicle-cautery and b) follicle-cautery (figure 3-1). Sur-

gery was performed on the flrst day of demonstrable estrus.

The animals were injected iv with 1 g sodium thiopentol

(Dipentol; Dianond Laboratories, Inc, Des Moines, lA) to

induce inltial anesthesia. A mixture of nitrous oxide, oxy-

gen and methoxyfluorane (Metofane; Pitman-Moore, Inc, Wash-

ington Crossing, NJ) delivered via a closed-circuit system

(Dziuk et al.,1964) was used to maintain anesthesia through-

out the surgical procedure.

A midventral laparotomy was performed and the ovaries

were located and exteriorlzed. In the aninals assigned to

the control group, the large follicles (7 to 10 nm in diane-

ter) and corpora albicantia on each ovary were counted and

recorded and the surgery was then terninated. In the treat-

ed animals, however, after recording the number of large

follicles and corpora albicantia on each ovary, the large

follicles were destroyed. This was accomplished hy

introducing the tip of a cautery needle into the lumen of

25

Day of cycle

0

0

16

0

0

16

/ / /

/ / /

/ / /

/ / /

Events

Estrus 1

^ ^

.

'///

/ / /

/ / /

XXX XXX XXX ^ \ XXX XXX XXX XXX 000 000 000

Estrus 2 (Pretreatment

^ cycle)

Begin blood sarapling

Estrus 3 (Treatment cycle)

^ ? 000 000 000 000 XXX XXX

xxxl xxxl

Estrus ^

V

Necropsy

PIGURE 3-1. EXPERIMENTAL DESIGN

each follicle and applying voltage sufficient to destroy the

follicular tissue.

After surgery, the animals were penned indoors for 24

h, then were returned to the outside lot. Daily estrous

checking and blood sanpling continued on both the control

26

and treated animals until a post-surgical estrus was

detected. In the treated group, additional blood samples

were obtalned on days 2, 4, 6, 8, 10, and 12 of the post-

treatment estrous cycle. All gilts were necropsied within

16 days after the post-treatment estrus. Reproductive

tracts were recovered and the number of corpora lutea on

each ovary was counted to determine ovulation rate in re-

sponse to the treatment. Data were analyzed using an un-

paired "t" test (Steele and Torrie, 1980).

Radioimmunoassay for progesterone was perforned using

the procedure of Clark et al. (1978). Serum (50, 100, or

200 microliters) was extracted with 6 ml petroleum ether.

Procedural losses were assessed and corrected for by recov-

ery of tritiated progesterone added to each sanple. Serum

extracts were incubated with 100 microliters GDN-337 antise-

rum (kindly provided by Dr. G.D. Niswender, Colorado State

University, Fort Collins) for 30 minutes before the addition

of tritiated progesterone, and then incubated at room temp-

erature overnight (16 to 20 h). Assay tubes were then

chilled in an ice bath for 30 minutes before the addition of

dextran-coated charcoal. Tubes were centrifuged at 2000 rpn

for 10 minutes at 4 C. The supernatant was decanted and

processed for scintillation counting. The progesterone

content of each sample was determined fron a standard curve

(range 0 to 500 pg) and expressed as nanograns progesterone

27

per milliliter of serum, using a computer program (Rodbard

and Lewald, 1970).

Results and Discussion

The means for reproductive endpoints before and after folli-

cle-cautery are shown in table 3-1. No difference (P>.05)

was observed in either pre-treatment ovulation rate or est-

rous cycle length between the two groups. The number of

follicles on the ovaries at surgery was also not signifi-

cantly different. It was observed, however, that follicle-

cautery resulted in a shortened (P<.01) length in the post-

surgical estrous cycle. The mean length of the third

estrous cycle in treated gilts was 12.9 days as conpared to

18.4 days in the control gilts.

Mean serun concentrations of progesterone are shown in

figure 3-2. Progesterone concentrations ranged from .5

ng/ml (minimum detectable concentration of the assay) to 16

ng/ml during the luteal phase in the sham follicle cautery

group. Pigure 3-3 illustrates the nean concentrations of

serum progesterone in the follicle-cautery group. Levels

remained at minimun detectable concentrations throughout the

period after follicle-cautery, then began to rise as the

28

T a b l e 3 - 1 . MEANS POR REPRODUCTIW ENDPOINTS BEFORE AND AFTER POLLICLE-CAUTERY ON THE DAY OP ESTRUS

IN GILTS

Sham . Reproductive follicle Follicle Pooled endpoints cautery cautery SE

Number of gilts 9 9

Number of corpora lutea at second estrus 11.5^ 13.4 1.16

Length of second estrus cycle, days 19.0^ IQ.4^ .036

Number of large follicles at third estrus ̂ 13.0^'^ 14.3 .546

Interval from treatment -, to estrus, days 18.4^ 12.9'' 1.09

Number of corpora lutea , at fourth estrus 12.Q^ 11.7^»'' 1.10

^Pooled standard error.

^N=8.

^'^Means within each row with different superscripts differ (P<.01).

^7 to 10 mm in diameter.

29

Figure 3-2. MEAN SERUM CONCENTRATIONS OF PROGESTERONE IN GILTS SUBJECTED TO SHAM FOLLICLE-CAUTERY.

Figure 3-3. MEAN SERUM CONCENTRATIONS OF PROGESTERONE IN GILTS SUBJECTED TO FOLLICLE-CAUTERY.

'^wK^

cn c lij Z

o d LU (f) LU

O o cr CL

16

14

12

10

8

6

4

2

1

r̂

1

m

. JL

1

Sham Follicle Cautery

^ '

»

1

1

1

1

1

1

l

1

I -

1

•T

l

I ••

t

1 1

SE= 1.824

í •

-

-

-

-

-

—

30

-2 2 4 6 8 10 12

DAYS OF ESTROUS CYCLE

14 16 18

C ' •

LU

Z

o D : LU \-<f) LU O

o CC CL

16

14 k

12

10

8

6

y/- T r SE = .689

Folíicle Cautery

X X X 2 4 6 8 10 12

DAYS OF ESTROUS CYCLE

V/ 'C-L

10

31

fourth estrous cycle was initiated. The absence of luteal

concentrations of progesterone indicates that the surgery

accomplished conplete follicular ablation and that no luteal

tissue was allowed to develop.

Several factors are involved in the developnent of fol-

licles and the onset of estrus in the gilt. Hansel and

Echternkamp (1972) reported that during the normal estrous

cycle, a rapid decline in progesterone occurs between days

14 and 16, with the animal returning to estrus 4 to 6 days

later. This drop in progesterone concentrations reflects

the period of luteal regression. The progesterone concentra-

tions in the control group of the present study denonstrate

a similar decline occurring between days l^ and 16. If the

corpora lutea are surgically removed during the luteal phase

of the cycle, the gilt will return to estrus within 6 days

(Anderson et al., 1966). In the sow, the lactational anes-

trus can be alleviated by weaning, after which, follicle

size increases and ovulation occurs within 6 days (Crighton

and Lamming, 1969; Clark et al., 1978). These data suggest

that porcine follicles require approximately 6 days to de-

velop to ovulatory status, which is comparable to the

follicular phase of the estrous cycle (Dziuk, 1Q77).

The discrepancy between the previously reported 6 days

and the 12.9 days indicated in this study can be explained

32

by examining the status of the ovary at the time of

follicular or luteal ablation. At the time of normal re-

gression of the corpora lutea or when the corpora lutea are

surgically removed, there are numerous macroscopic follicles

on the surface of the ovary that are readily available for

recruitment. However, on the day of estrus, other than the

few large follicles that have achieved ovulatory status, the

ovary is devoid of any follicles that are not in some stage

of atresia. When the large follicles are destroyed on the

day of estrus, the ovary is forced to draw from its pool of

microscopic follicles to recruit for the ensuing ovulation.

During the course of thls study, several treatment

gilts had to be dropped due to development of follicular

cysts following surgical manipulation. Pleming et al.

(1983) reported that any handling of the ovary, however min-

imal, could result in the formation of follicular cysts.

This was not the case in our findings, as none of the con-

trol animals developed cysts. Only tbe gilts that had un-

dergone the extreme insult of electrocautery were found to

be more prone to development of cysts.

CHAPTER IV

EPFECTS OP ALTERED SUCKLING INTENSITY AND GONADOTROPIN RELEASTNG HORMONE ON POSTPARTUM INTERVAL TN

SOWS

Summary

Thirty-two sows were randomly assigned to a 2 X 2 X 2

factorially designed experiment. The factors were: 1) no

altered suckling intensity (ASI) vs AST of the sow, 2) sa-

line vs 200 micrograms of gonadotropin releasing hormone

(GnRH) and 3) primiparous vs multiparous sows. AST was ac-

complished by removing sows fron litters for 12 h daily

starting 7 days prior to the scheduled weaning date (about 4

weeks). Tnjections (1 ml) of phosphate buffered saline or

GnRH were given sc on the first three days of AST. Sows in

the AST groups were moved from the farrowing house to the

breeding barn and penned adjacent to boars. Estrous activi-

ty was checked every 12 h. After the 7-day AST period, all

piglets were weaned and weaning weights were obtained along

with weights of piglets at one week post-weaning. Data were

analyzed by analysis of variance. The average (SE) interval

from beginning of treatment to onset of estrus, adjusted for

litter size, was 13.4 (?.S) days. No differences (P>.05) in

interval were observed due to AST, GnRH, parity or their

33

3^

interactions. None of the treatment factors or their

interactions affected (P>.05) average weaning weight per pig

(6.1 kg, SE=1.0). The weight of the pigs one-week post-we-

aning was affected (P<.03) by ASI. The average weight of

pigs in the AST group was 7.7 kg compared to 6.9 kg for the

control group. The results of this study suggest that

neither GnRH nor AST, alone or together, are able to de-

crease the postpartum interval in sows.

Tntroduction.

A basic obstacle to maximizing reproductive efficiency

of sows is the anestrus associated with lactation. Apart

from an anovulatory postpartum estrus shown by some sows

(Warnick et al., 1950; Baker et al., 1953), the lactation

period is generally characterized by ovarian quiescence.

Exogenous hormones alone, or coupled with altered suck-

ling intensity (AST), have been used with varying degrees of

success to initiate estrus during lactation. Complete remo-

val of pigs before day 21 of lactation may result in longer

intervals to fertile estrus, lowered ovulation rates and de-

creased pig performance (Self and Grummer, IQ'58). Altering

the suckling schedule has proven more beneficial than total

separation before day 21 (Walker and England, 1Q77; Britt et

al., 1Q78).

^5

Pregnant nare serum gonadotropin (PMSG) and hunan

chorionic gonadotropin (HCG) have effectively initiated ovu-

lation in both lactating sows and prepuberal gilts (Hausler

et al., 1980; Guthrie et al., 1978). Pollicle stinulating

hormone (PSH) and PMSG alone were unsuccessful in stimulat-

ing early ovulation In lactating sows (Cole and Hughes,

1946; Peters et al., 1969). Gonadotropin releasing hormone

(GnRH) has been used previously with other experimental

methods including sow grouping, boar contact and therapy

with PMSG and HOG (Guthrie et al.,1978).

The objective of this study was to deternine the ef-

fects of GnRH and ASI on postpartum performance in prinipa-

rous and multiparous sows.

Materials and Methods

Thirty-two Yorkshire x Hampshire x Landrace x Duroc

sows were randonly assigned to a 2 x 2 x 2 factorially de-

signed experiment. The factors were: 1) no ASI vs ASI, 2)

saline vs 200 micrograms GnRH and 3) primiparous vs multipa-

rous sows (table 4-1). Treatment was begun 7 days before

the scheduled weaning date which generally occurred 4 weeks

after farrowing. Hereafter, the period of treatnent will be

referred to as days 1 through 7 with day 7 being the day of

weaning. Tnjections of 200 micrograns GnRH or buffered

36

Table 4-1. EXPERTMENTAL DESIGN

Hormone treatment

No GnRH

GnRH

Primiparous

No ASI

n=3

n=3

Sows

AST

n=4

n=2

Multiparous

No AST

n=^

n=5

Sov;s

ASI

n=5

n = 5

saline were given sc on mornings 1, 2 and 3 before removal

of the sows from their litters. Altered suckling intensity

was accompllshed by removing the sows from the farrowing

crates and transferring them to the gestation barn where

they were housed adjacent to a boar for 12 h. Before re-

turning to their litters, the sows were checked for estrous

actlvity using intact boars. The altered suckling schedule

«as continued until day 7 when all litters were weaned. At

that time, all sows were transferred to the gestation barn,

and estrous checks were continued daily until all sows had

exhibited estrus. Creep feed was provided for all piglets

from day 14 of lactation until weaning. Weaning weights and

one-week post-weaning weights were obtained for all piglets.

Data were analyzed by analysis of variance (Steele and

Torrie, 1980) using the General Linear Models procedure of

SAS (SAS, 1982).

37

The GnRH used in this study was a gift from Dr. R.H.

Rippell, Abbott Laboratories, North Chicago, Illinois. The

lypholyzed GnRH was diluted with buffered saline to a con-

centration of 200 micrograms and stored at -20 C until used.

Results and Discussion

The results of this study are summarized in table 4-2

and appendix tables A-1 and A-2. The average day of lacta-

tion for the beginning of treatment was 24.2 days (not shown

in tabular form), with an average litter size of 8.^ pigs.

The average interval from the beginning of treatment to the

onset of estrus, adjusted for litter size, was 13.4 days.

No differences (P>.05) were observed in the interval due to

AST, GnRH, parity or their interactions. None of the treat-

ment factors were found to affect (P>.05) the average wean-

ing weight of the piglets (6.1 kg); however, altering the

suckling pattern of the litter was found to have an effect

(P<.03) on the one-week post-weaning weights. The average

weight in the AST group was 7.7 kg conpared to 6.Q kg for

the group that was not separated fron the sow.

These results indicate that neither ASI nor GnRH are

effective inducers of estrus in lactating sows; however,

other research has shown that GnRH can be successfully

utilized to induce ovulation in postpartun aninals. Cox and

38

T a b l e 4 - 2 . EPPECTS OP GnRH, ALTERED SUCKLTNG TNTENSITY AND PARITY ON POST-PARTUM REPRODUCTTVE ACTTVTTY TN SOWS

Treatment

Tnterval from Litter treatment to size estrus, days

Average Average weight 1 weaning week post-weight,kg weaning,kg

Hormone:

Vehicle GnRH

Parity:

Primiparous Multiparous

AST:

No AST AST

8.3 (17) 8.7 (15)

8.3 (12) 8.7 (20)

8.4 (16) 8.6 (16)

13.1 (15) 13.6 (14)

13.8 (11) 12.9 (18)

13.4 (13) 13.4 (16)

5.6 (17) 6.5 (15)

6.1 (12) 6.0 (20)

5.9 (16) 6.3 (16)

7.3 (16) 7.4 (15)

7.4 (11) 7.2 (20)

6.9 (l6)(i 7.7 (15)

SE

df

1.6

24

3.5

20

1.0

24

O.Q

2?

Adjusted by covariance for litter size.

Least-square means (number of sows).

^' Means in a column within treatment with different superscripts differ fP<.05).

Pooled standard error of the mean.

Degrees of freedom.

39

Britt (1Q82) stimulated estrus and ovulation in postpartum

sows with hourly injections of GnRH via anterior vena cava

cannula. The crucial factor in stimulatlng pituitary re-

sponsiveness to GnRH appears to be timing and concentration

of the hormone injected. Apparently 200 micrograms once

daily for three days was not sufficient to overcome the in-

hibitory effects of lactation and suckling. Knobil et al.

(I98O) experimented with different infusion rates of GnRH

into ovariectomized monkeys which had lesions within the

arcuate nucleus, so that secretlon of gonadotropins was vir-

tually absent. They reported that a constant infusion of

GnRH led to an abolition of pituitary response due to desen-

sitization or "down regulation" of receptors. They showed a

maximal response by adninistering GnRH at a rate of 1 ml/min

for 6 minutes once every hour.

Season may also have an effect upon the responsiveness

of the animals to treatment. There is some evidence that

during sum_mer months, alterlng the suckling intensity of

litters of primiparous sows results in a shorter interval to

estrus than during other times of the year (Britt and Levis,

1982) The AST in this study was carried out during November

and December for one group of sows, and during May for

another group of sows. Clark and Tribble (1982) showed that

during the summer months, primiparous sows have a longer

rebreeding interval than during other nonths.

40

Removal of sows from litters for 12 h daily did result

in significantly heavier pigs at one-week post-weaning.

This effect is due to the AST pigs tending to consume larger

quantities of creep feed before weaning, and therefore not

undergoing as severe a post-weaning lag as their control

counterparts.

CHAPTER V

GENERAL DTSCUSSION

Altering or nanipulating biological systens nust be

used in order to discover the normal workings of the systen.

The hope of the researcher is that the abstract of the re-

search itself can be translated into a workable solution to

a problen.

Electrocautery of follicles is research that is seem-

ingly so abstract as to have no applicable use. The impor-

tance of understanding the physiology of the ovary and ovu-

lation is the very foundation of attenpts to increase

ovulation rate and litter size. Studies are needed to un-

cover exactly what causes atresia of follicles and perhaps

neans to rescue follicles and pronpt them to ovulate. Des-

truction of the ovulatory follicles showed that tbe ovary

can recruit other follicles, and in a relatively short am-

ount of tine, ovulate nornally. Research into the endocri-

nology of the follicle at the cellular level will provide

some of the answers as to why some follicles wlll ovulate

and why others are destined to atresia.

Tnduction of estrus in lactating sows nay be used to

shorten the interval between litters, and thereby increase

the number of pigs produced per year. The inability of tbe

41

42

GnRH used in this study to produce ovulation is primarily

due to the low concentrations used. The short half-life of

GnRH combined with a single sc injection each day could not

achieve sufficient levels to induce estrus and ovulation.

Recently, studies have shown that hour y injections of GnRH

directly into the anterior vena cava produced fertile estrus

about ? days after initiation of treatnent in lactating sows

(Cox and Britt, 1Q82).

The final answer to achieving an effective use of hor-

mones to stimulate estrus must exist as a balance within the

current therapies. Hormonal stimulation with or without al-

tered suckling intensity can be a feasible protocol in a

commercial swine operation provided that a reasonable and

efficient therapy can be achieved. The solution will proba-

bly lie with the discovery of long-acting synthetic hornones

that can safely be used as feed additives.

LTTERATURE CTTED

Allen, E., W.B. Kountz and B.P. Prancis. 1925. Selective elimination of ova in the adult ovary. Aner. J. Anat. 34:445.

Anderson, L.L. I98O. Pigs. Tn: E.S.E. Hafez (Ed.) Reproduction in Parm Animals (4th Ed.). pp 358-386. Lea and Febiger, Philadelphia.

Anderson, L.L., G.W. Dyck and R.P. Rathmacher. 1Q66. Pituitary gonadotropic activities following luteal enucleation in the pig. Endocrinology 78:8Q7.

Astwood, E.B. 1941. The regulation of corpus luteun function by hypophyseal luteotrophin. Endocrinology 28:309.

Baker, L.N., H.L. Woehling, L.E. Casida and R.H. Grunmer. 1953. Occurrence of estrus in sows following parturition. J. Anim. Sci. 12:33.

Bherer, J., J. Dufour and P. Matton. 197^. Pate of the two largest follicles of the ovaries of a ewe as a result of the destruction of the largest follicle and(or) removal of the corpora lutea at two stages of the estrous cycle. Can. J. Physiol. Pharmacol. 54:7.

Bradbury, J.T., W.E. Braun and L.A. Gray. 1Q50. Maintenance of the corpus luteum and physiological actions of progesterone. Rec Prog. Horm. Res. 5:151.

Brinkley, H.J., H.W. Norton and A.V. Nalbandov. lQ64a. Role of the hypophyseal luteotrophic substance in the function of porcine corpora lutea. Endocrinology 74:9.

Brinkley, H.J., H.W. Norton and A.V. Nalbandov. lQ64b. Ts ovulation alone sufficient to cause formation of corpora lutea? Endocrinology 74:14.

Brinkley, H.J. and E.P. Young. 196Q. Effects of unilateral ovariectomy or the unilateral destruction of ovarian components on the follicles and corpora lutea of the nonpregnant pig. Endocrinology 84:12S0.

43

44

Britt, J.H. and D.G. Levis. 1982. Effect of altering suckling intervals of early-weaned pigs on rebreeding performance of sows. Theriogenology 18:201.

Britt, J.H., J.W. Parker and D.G. Levis. 1978. Preliminary results of field studies on reproductive problems in gilts and sows. North Carolina Pork Producers Conf. (22nd Annual), Jan 12-13 Raleigh, N.C.

Burger, J.P. 1952. Sex physlology of pigs. Onderstepoort J. Vet. Res. 2:3 (Suppl.).

Byskov, A.G.S. 1974. Cell kinetic studies of follicûlar atresia in the mouse ovary. J. Reprod. Pertil. 37:277.

Byskov, A.G.S. 1Q7Q. Atresia. Tn: A.R. Midgley and W.A. Sadler (Ed.) Ovarian Pollicular Development and Punction. pp 41-57. Raven Press, New York.

Carle, B.N. and W.H. Dewhirst. 1Q42. A method for bleeding swine. J. Aner. Vet. Med. Assoc 101:495.

Catchpole, H.R. 1977. Hornonal nechanisns in pregnancy and parturition. In: H.H. Cole and P.T. Cupps (Ed.) Reproduction in Domestic Animals (3rd Ed.). pp 341-368. Academic Press, New York.

Clark, J.R., D.J. Dierschke and R.C. Wolf. 1978. Hormonal regulation of ovarian folliculogenesis in rhesus monkeys: I. Concentrations of serum luteinizing hormone and progesterone during laparoscopy and patterns of follicular development during successive nenstrual cycles. Biol. Reprod. 18:77Q.

Clark, J.R., N.L. Pirst, A.B. Chapman and L.E. Casida. 1975. Ovarian follicular development during the estrous cycle in gilts following electrocautery of follicles. J. Anim. Sci. 41:1105.

Clark, J.R., D.E. Orr and L.P. Tribble. 1Q78. Tnterval from eaning to rebreeding in first and second litter sows.

•Texas Tech Univ. Agric Sci. Tech. Rep. No. T-5-138:6Q.

Clark, J.R., C.A. Kelly, D.E. Orr and L.P. Tribble. 1Q7Q. Poiliculogenesis in swine: Effects of follicle-cautery on subsequent ovulation rate and estrous cycle length. J. Anim. Sci. 49 (Suppl. 1):11.

4«=̂

Clark, J.R. and L.P. Tribble. 1Q82. Effect of parity and season on interval from weaning to rebreeding in sows. Texas Tech Univ. Agr. Sci. Tech. Rep. No. T-5-l65:l6.

Clark, J.R., S.G. Brazier, L.M. Wiginton, G.R. Stevenson and L.P. Tribble. 1982. Tine of ovarian follicle selection during the porcine estrous cycle. Theriogenology 18:6Q7.

Cole, H.H. and E.H. Hughes. 1946. Tnduction of estrus in lactating sows with equine gonadotropin. J. Anim. Sci. 5:25.

Cox, N.M. and J.H. Britt. 1Q82. Tnduction of fertile estrus in lactating sows with hourly injections of gonadotropin releasing hormone. J. Anim. Sci. 55 (Suppl. 1):4Q.

Crighton, D.B. 1970a. Tnduction of pregnancy during the lactation in the sow. J. Reprod. Pertil. 22:223.

Crighton, D.B. 1970b. The inductlon of pregnancy during lactation in the sow: The effects of a treatnent inposed at 21 days of lactation. Anim. Prod. 12:611.

Crighton, D.B. and G.E. Lamming. IQ69. The lactational anestrus of the sow: The status of the anterior pituitary system during lactation and after weaning.* J. Endocrinol. 43:507.

Dailey, R.A., J.R. Clark, N.L. Pirst, A.B. Chapman and L.E. Casida. 1Q72. Effects of high and low feeding at two stages of the estrous cycle on follicular development In gilts from four genetic groups. J. Anim. Sci. 35:1210.

Dailey, R.A., J.R. Clark, N.L. Pirst, A.B. Chapman and L.E. Casida. 1975. Loss of follicles during the follicular phase of the estrous cycle of swine as affected by genetic group and level of feed intake. J. Anim. Sci. 41:835.

Dailey, R.A., J.R. Clark, R.B. Staigmiller, N.L. Pirst, A.B. Chapman and L.E. Casida. 1Q76. Growth of new follicles following electrocautery in four genetic groups of swine. J. Anim. Sci. 43:175.

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46

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APPENDIX

51

52

Table A-1. ANALYSIS OF VARTANCE TABLES FOR REPRODUCTTVE ENDPOINTS TN SOWS

Source of variation

Hormone (H) Parity (P) ASI (A) H X P H X A P X A H X P X A Error

df^

1 1 1 1 1 1 1

24

Litter size

1.18 .82 .50 .67 .01 .95 .04

2.52

Mean squares Average weaning weight,kg

25.27 .46

5.94 .2 5.,.

22.32 1.98 12.83 5.25

Average weight 1 week post-weaning,kg

.16 1.78,, 23.66 6.34

10.69 .09

4.32^

Degrees of freedom,

^Error df=23.

'P<.05

53

Table A-2. ANALYSIS OF COVARIANCE TABLE FOR INTERVAL FROM TREATMENT TO ESTRUS IN SOWS

Source of

variation df*

Mean squares for interval from treatment to estrus, days

Hormone (H) Parity (P) AST (A)

P H H P H

X X X X

A A P X A

Regression Error

1 1 1 1 1 1 1 1

20

Degrees of freedom.

1 5

62 18 .01 • 17

6.47 21.33

• o Z j. A

92.10 12.16

^Adjusted by covariance for l i t te r size.

'p<.oi