GnRH induced LH release in suckled beef cows. II. The effects of exogenous corticoids and estradiol benzoate on luteinizing hormone release by GnRH

THERIOGENOLOGY

GnRH INDUCED LH RELEASE IN SUCKLED BEEF COWS. II. THE EFFECTS OF EXOGENOUS CORTICOIDS-AND,ESTRADIOL BENZO.ATE ON LUTEINIZING HORMONE RELEASE BY GnRH (a,b) H.

J.

Irvin

(c)

, V.

Departments

M. of

(d) PflantzR. E. Morrow(d), H. A. Ga:verick(c) .Dairy Science(-C) and Animal University of Missouri Columbia, MO 65211

B.

N.

Science

Day(d)

and

(d)

Received for Publication: April 10, 1981 aticepted:September 15, 1981 Summary In Experiment I, 24 suckled beef cows were assigned to 4 treatment groups (6 cows/group). Group I cows calved spontaneously. Parturition was induced in Groups 2, 3 and 4 with 20 mg dexamethasone (DEX) 8 to 12 _. days prior to expected calving date. Additionally, cows in Groups 3 and 4 received 8 mg triamcinalone acetonide (TA) 6 days prior to DEX treatment. Animals in Group 4 also received 10 mg estradiol benzoate (EB) with IA, and on alternate days ‘until DEX, when 20 mg EB was given. Gonadotropin releasing hormone (.GnRH, 100 pg) was given intramuscular (IM) Plasma LH increased (I’< .05) folto all cows on days 2 or 3 postpartum. lowing GnRH treatment in Groups 2, 3 and 4, but not in Group 1. LH release (area under the curve) following GnRH was greater (P< .05) for 4 compared to cows in Groups 1, 2 or 3, and differences in cows in Group LH release between Groups 1 , 2 or 3 were not significant. In Experiment II, 36 mature Hereford cows were assigned to a 2 x 3 factorial experiment (6 cows/group). Groups 1 and 2, 3 and 5, and 4.and 6 received 0, 100, or 200 pg GnRH (IM) at 78 hr postpartum, respectively. In addition, cows in Groups 2, 5 and 6 received 5 mg EB at 36 hr postpartum. Plasma LH concentrations were not different (.P c.05) among groups from 36 to 78 hr postpartum. A surge of LH in response to EB treatment was not detected at 54 to 62 hr (18 to 26 hr post EB), indicating a lack of rejponse by the positive feedback mechanism at this early time postpartum. Mean plasma LH concentrations were elevated 78 to 82 hr postpartum for Groups 3 through 6. Treatment with ED at 36 hr caused a significantly greater (PC .05) response to GnRH with 200 ug of GnRH releasing more LH than 100 ug of GnRH. ntroduction Postpartum pituitary responsiveness to GnRH is affected by highcirculating concentrations of estrogens and progesterone prepartum (1, 5, 22) and by suckling postpartum (4, 5, 19). Progressively increasing pulsatile releases of luteinizing hormone (LH) precede the return of cyclic ovarian activity in the postpartum period (ll), and’ these pulsatile relea,ses are decreased by suckling (3). LH release, in response to exogenous GnRH, increases with the time of treatment postaContribution Series Paper

from the Missourj Agriculture Experiment No. 8851. Approved by the Director.

bThis reserach was conducted Methods for Improvement of

NOVEMBER

as part Fertility

1981 VOL; 16 NO. 5,

Station.

of regional research in Cows Postpartum.

project

Journal NC-113,

513

THERIOGENOLOGY

partum

(8, 11, 12, 13). Pituitary LH content is low at calving and increases from I to 30 days postpartum (21). Estradiol-17B increases pituitary LH synthesis and pituitary GnRH responsiveness (2, 10, 16, 18). Two animals from a previous experiment (12), which were treated prepartum with estradiol benzoate (EB), triamcinalone acetonide (TA) and dexamethasone (DEX) responded to GnRH by releasing LH. Experiment I was conducted to determine which agents (DEX, TA, or EB) used to induce parturition were responsible for the increase in pituitary responsiveness to GnRH in early postpartum, suckled beef cows. Experiment II was conducted to determine the effects of EB upon the pituitary LH response to GnRH, and to define the GnRH dose dependent nature of this response. Experimental Experiment

Procedure

I

Twenty-four multiparous, Charolais x Hereford and Hereford cows were randomly assigned to four treatment groups (6 cows/group). Cows in Cows in Groups 2, 3 and 4 received an Group I calved spontaneously. intramuscular injection (IM) of 20 mg DEX (2 mg/ml Schering Corp., Kenilworth, NJ) 8 to 12 days prior to the expected calving date. Additionally, cows in Groups 3 and 4 received 8 mg TA (6 mg/ml, E. R. Squibb and Sons, Inc., Princeton, NJ) 6 days prior to DEX treatment. Animals in Group 4 also received 10 mg EB (2 mg/ml, Sigma Chemical Co., St. Louis, MO) diluted in corn oil, with TA and on alternate days until DEX when 20 mg EB was given (Figure 1). A 100 pg injection (IM) of GnRH (Abbott Labs., Chicago, IL) was administered 2 to 3 days postpartum and blood samples were collected prior to GdRH (0.0 hr) and at half hour intervals thereafter for 4.0 hr. Plasma samples prior to GnRH were assayed for estradiol-178, and LH was measured in all samples. Experiment

II

Thirty-six mature, Hereford cows were assigned by order of calving Groups 1 and to a two by three factorial experiment (.6 cows/group). 2, 3 and 5, and 4 and 6 received 0, 100 or 200 pg GnRH (IM) at 78 hr In addition, cows in Groups 2, 5 and 6 postpartum, respectively. Blood samples were collected prior received 5 mg EB at 36 hr postpartum. to EB treatment at 2 hr intervals from 54 to 62 hr postpartum (18-24 hr after EB). Plasma estradiol-17B was measured at 36, 54, 56, 58, 60, 62 and 72 hr postpartum. On sample collection days, cows and calves were moved to handling facilities, penned together, and the cows were removed for sampling. Cows were returned to calves between sampling periods (except for half hourly sampling) to avoid any limitation of suckling. ,Sampl ing Blood samples for vacutainers via a tail at 3000 x g for 15 min at -2OOC until assayed.

514

the 2 experiments were collected in heparinized and cooled in ice water until centrifuged vessel, at 4°C. The supernatant was decanted and stored

NOVEMBER 1981 VOL. 16 NO. 5

THERIOGENOLOGY Hormone

Assay

Plasma-estradiol-17$ described

by

recovery assay

of

Erb

et

al

(7)

estradioFl7B

variation,

was

assayed

and

validated

was

90.4%

respectively.

by

radioimmunoassay for

with

Plasma

this 6 and

as

laboratory 13.8%

(13).

intra

LH %* concentrations

and

were

The inter-

determined

by double-antibody-radioimmunoassay as described by Niswender et al (15) -and validated for this laboratory by Zaied et al (28). The intraand inter-assay coefficients of variation were m%-and 12.4%, respectively. Statistical

Analysis

Data

from

the

of variance (9) nificant-difference tions (24) were

two

experiments

were

analyzed

or

analysis of variance where (24) was used to compare used to analyze interrelationships

by split-plot-analysis Least sigappropriate. means, and simple correlabetween variables.

Results Experiment

I

All animals in this experiment calved according to treatment and without difficulty nor retained fetal membranes. Plasma estradiol-17B concentrations prior to GnRH treatment were greater (PC .05) in Group 4, than in Groups 1, 2 or 3 and averaged 92.7 + 2.0, 4.6 + 0.4, 5.6 + D.7 and 6.6 + 1.6 pg/ml, respectively (Table 1). This was-expected, since Group 4 was treated with E,B, and there were no differences among groups which were ml for nificant

did not receive EB. not different (PC .05) all

groups increase

(Table during

mean LH concentrations and 4 (Figure 2).

The

Mean plasma LH concentrations, among treatments and averaged

1). the

Concentrations sampling period

increased significantly peak LH release was

of (0

prior 1 .O 2

to GnRH, 0.1 ng/

plasma LH showed no sigto 4 hr) in Group 1, but

(P< greater

.05) (PC

in .05)

Groups 2, in Group

3 4

in comparison to Groups 1, 2 or 3 (11.6 + 2.1, 1.9 + 0.8, 5.8 + 1.7 and 5.5 2 2.0 ng/ml, respectively). Furthermore, the peak LH releases in Groups 2 and 3 were not different (.P< .05) from each other, but were higher than Group 1. Peak release of LH was highly correlated (r = .97; Pi .05) to area under the LH response curve (0 to 4 hr). Whenthis parameter was used to evaluate the release in LH response to GnRH, there were no differences between Groups 1, 2 and than Groups 1, 2 or 3 (6.4 + 1.5, 14.9 arbitrary units2). This suggests that between Groups 1, 2 and 3 with respect Experiment

at

36

(b)

Antisera Purified

but

Group

4 was

greater

(.P<

.05)

5.0

II

Plasma estradiol-17B hr postpartum (prior

7a)Antisera Eli Lilly

3,

+ 3.1, 14.9 + 6.0 and 29.8 + only minor dTfferences exist to LH response to GnRH.

for Co.

estradiol-17B

LH5-255 bovine

NOVEMBER

concentrations to EB treatment) generously

was generously LH (LER-1072-2)

1981 VOL. 16 NO. 5

were

not different and averaged

provided

provided by was supplied

by

Dr. by

Or.

5.4

among groups 2 1.1 pg/ml

Norman

Mason,

G. D. Niswender. Dr. L. E. Reichert.

515

THERIOGENOLOGY

Experimental

I

Groups

-a -7

-6

Treatment

III

II

r-

8 mgTA

I

-5 I -4 I E

-3

p

-2

1

-1

I

20 mg DEX

Cl 1 Parturition

+I

+3

Figure

1.

H

t

1 Parturition

Parturition

Parturition

I 100 rg GnRH

Preparatum Calving

100 rg GnRH

and

Postpartum

Spontaneously

100 rg

Treatment (Group

I)

/ 100ug~~~H

GnRH

Regimen or

Induced

for to

Beef Calve

(

Cows (Groups

II-IV).

Table

1.

Mean Prior

Plasma Concentrations of Estradiol-17B and LH to GnRH and LH Release Following GnRH in Suckled

Beef

Cows

Following

Induced

Parturition

pre-GnRH

(pg/ml)

LH

(ng/ml)

under

3

(units

LH curve

)

4.6

-+ o.4a

1.0

-+

.2a

6.41

2

5.6

+

o.7a

1.2

-+

.la

14.94

--F 3.06a

3

6.6

+

1.6a

1.1

-+

.2a

14.10

-+

5.Y8a

4

93.7

+

2.0b

0,.7

-+

.la

29.84

2

4.9gb

differ

within (PC

columns .05).

with

different

-+

he

1

a,b Means

516

&2B

I).

post-GnRH Area

Group

(Experiment

superscripts

NOVEMBER

1.47a

significantly

1981 VOL. 16 NO. 5

mg

Sample

Mean

b7CMean

aMeans

3

2

Sample

1

5 mg

o

EB

TabdIe

+

(ng/ml)

prior

SE

prior

within

GnRH

EB

columns

columns

to

to

E2B h/ml)

LH

(pg/ml)

(ng/mi)

Either

Given

E28

LH

Plasma

Mean'

within

2.

LH

+

+

.la

.gb

.la

with

are

+

2

1.7 +

3.9

1.6

54

(E2B)

.la

.sb

.la

.05)

superscripts

(P>

at

.ia

differ

(P<

various

+

.la

.gb

.la

(Hr)

-+

.2a

+

.4b

.la

Cows

3.3 + l.la

3.5 +

1.7

62

Beef

101.8 + 7.8'

Suckled

.7b

*Ia

94.5 + 9.3c

1.9 +

4.4 +

1.5

in II).

60

Postpartum

(Experiment

Times

88.1 + 6.5'

1.7 +

3.5 +

1.5

58

Postpartum

Postpartum

.05).

100.5 + 9.5c

1.7 L

.3b

4.Q +

Time

Hr

.la

36

+

56

at

1.6

(EB)

Concentrati,ons

Benzoate

105.6 2 lQ.3'

different

different

not

4.8 + .5'

I.7 +

5.9

1.7

36'

Estradiol

Estradiol-IB

5 mg

and

0 or

+

.la

.ab

.la

52.9 + 4.9'

1.8 +

2.7 -+

1.7

7R3

THERIOGENOLOGY

Figure

2.

Mean

Plasma

LH Following

GnRH Administration

parturition in Suckled Beef Cows induced 4) or Calving Spontaneously (Group 1). DEX = dexamethasone, TA = triamcinalone estradiol for all plasma

groups. estradiol-17B

(EB treated) estradiol-I78 postpartum

2-3 to Calve (Experiment acetonide,

Days

Post-

(Groups I). EB =

2-

benzoate. Thereafter levels

and continuing were elevated

as compared to Groups 1, concentrations were 2.7 for animals receiving either

through (PC .05)

78 in

3 and 4 (Table + 0.2 and 52.3 0 or 5 mg of

hr postpartum, Groups 2, 5 and

6

2). Mean plasma + 4.9 pg/ml at 78 hr FB 36 hr postpartum,

respectively. The three-way interaction of EB treatment, GnRH treatment and time of blood sampling was significant (PC .OI) in the split-plot analysis of variance of plasma LH concentrations. This interaction indicates that differences in plasma LH across time (36 to 82 hr postpartum) exist within and among treatment groups. Mean concentrations of plasma LH were not different among Groups from 36 through 78 hr postpartum (Table 2). No release of LH in response to EB treatment was observed 18 to 26 hr (34 to 62 hr postpartum) after EB administration in any of the treated animals. Following GnRH, plasma LH increased in Groups 3 through 6, but not in Groups I and 2 (Figure 3). Mean peak plasma LH concentrations were 1.6 2 0.3, 2.0 2 0.3, 8.7 + 1.3, 11.5 + 1.3, 15.0 + 7.5 and 29.3 + 5.7 ng/ml for Groups I through 6, respectively. O%hogonal comparisons

518

NOVEMBER

1981 VOL. 16 NO. 5

THERIOGENOLOGY

f

\

I

Group

0

s5

56

GnRH

gn9

: 2

54

EB

56

Time Postpartum,

Figure

3.

I

62 hrs

\ \

I

\ \

I I

200 100

60

\

I

;u9 200 100

:5

\

I

\.

I

7% 0

79 .s

1

2.

\

\

81

80 1.5

\

2.5

3

82 3.5

4

A GnRH

Mean Plasma LH Concentrations 36 hr Postpartum and GnRH at Cows (Experiment I I).

Following Estradiol 78 hr Postpartum in

Benzoate at Suckled Beef

showed that peak LH response to GnRH was greater (PC .05) in animals which received 5 mg EB and GnRH as compared to animals which received GnRH alone (Groups 5 and 6 versus 3 and 4, respectively). Furthermore, Groups 4 and 6 (200 pg, GnRH) had a significantly greater (PC .05) peak LH release than did Groups 3 and 5 (100 pg GnRH). When the area under the LH response curve (78 to 82 hr postpartum) was used to evaluate treatment response, slightly different results (with respect to responsiveness The areas for Groups 1 through 6 were 6.8 + 0.6, to GnRH) were evident. 8.8 + 1.0, 21.7 + 2.6, 30.2 + 4.0, 39.6 2 20.6 and 71.6 + 13.9 units2, respectively. Groups 5 or 6 were greater (PC .05) than croups 1 or 2, but there was no difference (PC .05) between Group 1, 2, 3 or 4, nor between Groups 3, 4 or 5. Discussion The major finding of Experiment I lies in the reduction of time which is required postpartum for the pituitary to regain responsiveness to GnRH and release LH. Differences in the areas under the LH response curve from Experiment I suggest that EB is chiefly responsible for increasing the GnRH induced LH release in suckled beef cows, 2 to 3 days postpartum. DEX may also have an effect on increasing the GnRH induced LH the mechanism for this effect is not release (Experiment I). However, known. Chantaraprateep and Thibier(6) demonstrated the negative action of DEX on basal and GnRH induced LH release in bulls. Dexamethasone

NOVEMBER

1981 VOL. 16 NO. 5

519

THERIOGENOLOGY

exerts its mg dose of

inhibitory DEX (6).

effect

at

the

pituitary

Suppression of tonic in increased stores

for

up to

LH release by of releasable

30 h.r after a 20 DEX (Experiment

may have resulted LH at the time ‘!? _ or GnRH administration after the effects of DEX had subsided. Estradiol benzoate given shortly after parturition (Experiment I I) had a significant positive effect upon pituitary responsiveness to GnRH at a time postpartum when the pituitary is marginally or not responsive to GnRH. Animals which received 5 mg EB prior to GnRH released signifiAddicanlty more LH in response to GnRH than animals not receiving EB. tionally, 200 ug of GnRH released more LH than 100 ug of GnRH, which is in agreement with others (1,10,16,17) for both estrogen enhanced LH release and GnRH dose reponse. Absence of an estrogen induced LH release 18 to 24 hr after EB administration (Experiment II) indicates that the hypothalamus is not This is in keeping with responding to estrogen at this time postpartum. other reports (18,23,27) and has been proposed to partially explain postpartum anestrus (23) with pituitary responsiveness to GnRH returning prior to hypothalamic responsiveness to estrogen (14,26,27). Recently, return of pituitary responsiveness to GnRH was delayed by treatment with high doses of estrogen alone, or in combination with progesterone (1). However, the effect of high concentrations of plasma estrogens and progesterone present in late gestation on hypothalamic sensitivity to estradiol-176 has not been established. Tang and Adams (25) investigated the alterations of estrogen binding sites in the hypothalamus and pituitaries of sheep which were grazed on Their findings demonstrated that the high concensubterranean clover. tration of phytoestrogens which were ingested while grazing the clover permanently altered estrogen binding in these tissues, which were relatively insensitive to estradiol-176 compared to normal ewes. Al though hypothalamic responsiveness to estrogen is not permanently affected in the postpartum cow, parallel changes in hypothalamic receptors might have occurred. References

(1)

G. F. and Garverick, Azzazi, F., Krause, GnRH induced LH Release by Steroids in Sci. (Submitted) . (1981).

(2)

Estradiol Beck, T. W. and Convey, E. M. J. Hormone Concentration in the Bovine.

H. A. Alteration Postpartum Cattle.

Control of Anim. Sci.

of J.

the Anim.

Serum Luteinizing -45: 1096-l 101.

(1977). (3)

Sucking and Four-Times Daily Carruthers, T. D. and Hafs, H. D. Influence on Ovulation, Estrus and Serum Luteinizing-Hormone, Milking; Glucocorticoids and Prolactin in Postpartum Holsteins. J. Anim. Sci. (1980). -50:919-525.

(4)

Carruthers, T. D., Convey, E. M., Kesner, J. S., Hafs, Cheng, K. W. The Hypothalamo-Pituitary Gonadotrophic J. Anim. Sci. and Non-Suckled Dairy Cows Postpartum.

H. I?. and Axis of Suckled

-51:949-957.

(1980). (.5)

520

Charnley, W. A., Findlay, J. K., Cummings, Is A., Buckmaster, Effect of Pregnancy on the LH Response and Goding, J. R. Endocrinology. Gonadotropin Releasing Hormone in the Ewe. 291-293. (1974).

NOVEMBER

to

J. M. Synthetic -94:

1981 VOL. 16 NO. 5

THERIOGENOLOGY

(6)

Chantaraprateep, P. and Thibier, M. Effects of Dexamethasone on Responses of Luteinizing Hormone and Testosterone to 2 Injections of Luteinizing Hormone-Releasing Hormone in Young Post-Pubertal _. Bul Is. J. Endo. (1978). -77:389-395.

(7)

Erb, R. E., Monk, E. L., Mollett, T. A., Malven, P. V. and Callahan, C. J. Estrogen, Progesterone, Prolactin and Other Changes Associated with Bovine Lactation Induced with Estradiol-178 and Progesterone. J. Anim. Sci. (1976) . -42:644-654.

(8)

Fernandes, L. C., Thatcher, W. W., Release in Response to GnRH During cows. J. Anim. Sci. -46:443-448.

19) (10)

Gill, J. Animals.

L. and Hafs, H. J. Anim. Sci.

Hausler, C. and Estradiol Anim. Sci.

D.

Wilcox, C. J. the Postpartum

and Call, E. P. Period of Dairy

of

Repeated

Measurements

of

(1971).

Interaction and Malven, P. V. in the Control of LH Release (1976). -42:1239-1243. L.

in

of Progesterone, Castrate Heifers.

(11)

Humphrey, Niswender, cows. J.

(12)

Irvin, H. J., Zaied, A. A., Day, B. N. and Garverick, H. A. Induced LH Release in Suckled Beef Cows. I. The Effects of Postpartum and Endogenous Reproductive Hormones on LH Release GnRH. Theriogenology (1981). -15:443-448.

(-13)

Kesler, D. J., Garverick, Bierschwal, C. J. Effect Hormones on GnRH-Induced

-45:797-803.

LH

(1978).

Analysis

-33:331-336.

W. D., Koritnik, D. G. D. Progesterone Anim. Sci. -43:290.

the

R., Kaltenbach, C. C., Dunn, and LH in Postpartum Suckled (1976) abstract.

H. A., Youngquist, of Days Postpartum LH Release in Dairy

GnRH J.

T. G. Beef

and

GnRH Days by

R. S., Elmore, R. G. and and Endogenous Reproductive Cows. J. Anim. Sci.

(1977).

04)

Nancarrow, C. D., Radford, H. M., Scaramuzzi, R. J. and Post, T. Responses to Injected Oestrogen in Suckled Cows. Theriogenology 8~192. (1977). Proc. Ninth Annual Meeting of Australian Society For Reproductive Biology, Melbourne, August 17-19, 1977.

(15)

Niswender, Nalbandov, Hormone.

(16)

Padmanabhan, V.) Kesner, J. S. and Convey, E. M. Effects of Estradiol on Basal Luteinizing Hormone Releasing Hormone (LH-RH) Induced Release of Luteinizing-Hormone (_LH) from Bovine Pituitary Cells in Culture. Biol. 18:608-619. Reprod. 0978).

G. D., Reichert, L. E. Jr., A. V. Radioimmunoassay for Endocrinology -84:1166-1179.

Midgley, Bovine (1969).

B,

A. R. Jr. and and Ovine Luteinizing

-

(17)

Padmanabhan, V. and Convey, E. M. Does and LH-RH Effect LH Release from Bovine Biol. Med. Cl 978‘1. -159:157-160.

NOVEMBER 1981VOL. 16 NO. 5

Time of Pituitary

Exposure Cells?

to Estradiol Proc. Sot.

521

THERIOGENOLOGY

(.I8)

Radford,

H.

in Suckling -54:4g-56.

M.,

Nancarrow,

(19)

Randel, R. D. and Calf Performances.

(20)

Reeves, J. J., Arimura, A. Responsiveness to Luteinizing in Anestrus Ewes Pretreated 4:88-92. (1971).

(21) Saiduddin, Relation Follicular -270:15-22.

C.

and Non-Suckling (1978).

of

D.

and

Beef

Welker, G. J. Anim.

W. Sci.

Mathner,

Cows

P.

E.

Postpartum.

Once Daily -42:301.

Fert.

on

Cow

and

Schal ly, A. V. Changes in Pituitary Hormone-Releasing Hormone (LH-RH) Biol. Repro. with Estradiol Benzoate.

S. A., Riesen, J. W., Tyler, W. J. and Casida, Postpartum Interval to Pituitary Gonadotropins, Development and Fertility of Dairy Cows. WI.

L.

E. Ovarian Res. Bull.

(1968).

D. and Zoltmier, K. E. Shams, (LH) and Follitrop;n (FSH) to the Administration (GnRH) in Pregnant and Postpartum Cattle Including Prolactin Suppression. Theriogenology -10:35-53.

Factors Reprod.

Function

Reprod.

Suck1 ing Effects (1976). abstract.

(22) Schallenberger,

(23) Short,

Ovarian J.

R. E., Randel, R. D., Staigmiller, R. Affecting Estrogen Induced LH Release (1979). -21 :683-689.

(24) Snedecor,

G. W. and Iowa State

Edition.

Cochran, W. University

E. Statistical Press. (1973).

Response of Lutropin of Gonadoliberin Experiments with

(1978).

B. and Bellows, in the Cow.

Methods.

R. Biol.

A.

Sixth

(25) Tang,

Changes in Oestradiol-176 Binding in B. Y. and Adams, N. R. the Hypothalami and Pituitary Glands of Persistently Infertile Ewes Previously Exposed to Oestrogenic Subterranean Clover--Evidence of (1978). Alteration of Oestradiol Receptors. J. Endo. -78:171-178.

P. J. and Findlay, J. (26) Wright, 178 (Ea! in Postpartum Ewes. Proc. Biology,

rnth Annual Melbourne,

Meetrng August

L. LH Release Due to LH-RH or Estradio Theriogenology. 8:191. (1977). of Australran Society for Reproductive 17-19, 1977.

Garverick, H. A., Bierschwal, C. J., Elmore, R. G., A. A., Youngquist, R. S. and Sharp, A. J. Effects of Ovarian Activity and Endogenous Reproductive Hormones on GnRH-Induced Ovarian Cycles J. Anim. Sci. ~:508-513. in Postpartum Dairy Cows. (1980).

(27) Zaied,

(28) Zolman

J., Luteinizing Endogenous

522

Convey, Hormone Steroids.

Relationships Between the E. M. and Britt, J. H. Response to Gonadotropin Releasing Hormone and J. Anim. Sci. =:355-359. (1974).

NOVEMBER

1981 VOL. 16 NO. 5