endocrine control of estrous cycle in mithun (bos frontalis)

Endocrine control of estrous cycle in mithun

(Bos frontalis)

A. Dhali a,*, D.P. Mishra b, A. Mech a,M. Karunakaran c, C. Rajkhowa a

a National Research Centre on Mithun, Jharnapani, Medziphema 797106, Nagaland, Indiab Sperm Biotechnology Laboratory, N.I.I., New Delhi 110067, India

c ICAR-RC-NEH Region, Medziphema 797106, Nagaland, India

Received 14 January 2005; received in revised form 4 April 2005; accepted 1 May 2005

Abstract

The objective of the present study was to establish the profiles of luteinising hormone (LH),

follicle stimulating hormone (FSH), estradiol 17b (E2) and progesterone (P4) secretion and their

interrelationships during the natural estrous cycle of mithun (Bos frontalis). Daily blood samples

were collected from second or third postpartum estrous cycles for determination of plasma

concentrations of LH, FSH, E2 and P4. Concentration of P4 was found to be lowest on the day

of estrus. It increased following estrus, attained the highest concentration on day 11 and decreased

thereafter. Concentrations of LH and FSH varied significantly ( p < 0.01) during the first and last 6

days of the cycle and their variations were found to be synchronised. Both LH and FSH attained a

biphasic peak during the estrous cycle. This biphasic peak lasted on from day �5 to day 3 of the

cycle. The variations in maximum LH and FSH concentrations of both the phases did not differ

significantly. During the entire estrous cycle, the E2 concentrations attained either one peak or two

peaks. The first peak, approximately on day 4 before estrus was common in all animals. One

additional peak was found on the day of estrus in 45% animals. A significant ( p < 0.01) negative

relationship was found between P4 and, LH and FSH during the first and last 6 days of cycle. But a

significant ( p � 0.01) negative relationship between E2 and, LH and FSHwas found only during the

last 6 days of cycle. The results suggest a negative feedback mechanism on LH and FSH release by

E2 and P4 during the respective phases of cycle. A sustained increase in LH and FSH levels during

www.journals.elsevierhealth.com/periodicals/the

Theriogenology 64 (2005) 2010–2021

* Corresponding author. Tel.: +91 3862247341; fax: +91 3862247341.

E-mail address: [email protected] (A. Dhali).

0093-691X/$ – see front matter # 2005 Elsevier Inc. All rights reserved.

doi:10.1016/j.theriogenology.2005.05.002

the period before estrus was probably necessary for the final maturation of ovulatory follicle and

subsequent ovulation.

# 2005 Elsevier Inc. All rights reserved.

Keywords: Estrous cycle; Luteinising hormone; Follicle stimulating hormone; Mithun

1. Introduction

Scientific mithun (Bos frontalis) rearing is a new introduction into animal production

science.Mithun, the ‘‘cattle ofmountain’’, probably originatedmore than 8000 years ago and

is presumed to be the domesticated form of wild gaur [1]. This rare bovine species is mainly

found in the North-East hill region of India and in parts of Bhutan, Myanmar, China and

Bangladesh. Mithun rearing has become an important occupation at an altitude of 1000–

3000 mMSL because of limited arable agriculture [2]. The multifarious utility of mithun is

well recognized. This species is primarily used as a beef animal. Consumers considerMithun

meat as more tender and superior over the meat of any other species except pork. Besides,

mithun is also reared as a pack animal and its utility as milch animal has also been explored

recently. At present, farmers rear this animal under free grazing condition in its natural

habitat. In recent days, initiatives have been taken to popularise the economicmithun farming

under semi intensive condition with control breeding programme.

In any animal production system, the target of one calf per year per female could only be

achieved with proper detection of estrus and subsequent breeding. A good understanding of

the endocrine mechanisms, which control estrous cycle, is necessary to implement

effective reproductive management in this species. However, information pertaining to the

endocrinology of mithun estrous cycle is obscure at present. Therefore, the present

investigation was planned with the objectives of establishing the profiles of luteinising

hormone (LH), follicle stimulating hormone (FSH), estradiol 17b (E2) and progesterone

(P4) during the natural estrous cycle of mithun and understanding the temporal

relationships among these hormones during different phases of estrous cycle.

2. Materials and methods

2.1. Animals and management

The experimentwas conducted on 11 adult postpartummithun cows of age 4–7 years (1–3

lactations), maintained at the Institute Mithun Farm, N. R. C. on Mithun, Medziphema,

Nagaland, India. All experimental protocols and animal care met IACUC regulations.

Animalswere allowed for free grazing from0600 to 1600 h daily onmixed pasture (23%DM

and 12.2% CP). Two kilogram concentrate (88% DM and 16.5% CP) fortified with mineral

mixture and salt was offered to all the animals daily during eveningwhen theywere tied in the

shed. Animals had free access to water throughout the day. Animals were detected for estrus

since day 30 postpartum onwards. Estrous detection was done twice daily with healthy and

fertile mithun bulls during early morning and late evening.

A. Dhali et al. / Theriogenology 64 (2005) 2010–2021 2011

2.2. Blood collection

Daily blood samples (5 ml) from animals in their second or third postpartum estrous

cycle were collected to estimate and monitor the plasma LH, FSH, E2 and P4 changes.

Samples were collected by means of jugular vein puncture in heparinised (20 IU/ml blood)

polystyrene tubes and kept on ice immediately after collection. Plasma was separated

within 1 h of collection by centrifugation (1200 � g) for 20 min at 4 8C and stored at

�20 8C until assayed for hormones.

2.3. Hormone assay

Plasma FSH concentration was measured by using a validated radioimmunoassay

procedure [3]. USDA-bFSH-1-2 (bFSH) was served as both reference standard and tracer.

bFSH was radioiodinated by a modified method [4]. Briefly, 5 mg bFSH were dissolved in

20 ml 0.3 M phosphate buffer (pH 7.5) and was reacted with 800 mCi 125I (Amersham

Corp., Arlington Heights, IL) and 0.8 mg chloramines T for 5 min. This reaction was

stopped by addition of 1 mg sodium metabisulfite, chromatographed on Sephadex G100

and eluted with 0.05 M PBS. The peak fraction of radioiodinated bFSH was diluted to

20,000 cpm/100 ml in assay buffer (PBS containing 0.1% BSA and 0.1% sodium azide, pH

7.4). Unknowns and standards (100 ml) were pipetted into borosilicate glass tubes and

incubated with 100 ml of rabbit anti-oFSH, NIDDK-oFSH-I, dilution 1:80,000 (AFP-

C5288113) and 100 ml (20,000 cpm) bFSH tracer for 24 h at 4 8C. After this incubation,100 ml rabbit anti-gamma globulin sheep serum (dilution 1:10) was added and incubation

continued for 2 h at room temperature. Tubes were washed with distilled water and

centrifuged (3200 � g) at 4 8C for 10 min. Supernatants were decanted and radioactivity in

each pellet was determined using a gamma counter. The sensitivity of the assay was

0.25 ng/ml. The intra- and inter-assays CVs were 4.9 and 12.3%, respectively.

Plasma LH was measured by using a sensitive enzymeimmunoassay procedure

described previously [5]. Briefly, duplicate of 20 ml of unknown plasma samples or bovine

LH standards (USDA-Blh-B6, prepared in hormone free plasma collected on day 4

postpartum) ranging from 6.25 to 400 pg/20 ml/well were simultaneously pipetted into the

respective wells along with 100 ml of LH antibody (USDA-309-684P-Beltsville, USA)

diluted to 1:160,000 in assay buffer (pH 7.4). The plates were incubated overnight at room

temperature after initial 30 min of constant agitation. On the next day, the plates were

decanted and washed twice with washing solution before addition of 100 ml of biotinyl LH

conjugate diluted to 1:400 in assay buffer. The plates were further incubated for 30 min

under constant agitation followed by decantation and four time washing with washing

solution. Then, 20 ng of Streptavidin Peroxidase (Sigma, Germany) in 100 ml assay buffer

was added to all the wells and the plates were wrapped in aluminium foil and incubated

further for 30 min under constant agitation. All the steps were performed at room

temperature. The plates were then washed five times with the washing solution and

incubated further in dark for 40 min after addition of 150 ml of substrate solution per well.

Substrate solution was prepared by adding 17 ml substrate buffer (0.05 M citric acid,

0.11 MNa2HPO4, 0.05% ureum peroxide; pH adjusted to 4.0), 340 ml 3,30,5,50-tetramethyl

benzidine (Sigma, Germany) and 12.5 mg/ml dimethyl sulfoxide (Sigma, Germany). The

A. Dhali et al. / Theriogenology 64 (2005) 2010–20212012

reaction was stopped by the addition of 50 ml 4N H2SO4 and the colour developed was

measured at 450 nm. The sensitivity of the assay was 0.31 ng/ml. The intra- and inter-

assays CVs were 4.9 and 12.3%, respectively.

Plasma P4 concentration was measured by using a commercially available Progesterone

Enzyme Immunoassay Kit (EIA kit, Assay Design Inc., USA). The kit was validated for

estimating P4 in mithun plasma according to the method described earlier [6]. Sensitivity

of the assay was 0.25 ng/ml. The intra- and inter-assays CVs were 5.5 and 9.2%,

respectively.

Plasma E2 concentration was measured by using a commercially available EIA kit

(TKE2, Diagnostic Products Crop, Los Angeles, CA). The kit was validated for estimating

E2 in mithun plasma according to the method described earlier [7]. Sensitivity of the assay

was 2.5 pg/ml. The intra- and inter-assays CVs were 4.6 and 11.5%, respectively.

2.4. Statistical analysis

P4 concentrations on different days of estrous cycle were plotted for individual animal

to confirm the cyclicity and estrous cycle length. Estrous cycle length was defined as the

interval between two subsequent estruses. Animals were considered to be in estrus when

plasma P4 concentrations were below 0.5 ng/ml and bull exhibited mounting behaviour.

Further individual patterns of LH, FSH and E2 concentrations on different days of estrous

cycle were plotted to identify the occurrence of peak concentrations and the

interrelationships among these hormones. The basal concentration of each hormone for

individual animals was defined as the mean of all concentrations excluding the

concentrations, which were �overall mean + 1S.D. Changes in plasma LH, FSH, E2 and

P4 concentrations during estrous cycle were analysed by repeated measure ANOVA using

SPSS software package [8]. The model included the main effects of day and animal. The

Duncan multiple-range test was used for detection of significant differences among means

[8]. To study the relationship of E2 and P4 with LH and FSH, the Pearson correlation

analysis was performed [8]. The variations in peak concentrations of LH, FSH and E2 were

analysed using GLM procedure [8].

3. Results

3.1. Profile of P4 and estrous cycle length

Average P4 concentrations (n = 11) on different days of estrous cycle are depicted in

Fig. 1. P4 concentrations on different days in two subsequent estrous cycles are presented

in Fig. 2. As expected, P4 concentration was found to be lowest ( p < 0.01) on the day of

estrus followed by a gradual increase till day 6. From day 7, P4 concentration was started

increasing sharply and attained peak on day 11. The concentrations decreased thereafter

and reached to the lowest level again on next estrous day. P4 profiles indicated that all the

experimental animals were normally cyclic. A significant ( p < 0.01) negative relationship

was found between P4 and, LH and FSH during first and last 6 days of cycle (Table 1). The

average estrous cycle length in mithun was observed 21.2 � 0.3 days (range 19–23 days).


3.2. Profile of LH

Average LH concentrations (n = 11) on different days of estrous cycle (Fig. 1) varied

significantly ( p < 0.01). LH concentrations on different days in two subsequent estrous

cycles are presented in Fig. 2. In all animals, a biphasic LH peak was found during the cycle.

This biphasic LH peak lasted on from day �5 to day 3. The variation in maximum LH


Fig. 1. Plasma concentrations of progesterone (P4), LH and FSH on different days of estrous cycle in mithun

(n = 11).

Table 1

Correlation of estradiol 17b (E2) and progesterone (P4) with LH and FSH during different phases of estrous cycle

in mithun

Particulars 0 to 5 Days 6 to 14 Days �6 to �1 days

r p r p r p

Correlation of E2 with

LH 0.40 0.00 0.08 0.46 �0.30 0.01

FSH 0.28 0.06 �0.04 0.68 �0.36 0.00

Correlation of P4 with

LH �0.57 0.00 0.01 0.91 �0.64 0.00

FSH �0.67 0.00 0.19 0.28 �0.53 0.00

Day 0 indicates the day of estrus; negative sign indicates the day before onset of estrus; r indicates the Pearson

correlation coefficient; P indicates the probability value.

concentrations of both the phases (14.66 � 0.52 and 14.94 � 0.84 ng/ml) did not differ

significantly (Table 2). Representative individual variation in LH concentrations on different

days of cycle is presented in Fig. 3. LH concentration was started increasing from day 6

before estrus and attained the first peak concentration on day 2 or 3 before estrus. The LH

level decreased thereafter and was followed by the second peak concentration on the day of

estrus. After estrus, LH concentration came down to basal levels on day 4 or 5 of the cycle.

3.3. Profile of FSH

Average FSH concentrations (n = 11) on different days of estrous cycle are presented in

Fig. 1. FSH concentrations on different days in two subsequent estrous cycles are presented


Fig. 2. Plasma concentrations of progesterone (P4), LH, FSH and estradiol 17b (E2) on different days in two

subsequent estrous cycles in mithun (n = 4); panel C1, cycle 1; panel C2, cycle 2.

Table 2

Occurrences and amplitudes of LH and FSH peak concentrations on different days of estrous cycle in mithun

Particulars First peak concentration Second peak concentration

Occurrence (day) Amplitude (ng/ml) Occurrence (day) Amplitude (ng/ml)

LH �2.5 � 0.15 14.66 � 0.52 0.0 � 0.0 14.94 � 0.84

FSH �2.3 � 0.24 5.91 � 0.17 0.0 � 0.0 6.52 � 0.22

Day 0 indicates the day of estrus; negative sign indicates the day before onset of estrus.

in Fig. 2. A significant ( p < 0.01) variation in FSH concentrations was found during the

first and last 6 days of the cycle. A synchronised variation in LH and FSH concentrations

was found during these phases (Fig. 1). In all animals, a biphasic FSH peak was observed

during the cycle. This biphasic FSH peak lasted on from day �5 to day 3 of the cycle. The

variation in maximum FSH concentrations of both the phases (5.91 � 0.17 and

6.52 � 0.22 ng/ml) did not differ significantly (Table 2). Representative individual

variation in FSH concentrations on different days of the cycle is presented in Fig. 4. It was

observed that the FSH level was started increasing from day 5 before estrus. The first peak

FSH concentration was found on day 2 or 3 before estrus followed by the second peak

concentration on estrous day. FSH came down to basal level on day 3 or 4 of the cycle.

3.4. Profile of E2

Mean concentrations of E2, FSH and LH on different days of the estrous cycle are

plotted separately for the animals showing one E2 peak and two E2 peaks (Fig. 5). In

animals (5 out of 11) with two E2 peaks, the first peak (20.32 � 3.61 pg/ml) was observed

approximately on day 4 before estrus followed by a second peak (19.23 � 2.14 pg/ml) on

the day of estrus. In animals (6 out of 11) with one E2 peak, the peak concentration


Fig. 3. Representative individual variation in plasma LH concentrations on different days of estrous cycle in

mithun (n = 4).

(18.92 � 3.76 pg/ml) was found approximately on day 4 before estrus. Amplitudes of these

E2 peaks did not differ significantly within or among animals. In all these animals, a

biphasic peak of FSH and LH was observed during the cycle. The first peak of LH and FSH

was observed on day 2 before estrus followed by the second peak on estrous day. The

variation in amplitude of the peak FSH concentration on estrous day and the variation in

amplitudes of both the LH peak concentrations did not differ significantly among animals.

But the peak FSH concentration, which observed before estrus, was found to be

significantly ( p < 0.05) higher in animals with two E2 peaks. A significant ( p � 0.01)

negative relationship was observed between E2 and, LH and FSH during last 6 days of the

cycle. While a significant ( p < 0.01) and nearly significant positive relationship was

observed between E2 and LH and, E2 and FSH, respectively, during first 6 days of cycle

(Table 1). E2 concentrations on different days in two subsequent estrous cycles are

presented in Fig. 2.

4. Discussion

In the bovine estrous cycle, FSH and LH promote follicular development, follicular

maturation and ovulation. Whereas the variation in E2 concentrations during different


Fig. 4. Representative individual variation in plasma FSH concentrations on different days of estrous cycle in

mithun (n = 4).

phases of estrous cycle indicates the state of follicular development [9]. The current study

was conducted to establish the changes of LH, FSH, E2 and P4 concentrations in peripheral

plasma during estrous cycle in mithun and to establish the temporal relationship among

these hormones. In all experimental animals P4 concentration was found lowest on the day

of estrus followed by a subsequent increase and occurrence of peak during mid cycle. After

peak, P4 concentration was decreased and reached to the lowest level again on the next

estrous day. This confirmed that ovulation took place in all the experimental animals after

estrus followed by a functional CL development. A significant negative relationship

between P4 and, LH and FSH during early and late phases of cycle was observed. The

results indicated that P4 influenced the LH and FSH release through a negative feedback

mechanism. Earlier studies also indicated that P4 inhibits LH release in cattle [10] and

sheep [11]. GnRH secretion was also found to be decreased after progesterone treatment in

sheep [12].

In all the experimental animals, a biphasic LH peak was observed during the entire

estrous cycle. LH concentration was started increasing from day 6 before estrus and

attained peak concentrations approximately on day 2 or 3 before estrus followed by the

next peak concentration on estrous day. In contrast to the present observation, in cattle the

plasma LH concentrations are reported to be low for most part of the estrous cycle with a

peak or surge at about the time of estrus [9,13]. The preovulatory LH surge has been found

closely associated with the process of ovulation in cattle [9,14]. Besides, preovulatory LH


Fig. 5. Plasma concentrations of FSH, LH and estradiol 17b (E2) on different days of estrous cycle in mithun;

panel A, animals showing two E2 peaks during entire cycle (n = 5); panel B, animals showing one E2 peak during

entire cycle (n = 6).

surge initiates the luteinisation of granulosa cells and theca cells of the ovulatory follicle

[9]. Increased LH pulse amplitude during end of the growth phase of the dominant follicle

is important for its development in later stages [15]. In mithun during the late phase of

estrous cycle, a sustained increase in LH level was probably crucial for the final maturation

of the ovulatory follicle. The peak LH concentration on the day of estrus is probably the

preovulatory LH surge.

A biphasic FSH peak was found during estrous cycle in mithun. FSH concentration

closely followed the LH pattern observed during the cycle. The occurrences of LH and FSH

peak concentrations were simultaneous. In cattle, FSH concentrations attain peak levels on

the day of estrus [16]. In the present study, FSH concentrations fluctuated over the days

after estrus, which was probably associated with the waves of follicular development that

occurred throughout the cycle. It is established that there are two or three waves of

follicular development in bovine estrous cycle [17]. Dominant follicle of each wave except

ovulatory follicle that develops in the presence of a CL eventually regresses in absence of

preovulatory LH surge. However, dominant follicle that develops in the environment of CL

regression completes the final stages of development and ovulates following an exposure to

LH surge [18]. Before each wave of follicular emergence, a rise in plasma FSH above basal

level occurs and as follicles develop, the FSH concentration declines [16,18,19]. A rise in

vena cava E2 level is found to be associated with the emergence of each follicular wave in

the bovine estrous cycle. E2 level rises further during follicular deviation of each wave.

After deviation, only the dominant follicle continues to grow. Simultaneously at the same

time, the subordinate follicles start regressing and vena cava E2 level decreases [15]. In

mithun following estrus, probably the first follicular wave emerged in the environment of

low P4 and high FSH concentrations. The dominant follicle of first wave probably did not

attain the final maturation. Possibly, this dominant follicle regressed on subsequent days

because of its continued and close temporal dependency on FSH. It eventually became

susceptible to low FSH concentrations during the luteal phase of the cycle [16]. The second

follicular wave probably emerged approximately on day 14 of the cycle and was associated

with the increasing E2 concentrations in the peripheral circulation. Dominant follicle of

this wave probably went through final maturation in the environment of sustained increase

in LH and FSH levels and ovulated following estrus. In cattle, it has been found that in a

three follicular wave cycle, the third wave emerges approximately on day 8 before estrus

[20].

In mithun estrous cycle, the E2 profile showed an unique trend. E2 concentration

attained either one peak or two peaks during the cycle. The first peak that observed on day 4

before estrus was common in all animals. In 45% animals, one additional peak was

observed on the day of estrous. In cattle, E2 concentrations remain low for most part of the

estrus cycle. It rises on the day 4 before estrus and attains peak a day before or on the day of

standing estrus [21]. Dominant follicle is responsible for fluctuation in circulatory estradiol

during follicular waves [22]. In cattle, the rise of E2 is correlated with increasing size of the

dominant follicle [9]. In the present study, in 55% experimental animals, the second E2

peak was absent. Probably in these animals, either size of the ovulatory follicle was smaller

and it produced comparatively less E2 or the first follicular wave following estrus was

absent and resulted in no secretion of E2 around estrus. On the contrary, in the other 45%

experimental animals, two follicular waves took place (around day 0 and day 14) giving


two E2 secretion peaks. In mithun during late phase of estrous cycle, E2 exhibited a

significant negative relationship with LH and FSH. This suggests that E2 exerts a negative

feedback on LH and FSH secretion during this phase. However, a positive relationship was

observed between E2 and, LH and FSH during day 0 to 5. In cattle, it is reported that high

the E2 concentrations during the follicular phase of the cycle has a positive feedback

mechanism on hypothalamic GnRH secretion that in turn increases the LH and FSH

release. While low concentrations of E2 during luteal phase by developing follicle acts

synergistically with P4 to exert a negative feedback on LH secretion [9,10,23].

On the basis of the results of the present study and the information available in literature,

the following hypothetical model for the endocrine events and associated follicular

development and ovulation in mithun estrous cycle is proposed. Following estrus in the

environment of low circulating P4 and high FSH levels, the first follicular wave emerges.

The follicles of the first wave regress during luteal phase without the exposure of sufficient

FSH required for its final maturation. From the hormonal profiles, it is evident that the

second follicular wave emerges around day 14 of the cycle and is probably associated with

the increasing E2 concentrations in peripheral circulation. As CL starts regressing from day

11 onwards, circulating P4 level starts coming down. Low P4 in turn allows FSH to

increase from day 16 of the cycle. The increase in FSH concentration from this time

onwards helps in selection of the ovulatory follicle of the second follicular wave. The

deviation of ovulatory follicle of second wave takes place most probably around day 18 of

the cycle. The circulatory E2 concentration decreases thereafter along with the regression

of subordinate follicles. During the late stage of cycle, low P4 concentration possibly

stimulates the LH rise at around the same time of FSH, which follows the same pattern of

FSH rise. High level of LH during the late phase of cycle helps in the final maturation of

ovulatory follicle along with FSH and makes the environment for functional CL

development following ovulation. On the day of estrus, low P4 concentrations along with

physiological levels of circulating E2 induce the preovulatory LH surge and ovulation

takes place. Basing on the data obtained from the present investigation, it can be

hypothesized that following ovulation, the functional CL develops and peripheral P4 starts

increasing gradually which exerts a negative feedback on LH and FSH levels subsequent to

estrus. However, further ultrasonographic studies are required to confirm and precisely

define the occurrence and timing of different follicular waves. This will be of importance to

explain the variations in LH, FSH and E2 concentrations more accurately throughout the

estrous cycle in mithun.

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