Introduction
Cows and buffaloes are mainly kept for milk production
In India there is 190 million cattle & 109 million buffalo population and more than 175 million tons of milk production
More than 75% of total dairy animal population are females but have huge number of dry animals
From this data we can estimate that per animal milk production is not beyond 2 lit per day
Which means dry animal population is very large
It is because dairy animal infertility is big issue in India, reproductively capable animals do not calved regularly
When dairy animal comes in milk?
Dairy animal gives milk after calving
There are many factors which influence pregnancy and calving
Understanding estrous cycle is basic thing which could help in reproductive management of dairy animals
A dairy farmer cannot leave this subject for experts because day to day dairy management need knowledge of biology of cows
Understanding milking physiology is key to success in dairy farming
Now we are going to understand estrous cycle of cows and buffaloes
Why knowledge of estrous cycle is important?
To know the reproductive patterns of dairy animals
To know proper timing of AI or mating for successful conception
To understand infertility issues in dairy animals
To understand the working and uses of commercial hormone preparations available in the market
To plan the calving according to the best season of sales and milk prices
In short regular and healthy estrous cycle is foundation pillar of profitable dairy farming
Estrous Cycle
Farm animals exhibit limited period of sexual receptivity after every fixed period of interval
Sexual receptivity means animal is ready for mating and get pregnant
This limited period of receptivity is called estrus and rhythmic cycle of its various phases is called estrous cycle. Its is usually 21day long.
According to the duration there are 4 main stages
Estrus
Metestrus
Diestrus
Proestrus
Estrous Cycle in Cows
Once the animal reaches puberty and starts reproductive cycling there is a repeated pattern of hormone production. Estrous cycle in cows and buffaloes are of 21 days.
There are 2 phases according to the production of hormones
Follicular phase: This phase is dominated by Estrogen
Luteal phase: This phase is dominated by Progesterone
Balance and timely production of these 2 hormones is the key to successful reproduction in dairy animals
Now we will learn about hormones and their functions in cows and later learn some minor differences in buffalo
Hormones of Estrous Cycle
Gonadotropin Releasing Hormone (GnRH): It is a hormone secreted in brain (hypothalamus) which govern overall reproduction pattern through production of other hormones
Luteinizing Hormone (LH): It is secreted from brain (pituitary) helps in ovulation in large doses and required for maintenance of progesterone production from CL
Follicle Stimulating Hormone (FSH): It is secreted from brain (pituitary) helps in development of follicle up to the stage from which egg is released
Estrogen (E2): It is a hormone released from follicular cells under the influence of FSH. This hormone mainly govern sexual receptivity during estrus.
Progesterone (P4): It is a hormone released from luteal cells under the influence of LH. This hormone mainly maintained pregnancy and absence or reduction of it causes abortion.
Follicles
Follicle is fluid filled cavity in ovary develop cyclically every 21 days
Follicle perform dual functions
1st fxn: Balanced production of steroid hormones that maintained the development of genital tract
2nd fxn: Production of fertilizable oocyte (egg) which on interaction with sperm generate
When female calf born
It already has follicle reserve of about 100,000 follicles in its immature ovary
At sexual maturity (heifer stage) follicles start developing
Growing follicles have one of 2 fates
Either undergoes atresia or ovulate an egg for fertilization
Most of the follicles degenerate and destroyed during development. Only few developed into ova (egg) and ovulate
In HF cows for every 1 ovulation 40 to 45 follicles are recruited but in buffaloes only 4-5 follicles are recruited
Follicular Wave & GnRH
Usually heifers and cows has large pool of primordial follicles which start develops under the influence of sex hormones at puberty.
Estrous cycle is of 21 days duration which means at every 21st day primordial follicle develops into egg/ova which if get fertilized by AI or natural mating can develop into calf
During this period 2 or 3 waves of follicle development occurs
Final phase requires 3-4 days for follicle maturation, in this phase initially slow GnRH pulse initiate growth & later GnRH frequency increases for rapid development & ovulation. At this time CL starts regressing which allows increase pulse of GnRH
Why only one follicle is developed naturally ?
In IVF & embryo transfer technology in cows super ovulation occurs which means many ovulatory follicles simultaneously develops and ovulate. It occurs because of artificial hormone injection at high doses but in natural conditions only one follicle develops and rapidly developing follicle inhibit development of other growing follicles.
Largest growing follicle produces inhibin hormone in large quantities which inhibit other follicles. Inhibin reduces secretion of FSH from pituitary.
At rapid growing final stages of follicle it is strictly required fine GnRH environment and absence of this causes immediate atresia (follicular death)
Function of FSH and LH
FSH increases growth and development of graffian follicle (GF), this follicle contains ovum inside it
When graffian follicle matures it become ready to release ovum into fallopian tube
Cells of GF before releasing ovum produces estrogen (E2) under influence of FSH
Just before release of ovum when GF ripen completely, FSH start decreasing and there is surge of LH from pituitary
LH causes loosening of cells around ovum and causes its release
After release of ovum the remaining structure of GF transform into Corpus Luteum
But after releasing ovum same cells gets transformed and start producing progesterone (P4) under influence of LH
Best time of mating or AI
For maximum conception rates insemination must be done between 4 to 16 hours after first signs of estrus
Insufficient LH or LH receptors on follicle / Follicular Cyst
Usually before ovulation LH hormone suddenly increases in blood many folds
This helps in ovulation and lutenization of the granulosa cells and helps in its transformation
LH receptors slowly start increasing at the surface of granulosa cells this process is called as lutenization
But sometimes insufficient LH from pituitary may not be able to luteinize follicular granulosa cells and follicle not at all ovulate
When follicle do not ovulate and cells do not luteinize they continuously secretes estrogen, this is called FOLLICULAR CYST
Animal may not show proper estrous cycle or remain in continues estrous or become nymphomaniac
In addition to insufficient LH, the causes of follicular cysts are unclear, their development has been linked to genetics, nutritional factors of under- or over- feeding, high milk production (increased steroid metabolism), and stress.
Luteal Cyst
Luteal cysts are also anovulatory follicular cyst with the difference in wall thickening and presence of some luteal cells
Basic causes are same as that of follicular cyst
In this condition LH hormone release is somewhat greater than follicular cyst condition which causes luteinization of cells but lesser enough that do not cause ovulation.
LUTEAL CYST is extension of FOLLICULAR CYST such that the non-ovulatory follicle is partially luteinized spontaneously or in response to hormonal therapy.
Clinically it is very difficult to differentiate by per rectal palpation and rupture during palpation may cause serious problems
Many commercial products are available to treat follicular cyst
Hormonal treatment of a follicular cyst with GnRH analogue like BUSERELIN will generally cause the cyst to luteinize and subsequent treatment with prostaglandin F2α analogue like CLOPROSTENOL or DINOPROST will resolve the luteinizing cyst by triggering luteolysis.
Estrogen
In summary, estrogen is used by the follicle(s) and
(1) to stimulate the growth and development of the granulosa
(2) to signal the hypothalamus and anterior pituitary as to the readiness of the follicle(s) for ovulation.
(3) It increases the motility of genital tract and helps in sperm movement
Corpus Luteum
Main function of CL is the production of progesterone
Follicle after ovulation convert into Corpus Luteum
Pre-ovulatory LH surge converts granulosa cells into luteal cells.
Only difference between above 2 is granulosa cells produces estrogen and luteal cells produces progesterone
LH maintains CL in both pregnant as well as non pregnant animals
It maintained up to 15 days and then regress slowly if animal is not get pregnant
If animal gets pregnant it maintained and release progesterone for the maintenance of pregnancy.
Corpus Luteum Regression & PGF2 alpha
Uterus cells are the source of PGF2 alpha which causes CL regression
PGF2 alpha also thought to release luteal oxytocin which helps in CL regression
Further release of oxytocin initiate uterus wall to release more PGF2 alpha
CL regression is normal phenomenon but it may get regress early (before time) or delayed or may not produce sufficient progesterone which may affect estrous cycle of early embryonic mortality
Clinical significance of CL
CL life (14-15days in non pregnant animals) and progesterone production should be normal
If animal gets pregnant then it blocks PGF2 alpha production & prevent CL regression which is normal
In buffaloes CL may not produce sufficient progesterone due to deficient LH and its receptors, which causes early abortion and repeat estrous cycle.. That is why external progesterone injections may prevent abortions in buffaloes
Sometimes uterine infection may irritate uterine wall which results in significant synthesis and release of PGF2α and shorten estrous cycle (less than 21 days) and in such cases we often seen that after AI animal again comes into heat
Again in buffaloes mainly and also in exotic cows when sufficient PGF2α is not produced then CL wont get regress and continually produces progesterone which do not allow animal to come in heat this condition is called as persistent CL. In this case lutalyse injection may be helpful.
