Reproductive cycle in farm animals

Reproduction in female is a complicated process, and normal reproduction ivolves synchronization of many functions. It is subject to many adverse effects at certain stages. Different aspects of reproduction in cows and buffaloes along with the physiological mechanisms involved are discussed in the following paragraphs.
Puberty or sexual maturity
Puberty is defined as the age at which reproduction becomes possible. In the female this is usually considered to be the age of first estrus. Puberty occurs before full mature body size is attained. The average age at puberty in cows is average 34 months and in buffaloes is 38 months. In cross breeds, the age at puberty is 9 - 18 months.

Trait	Species
	Sahiwal Cow	Holstein Cow	Buffalo	Sheep	Goat	Horse
Age at first service (days)	1030-1052	596-628	1137-1151	195	195	495
Gestation Period (days)	282	282	310	144-151	146-152	340
Dry Period (days)	298	-	191
Service Period (days)	154-221	144	215
Calving interval (days)	500	426	520
Length of estrus cycle (days)	21	21	21	16-17	20-21	21
Duration of estrus (hours)	17	17	17-24	22-36	24-48	4-8 Days
Post-estrus time ovulation (hours)	10-11	10-11	18-45	24-30	24-36	1-2 Days
Best time to Breed	Late estrus or shortly after Estrus	Late estrus or shortly after Estrus		Middle of Estrus		Every other day beginning 2nd day of Estrus
Fertile life of sperm in the female genital tract (hours)	30-48	30-48		30-48		72-120
Fertile life of ovum (hours)	20-24	20-24	N/A	10-25	N/A	6-8

The onset of puberty depends upon a change in the balance between the output of gonadotropins from the gonads and growth hormones secreted by the anterior pituitary gland. The pituitary gland is located in a bony depression at the base of the brain. It consists of two distinct parts, known as the anterior (front) and the posterior (rear) pituitary glands. The pituitary is controlled by certain physiological functions involving the gonads and the anterior pituitary gland, and is influenced by several environmental (season, temperature, nutrition) and genetic factors.
Breeding season
The term breeding season refers to the period of time during the year when females of a species come into estrus. Most species of wild animals have a definite breeding season which is initiated at a time when environmental conditions are conducive to the survival, growth, and development of the young one after birth. For example, the gestation period in sheep and goats is 144 - 152 days, and the breeding season in these species starts in fall so that the young are born in spring. In cattle, where pregnancy on average lasts for nine months, the female breeding season occurs in the early summer so that the calves are born in the spring when the temperature is optimum for the growth of the newborn. Green fodder and forages are also available, which support sufficient milk production by the dams.
Domestication of farm animals has resulted in some variation of breeding season. Due to the intervention of modern production techniques, farm animals are now capable of breeding throughout the year. Among the farm animals, cows and buffaloes are polyestrous, showing diestrous cycles throughout the year. Mares, ewes, and does are seasonally poly estrous, showing diestrous cycles during a definite breeding season. The breeding season of ewes and does is confined largely to fall and winter. In Indian sub continent, buffaloes breed from October to December, when day length is decreasing. The stimulus for the initiation of reproduction during certain seasons results from the action of light, via the eye and optic nerve, on the pituitary gland. This stimulus causes the pituitary to release gonadotropic hormones, which activate the functioning of the gonads. One of the limiting factors influencing the productivity of the buffalo is the seasonality of breeding, which may be due to either anestrus or silent estrus during the hot, dry months of summer.
Estrus Cycle
Estrus occurs in non-pregnant females in characteristic rhythmic cycles. The interval between two consecutive estrus periods is known as the estrus cycle. The estrus cycle in farm animals is characterized by cyclic changes in the morphology of the reproductive organs as well as in the behavior of the animals.
The estrus cycle is divided into four different phases; these are known as proestrus, estrus, metestrus and diestrus. 

Proestrus is the phase of estrus in which the reproductive system is beginning preparation for the release of the mature ovum from the ovary. Estrus is defined as that phase of the cycle when females are receptive to coitus. Metestrus is the short transitional stage following ovulation in which the effects of estrogen are declining. The recently ruptured follicle is reorganized, and the secretion of progesterone is rising. During this phase, the uterus gets prepared for the implantation of the embryo. If pregnancy does not occur, the reproductive tract goes into a period of rest called diestrus. In cows and buffaloes the estrus cycle averages 21 days in length. Sheep and goats usually have a shorter estrus cycle ranging from 16 - 21 days.
Signs of estrus are somewhat similar in females of different species. In cows and buffaloes, estrus is characterized by the manifestation of heat. The cows may bawl frequently, become very active and restless, or mount other females and remain mounted. The vulva swells and the vestibule may be come deep-red. The swelling subsides with the end of estrus. There is marked discharge of clear, viscid mucus secreted by the cervix from the vulva. The excessive secretion at estrus acts as a lubricant during copulation.
Ovulation: Ovulation may be defined as the discharge of the ova from the Graafian follicle. Ova develop from germ cells in the ovaries. Numerous germ cells are present in the ovaries at birth, but few of the follicles surrounding them mature to rupture and shed ova. Follicles grow under the influence of the follicle stimulating hormone (FSH). The layers of follicular tissue become thinner with growth, the follicle rupturees and ova are released. This process is stimulated by the luteinizing hormone (LH). The released ova are swept by muscular and ciliary movements into the oviduct towards the uterus. If mating or articficial insermination takes place, spermatozoa and ova meet in the oviduct.
Esturs and ovulation are more or less synchronized in females to increase the chances of fertilization, the union of an ovum with a spermatozon. Synchronization of ovulation and insemination is essential because the life of the ovum once it is shed and that of the spermatozoa in the female reproductive tract is limited to a few hours.
Gestation: Gestation is the process that begins with the fertilization of the ovum by the sperm and terminates with the birth of a young one. The period of gestation varies considerably among species of farm animals. The average gestation period of cattle is 282 days, buffaloes 310 days, and sheep and goat 151 days. Generally, male calves are carried one day longer than female calves.
Parturition: Parturition is the physiological mechanism that enables the uterus to expel fetus after a certain period of development and nourishment. The process is called "foaling" in mares, "calving" in cows and buffaloes, "lambing" in ewes, and "kidding" in goats.
Signs of Parturition. The dam shows a number of signs of approaching parturition, which are usually divided in to following four stages.
(a) Preliminary stage: This stage may last for hours or even days. The ligaments in the pelvic region and around the tail head begin to relax and sink a few days before calving. The dam exhibits difficulty in movement. The external genitalia are swollen, enlarged, and flabby, and strings of mucous are seen. The mammary glands are enlarged and become tender. A clear, waxy fluid oozes out of the teats when pressure is applied.
(b) Dilation of the cervix: This stage merges with preliminary stage. Uneasiness increases and the animal may lie down and get up frequently. Frequency of micturition (urination) increases. At this time the cervix is fully dialted, and the next stage follows without any appreciable break in the sequence of events.
(c) Expulsion of the fetus: This staage begins with the fetus entering the dilaated cervix and the birth canal. During this time, uterine contractions occur at about two-minute intervals and the amnion is not ruptureed. There is a rush of fluid from the uterus when the water bag rupturees, and the animal appears relieved. Normally, the forefeet, with the muzzle lying behind and over them, appear at the vulva. The young one is expelled from the fetal membranes by a forceful and painful effort by the mother.
(d) Expulsion of membranes: The expulsion of the fetal membranes is commonly known as the delivery of the afterbirth. COntinued contractions of the uterus disjoin the fetal cotyledons, and the placenta is expelled within half an hour to eight hours after partuirition.

Reproductive Hormones

Hormones are organic substances secreted by certain specialized cells (glands) in the body, which are diffused or transported to some other part of the body and bring about certain changes. A number of hormones are directly or indirectly involved in vaarious aspects of reproduction. The secretion of these hormones is essential for the maintenance of a proper internal environment to ensure successful reproduction. The hormones involved in reproduction may be divided into two groups: 

(a) Primary hormones of reproduction

(b) Secondary hormones of reproduction

(a) Primary hormones of reproduction:

Gland	Hormone	Important Function
Anterior Pituitary	Follicle stimulating hormone (FSH)	Spermatogenesis; ovarian follicular growth.
	Lutenizing hormone (LH) (ICSH)	Androgen release; ovulation.
Posterior Pituitary	Oxytocin	Parturition; uterine contractions; milk letdown.
Testis	Testosterone	Maintenance of male reproductive duct system and secondary sexual characteristics; male sexual behavior; spermatogenesis
Ovary	Estradiol	Maintenance of female reproductive duct system and secondary sexual behavior; mammary gland stimulation.
	Progestrone	Implantation; pregnancy maintenance; mammary gland stimulation.
	Relaxin	Relaxation of uterine cervix and pubic symphysis; inhibition of uterine contractions.
Placenta	Human chorionicgonadotrophin (HCG [primate])	LH-Like
	Pregnant mare's serum (PMS [horse])	FSH-Like

(b) Secondary hormones of reproduction:

Gland	Hormone	Important Function
Anterior Pituitary	Somatotropin releasing hormone (STH)	Body growth; protein synthesis.
	Thyroid stimulating hormone (TSH)	Stimulation of thyroid gland; thyroxine release and iodine uptake by thyroid.
	Adrenocorticotropic hormone (ACTH)	Stimulation of adrenal cortex; release of adrenal corticoids
Posterior Pituitary	Vasopressin (antidiuretic horomne) (ADR)	Water balance.
Thyroid	Thyroxine	Body growth; development of maturation; oxidation of feeds.
Adrenal Cortex	Aldosterone	Electrolyte and water metabolism.
	17-OH corticoids (cortisone) (cortisol) (corticosterone)	Carbohydrate, protein, and fat metabolism.
Pancreas	Insulin	Carbohydrate, fat, and protein metabolism.
Parathyroid	Parathormone	Calcium and Phosphorus metabolism.

 

The primary hormones are directly involved in various aspects of reproduction, such as spermatogenesis, oogenesis, and ovulation. The secondary hormones are needed for the normal functioning of the organisms which makes it possible for reproduction to occur.

Reproduction In Farm Animals: Female Reproductive System

Female Reproductive Tract - Cow (Courtesy: fao.org)

The female not only contributes the female sex cell (ovum) but also provides the necessary environment in which the new individual is nourished during the early days of its life. These functions are performed by the primary and secondary organs of the reproductive system. A pair of ovaries are the primary organs of reproduction and produce not only ova but also the hormones required for the normal sexual behavior of females. The secondary organs of reproduction are a pair of oviducts (Fallopian tubes), the uterus, the cervix, the vagina, and the vulva. The mammary glands are also considered an accessory sex organ as they are closely related to the completion of reproduction by the nourishment of the young one.

The development of the reproductive organs starts before birth during the embryonic stage, but they remain quiescent. After birth, the reproductive organs develop gradually until the female attains a specific weight at a certain age when she becomes sexually mature, i.e. capable of producing viable gametes. The age at which a female becomes sexually mature is called puberty.

(a) Ovaries: The cow or buffalo has two ovaries situated on either side of the medial line, deep in the pelvic cavity almost at the border of the abdominal cavity. The right and left ovaries are connected with the uterine cornua of each side by tortuous Fallopian tubes. The ovaries of the buffalo are irregularly oval or spherical in shape and variable in size, while the ovaries of the cow are usually ovoid or spheroidal. The size and weight of active ovaries change frequently due to the formation of Graffian follicles and corpora lutea. A functional ovary develops from outer germinal epithelium from which the Graafian follicle develops.

The ovaries are the primary organs of reproduction because they produce not only the egg cells (ova) but also the sex hormones essential for female sexual activities. Willingness to mate (libido), ovulation, and many processes related to development, maintenance, and delivery of the calf are regulated by the female sex hormones. Hormone production in the ovaries is primarily controlled by the gonadotropic hormones of the pituitary gland. Under the stimulation of the pituitary hormones, the ovaries produce a follicular stimulating hormone (FSH), which is an estrogenic substance. Estrogens induce the psychic manifestations of estrus (heat). They also bring about glands of the cervix to secrete, and promote development of the mammary duct system. The characteristics behavior of the female and the secondary sex characteristics of the female are largely due to the influence of estrogens.

When ovulation takes place, the ruptured cells form the corpus luteum. The corpus luteum becomes a temporary endocrine organ under the influence of the luteinizing hormone  (LH) of the anterior pituitray. The corpus luteum secretes a hormone called progestrone which prepares the endometrium for the implantation of fertilized ovum and the maintenance of pregnancy. Progestrone also prevents ripening of additional follicles and ovulation, thus preventing the occurrence and manifestation of heat. It also stimulates the growth of the alveoli of the mammary glands and depresses the production of luteinizing hormone by the pituitary. The corpus luteum also produces another hormone known as relaxin, which causes decalcification of the pelvis. This results in the relaxation of the birth canal during parturition.
(b) Tubular Genitalia:
The tubular genitalia consist of the oviducts or Fallopian tubes, the uterus, the cervix, the vagina, and the vulva.
Oviduct or Fallopian tube. The oviduct is a tortuous, tubular structure of muscular and epithelial tissues. It reaches from the ovaries to the tapered end of the uterine horn. In cattle of different ages, the oviduct varies in length from 20 - 35 cm, while the length of the oviduct in buffalo varies from 20 - 25 cm. On the basis of diameter and internal structure, the oviduct consists of three parts: the isthmus or narrow portion which adjoins the uterine horns; the ampulla, which is slightly wider; and the infundibulum, which opens into the peritoneal cavity.
Uterus. The uterus consists of a body and two "horns". It is a muscular and irregularly tubular organ, the size of which increases during successive pregnancies. In heifers, it is situated 25 - 40 cm from the vulvular opening, just anterior to the cervix.
Cervix. The cervix or the neck of the uterus is a small muscular tube which extends from the uterus to the vagina. The muscular wall of the cervix is thick and its mucosa contain a large number of mucus-secreting cells. The mucosa form thick folds; opposite folds tend to overlap, forming a spiral passageway through the cervix. This makes possible the tight closure of the cervix with a cervical plug that is formed during pregnancy. This structure and secretions from the mucus secreting cells protect the uterus from microorganisms commonly present in the vagina. The cervical muscles relax during estrus and at the time of parturition. At parturition the cervix dilates, allowing the calf and fetal membranes to be expelled.
Vagina. The thin but elastic-walled tubular connection of the cervix with the vulva is called the vagina. It is a common passage for the reproductive and the urinary tract. The epithelial lining of the vagina undergoes changes during the estrus cycle. During natural breeding, semen is deposited in the vagina near the cervix.
Vulva. The external opening of the female genitalia is called the vulva. It includes the clitoris and the vestibule. The clitoris is a structure of erectile tissue (homologous with the male penis) located just inside the lower junction of the vulvular lips. The external opening of the reproductive tract of the cow lies just below the anus. It is about 12 cm long and has a wide, rounded dorsal angle and a narrow ventral angle. The thick labia majora are covered with fine hair.

Reproduction In Farm Animals: Male Reproductive System

One of the fundamental laws of nature is that every species makes a vigorous attempt to reproduce its kind. The higher animals reproduce sexually. This involves the production of potentially differentiated sex cells by the male and female. In animals, this has led to the development of highly specialized body parts for the specific purpose of reproduction.

Male Reproductive System:

Male reproductive tract of a bull (Courtesy: iaea.org)

The primary sex organs of the male are the two testes, which are normally located in an external sac of skin called the scrotum. The secondary sex organs are the duct system (the vas deferens), the epididymis, and the penis. The penis is trans-versed by the urethra, which is common passageway for urine and semen. The semen is composed of sexual secretions and spermatozoa. Besides the reproductive tract, there are a few accessory organs like the prostate gland, a pair of seminal vesicles, and two Cowper's glands, or bulbo-urethral glands. The primary, secondary and accessory sex organs are collectively called male reproductive tract.

(a) Scrotum: The scrotum is a two-lobed sac developed from the invagination of inguinal skin to accommodate the testes. The scrotum is pendulous and situated just behind the rear part of rudimentary teats in the inguinal region. Although the scrotum is apparently divided into two almost equal halves by the median vertical band, the left half is slightly longer and more voluminous than the right half.

The main function of the scrotum is to support and protect the testes suspended by the spermatic cord in the scrotal sac. The scrotum functions as a heat-regulating mechanism in the male. It keeps the testicles 4 - 5 degree celsius below normal body temperature. This lowered temperature is essential for spermatogenesis. The large number of sebaceous and sweat glands on the scrotum help in lowering the scrotal temperature. During the hot season, the thermoregulatory action of the tunica dartos muscles causes it to relax, allowing the scrotum to elongate, dropping the testes far from the heat of the body. During the cold season, the scrotal muscle contracts, retracting the scrotum and bringing the testes nearer to the body. This thermoregulatory action, however, does not begin until the animal approaches puberty.

(b) Testes: The testes are the primary sex organs in the male. They are found in pairs suspended in the scrotal sac by the spermatic cord outside the abdominal cavity in the inguinal region. Each testis is an independent unit, separated from the other in the scrotal sac. The testes are firm and compact masses of parenchymatous tissue. In the buffalo bull, the average length, breadth and circumference of the testes without epididymis have been reported to be 7.73, 4.32, and 12.22 cm respectively.

The testes develop within the abdominal region near the kidneys. They commence their descent from the abdomen into the scrotal pouches during fetal development. Migration is normally completed by the time of or soon after birth. Either one or both of the testes may, however, fail to descend into scrotum during maturity. This condition is known as unilateral or bilateral cryptorchidism. Bulls affected by bilateral cryptorchidism are sterile. This condition is thought to be an inherited trait, hence such bulls are not selected for further breeding.

The main functions of the testes are,

(i) Production of viable, potentially fertile spermatozoa,

(ii) Production of androgens or the male hormone, testosteroone.

The seminiferous tubules produce spermatozoa from the germinal epithelial layer by a series of cell division. The seminiferous tubules join to form the rete testes in the mediastinum, and these, in turn, lead in to a dozen efferent ducts, the vasa efferentia, which finally converge at the dorsal part of the mediastinum to form the begining of the epididymis.

The main sex hormone, tesstosterone, is secreted by the Leyding cells or interstitial cells of the testes. The secretion of this hormone is regulated by the luteinizing hormone of the anterior pituitary gland. Testosterone is responsible for the development and maintenance of the functions of the male reproductive tract, secondary sex characteristics and sexual behaviour.

(c) Epididymis: The epididymis emerges from the joining of the vas efferentia at the dorsal part of the testis. It is a very long single duct, highly vonvoluted and appearing as a mass of tubes. It is comprised of three parts: the caput epididymis (head), the corpus epididymis (body) and the cauda epidiymis (tail). The tail of the epididymis opens into the vas deferens.

Throughout most of its length, the epididymal tube is lined with secretory cells. Spermatozoa accumulate in the epididymis and mature during their passage through it. In the epididymis, the spermatozoa mature and become able to move spontaneously and fertilize the ovum (egg) when they come in contact with it.

(d) Vas deferens (ductus deferens): The vas deferens is a tube emerging from the tail end of the epididymis. It starts from the base of the testes, extends upward, and in association with the spermatic cord runs through the inguinal ring, where it separates itself from the arteries, veins, nerves, and other cord tissues. It passes through the abdominal cavity towards the pelvis and finally empties  into the urethra. The lumen of the vas deferens is narrow and lined with mucous membrane. The wall is made up of longitudinal and circular layers of involuntary muscles covered by the outer layer of the peritoneum. The muscles of the vas deferens contract involuntarily during ejaculation of semen and help in the expulsion of spermatozoa. In the pelvic region, the vas deferens enlarges to form the ampulla of Henle which is 4 - 7 inches long. The ampulla has numerous glands, and spermatozoa often accumulate heree before ejaculation. The glands of the ampulla secrete fructose and citric acid which provide nutrition for the spermatozoa.

(e) Urethra: The urethra is the common passage for the excretion of urine and semen. It extends through the pelvis and the penis and ends at the tip of the glans penis as the external urethral orifice. In the urethra, spermatozoa mix with the seminal plasma of the accessory sex fluids at the time of ejaculation.

(f) Penis: The penis is essentially composed of erectile tissue. It is divided into three portions: the attached portion is called the root, the main portion is called the body, and the free portion is called the glans penis. The erectile tissue is a sponge-like system of blood vessels which becomes filled with blood under pressure when the bull is sexually stimulated. This helps the penis to enlarge and become rigid, thus enabling it to enter into the vagina of the female.

The penis of the bull contains very little erectile tissue. The length of the penis from the neck of the pelvic urethra to the tip ranges from 65 - 111 cm. The penis is a cylindrical organ with a tapering end; the tapering portion opens at the angular end of the triangular sheath. The sheath of the buffalo bull is a pendulous triangular fold of skin extending backward from the umilicus. When the bull is not sexually excited, the penis is in a "S" - shaped from tknown as a sigmoid flexure. During erection, the longitudinal flexure straightens, thereby increasing the length of the organ. The erector muscles pull the penis against the pelvis and aids in erection by compressing the veins of the penis. The retractor muscles help to return the extended penis to flexed state.

(g) Accessory sex glands: The accessory sex glands of the male reproductive system are a pair of seminal vesicles, the prostate gland, and Cowper's glands or transverse urethral glands. They provide the bulk of seminal plasma.

Seminal Vesicles. Each of the two seminal vesicles are located on either side of the ampulla. The seminal vesicles are lobulated and highly secretory. They open either above or below the opening of the vas deferens. The secretions of the seminal vesicles contains a large amount of fructose and citric acid, which are required for the nourishment of spermatozoa. 

Prostate gland. The body of the prostate is situated in front of the vesicula seminalis on the dorsal surface of the pelvic urethra near the neck of the urinary bladder. It secretes a mineral rich fluid.

Cowper's glands or transverse-urethral glands. The Cowper's glands are two in number and lie one on each side of the pelvic urethra, partially buried in the transverse-cavernous muscle. They produce a viscid, mucous like lubricating substance.

Semen. The reproductive sexual fluids containing spermatozoa (product of seminiferous tubules of testes), secoretion of the epididymis, and the secretions of the accessory sex glands mixed in the urethra is known as semin. The semen is creamy-white to yellowish in color, and its consistency varies with the number of spermatozoa in the semen. A buffalo bull produces 1.5 - 6 ml of semen per ejaculation.

The discharge of semen during mating with a female or in an artificial vagina is called ejaculation. Nervous stimulation creates muscular contractions of seminal tract which ejects spermatozoa into the penis through the pelvic urethra. As a result of rhythmic contractions of the urethra, the semen is forced out through the orifice of the penis with a sudden thrust of very short duration.