Practical implications of comparative digestion in farm animals

Monogastric (non-ruminant) animals cannot digest fibrous feed (cellulose, hemicellulose) and depend on high quality, low-fiber concentrate diets. Among the monogastrics, horses and rabbits, due to their well developed caecum is less efficient than in the rumen. Therefore, horses should be fed good quality forages. On the other hand, ruminants, because of efficient microbial fermentation in the rumen, have acquired the ability to digest a wide range of roughages, including poor quality cereal straws and other crop residues. Ruminants can also utilize non protein nitrogen from sources such as urea as a dietary crude protein and convert it to a protein of high biological value (microbial protein).
The unique digestive features of ruminants enable them to utilize agro-industrial by products and wastes which cannot otherwise be used by monogastrics, including man. As ruminants depend largely on the fermentation products of the rumen (VFA's and microbial protein), conditions in the rumen should be optimal at all times to support efficient fermentation. This can easily be achieved by taking care of the nutrient requirements of rumen microbes. Recent developments in ruminants nutrition have emphasized that these animals should be fed in such a way that their capacity to utilize roughages and non-protein nitrogen is exploited to the greatest extent possible, and at the same time high quality feeds (such as good quality feeds and starches) should be protected from rumen fermentation so that they escape from the rumen and are efficiently digested and abosrbed in the small intestine. Unlike ruminants, monogastrics cannot synthesize amino acids and vitamins B and K. Therefore they depend on feed resources for these nutrients.

Functions of Various Parts of Digestive Tract: Large Intestine

The large intestine is the terminal part of the alimentary canal; it consists of the caecum (blind sac), colon, and rectum. The size of the large intestine varies considerably in farm animals. In horses it constitutes 46% of the total volume of the digestive tract. In ruminants, it represents only 10% of the total digestive tract capacity. The large intestine serves as a major site for the absorption of water, sodium, and chloride. Other salts such as potassium, phosphorus, and magnesium can also be absorbed from the large intestine. The large intestine has only mucous secretions and no enzymatic activity.
Undigested materials which escape the digestive processes in the stomach and small intestine are subjected to microbial digestion in the caecum. In monogastric animals, particularly horses and rabbits, the caecum serves as a major site for digestion of fibrous materials. This is accompanied by a variety of microflora. However, microbial digestion and absorption of end products in the caecum is not as efficient as it is in the rumen, and therefore does not nutritionally benefit the animal as much. The microbial protein synthesized in the caecum is mostly excreted in the faeces. The colon is part of the large intestine extending from caecum to rectum. The rectum is a dilatable tube serving as a storage place for feces until it is excreted.
Anus: 
The anus is the opening at the posterior end of the alimentary canal under the root of the animal's tail. The anal canal or lumen is normally closed by the contraction of sphincter muscles. During defecation, this canal is opened and undigested material (excreta) is passed through the anus.
Note: Next post on this blog will cover, "Practical Implications of Comparative Digestion in Farm Animals"

Functions of various parts of the digestive tract: Stomach - 2

Earlier Post: Functions of various parts of the digestive tract: Stomach
Omasum: The third compartment of the ruminant stomach is known as the omasum. It constitutes about 12.6% of the total volume of the stomach in cattle, buffaloes, sheep and goats. The inner lining of the omasum is raised into longitudinal folds are pillars, which greatly increase the surface area. The principal function of the omasum is to absorb water and reduce size of the feed particles. The contents of the omasum are much drier than those of the rumen and abomasum. Because of the small size of the omasum, its contents are not retained for long and are passed on to the abomasum.
Abomasum: This is the fourth and last chamber of the ruminant stomach. It resembles the simple stomach of the monogastric animals and is the only glandular part of the ruminant stomach. The inner lining of the abomasum has folds which are more prominent in the glandular region. It secretes hydrochloric acid and pepsin. The acidic environment of the abomasum causes destruction of microbial cells which are subsequently digested by the pepsin secreted into the abomasum. The rate of abomasal secretion is influenced by the volume and composition of ingesta entering the abomasum. The digesta leaving the abomasum (in a ruminant) and the simple stomach (in a non-ruminant) is called chyme.
Development of the stomach in young ruminants: In ruminants, the rumen is small and non - functional at birth. The abomasum, on the other hand is comparatively large and well developed. As the animal grows, the rumen gradually develops and attains a size which is 10 times greater than the abomasum in adult animals. Since the rumen does not play any role in digestion during early life, the milk suckled by the calf bypasses the rumen and goes directly in to abomasum through a special mechanism, the oesophageal groove. In the abomasum an enzyme, rennin, is secreted which has a strong coagulating action on milk. This helps in retaining the milk for longer time in abomasum for further digestion. The oesophageal groove is a tube like structure with a slit. Suckling and swallowing of milk and other liquids causes closure of the oesophageal groove, thus directing the liquid food to the abdomasum. Ingestion of solid food causes the oesophageal groove to open, allowing the feed to drop in to the rumen. As the intake of growing calf gradually changes from milk to solid feed, the stomach compartments, particularly the rumen, increase in volume, formation of papillae on the internal surface, and establishment of rumen micro-flora. Early development of the rumen can be encouraged by offering good quality solid feed after two weeks of age. This can also help in early weaning of  young calves.
B. Non - Ruminants (Mono Gastrics): In these animals, the stomach is a "V" - Shaped muscular sac. Muscular contractions of the stomach wall help in breakin gdown the feed particles, and at the same time cause mixing of the stomach contents. About two thirds of the inner surface of the stomach wall has glands which secrete gastric juice containing mainly hydrochloric acid and the enzyme pepsin. The pepsin digests protein into peptides and amino acids and requires acidic medium for its action. Hydrocholoric acid helps in maintaining acidic pH in the stomach. The feed is exposed to gastric enzymes ofr some time, and the semi-digested feed is passed to the small intestine through an opening controlled by sphincter muscles.

Note: Next post on this blog will be comprised of details about "Small Intestine".

Functions of various parts of the digestive tract: Mouth, Oesophagus

Mouth:

The primary function of the mouth include prehension (grasping), mastication (chewing), and mixing of feed with saliva as preparatory steps for subsequent digestion in the remaining parts of the alimentary canal. The lips, tongue, and teeth are used to pick and hold feed while it is in the mouth. The teeth grind the feed, and the tongue mixes it with saliva. The lips of sheep, goats, and horses are flexible, while those of cattle and buffaloes are relatively stiff and immobile. In horses, the upper lip is very sensitive and mobile and is used to feel the feed and direct it towards the moouth. Horses also have both upper and lower incisors which cut herbage like scissors and help in prehension. The mastication of feed with grinding molars is also very efficient in horses. In cattle, buffaloes, and sheep, the upper incisors are absent. Instead, they have a hard pad called the dental pad. During grazing, the tongue is used to guide feed into the mouth, and plants are grasped between the dental pad and lower incisors and torn away.

Saliva is a watery solution containing small amounts of mucin, inorganic salts and enzymes. It is secreted into the mouth from three pairs of salivary glands (paratoid, sub-maxillary, and sub-lingual). Secretion of saliva increases during mastication of feed. Cattle and buffaloes produce 130 - 200 litres, sheep and goats 2 - 3 litres, and horses 10 - 12 litres of saliva per day. Saliva lubricates the ingested feed and thus facilitates swallowing. It also helps in maintaining optimum pH in the rumen and provides a mechanism for recycling urea in ruminants.

Oesophagus:

This is a muscular tube extending from the back of the mouth (pharynx) to the stomach. Ingested feed is forced through the oesophagus by its wavelike muscular contractions.

(Note: In next Post, find about Function of stomach in digestive system of farm animals)

Parts of digestive tract

The digestive tract, also known as the alimentary canal, is a tube like structure extending from the mouth to anus. The different parts of the digestive tracts of ruminants and non-ruminants are illustrated in figures. The alimentary canal is comprised of the following parts,

1. Mouth (lips, tongue, gums and teeth) and pharynx
2. Oesophagus
3. Stomach (In ruminants, the stomach includes the rumen, reticulum, omasum, and abomasum)
4. Small intestine (duodenum, jejunum, and ileum)
5. Large intestine (caecum, colon, and rectum)
6. Anus

In addition to the above, the following accessory organs/glands also take part in the process of digestion: salivary glands, liver, gall bladder, and pancreas.

Among domestic animals, the proportion of the digestive tract to total body weight is higher in ruminants than in non-ruminants, as shown in table. Horses, for example, have a large caecum (15.6% of the total alimentary tract) where extensive microbial fermentation of plant materials takes place. Similarly, rabbits also have a well developed caecum. In ruminants, the stomach compartments (rumen, reticulum, omasum, and abomasum) occupy the major part (62%) of the digestive tract. Large variations in the proportion of different parts of the digestive tract in farm animals reflect differences in their feeding habits. These variations have evolved over time, and the animals have gradually adapted to various feed resources.

	Cattle	Buffaloes	Sheep/Goats	Horses	Dogs
Body Weight (Kg)	450	600	50	450	20
	Volume in liters, and proportion of body weight (percent)
Stomach
Reticulo-rumen	97.6 (48)	130.3 (48)	10.7 (56.7)	-	-
Omasum	15.8 (7.6)	20.9 (7.6)	0.6 (3.3)	-	-
Abomasum	11.8 (5.8)	15.7 (5.8)	1.3 (6.7)	8.0 (8.9)	1.0 (63)
Small Intestine	50.9 (25)	67.7 (25)	3.8 (20)	27.0 (30.0)	0.37 (23.0)
Caecum	7.7 (3.8)	10.3 (3.8)	0.6 (3.3)	14.0 (15.6)	0.02 (1.0)
Large Intestine	19.7 (9.5)	26.0 (9.5)	1.9 (10.0)	41.0 (45.6)	0.21 (13.0)
Total	203.5	270.9	18.9	90.0	1.6

Introduction to Digestion in Farm Animals

Digestion in animals involves mechanical, enzymatic, and microbial processes in the gastrointestinal tract, which convert large feed particles to a size which can be absorbed, transported, and used by the animals. The digestive systems of all farm animals are not the same, rendering them less competitive and able to adapt to a wide range of available feed resource niches. Based on the structure and functions of the digestive tract, farm animals are divided into two major groups:  ruminants or polygastrics, e.g. cattle, buffaloes, sheep, and goats; and non-ruminants or monogastrics, e.g. horses, donkeys, rabbits, dogs and cats. The key difference between these two groups of animals lies in the structure of the stomach. Ruminants, as opposed to non ruminants, have three additional stomach compartments where digestion takes place in different environments. In general, for farm animals the processes associated with digestion include prehension, ingestion, grinding or mastication, digestion of feed, absorption of nutrients, and excretion of waste products.

Artificial Insemination (AI)

Artificial insemination (AI) is the deposition of semen in the cervix by the artificial means. It is a useful technique devised for the genetic improvement of farm animals. Artificial insemination is widely used for breeding cattle, buffaloes, sheep, goats, horses, dogs and a variety of laboratory animals.
Advantages of artificial insemination

• The greatest advantage of artificial insemination is the opportunity to spread superior germ plasm by the wide use of carefully tested and selected sires. On average, a bull can sire 50,000 progeny per year through artificial insemination compared to 40 - 50 progeny through natural mating. 
• Artificial insemination plays an important role in the control of various diseases, particularly the venereal diseases disseminated by natural mating. 
• It helps to maintain complete and accurate breeding records, leading to better herd management and the identification of infertility problems. 
• Artificial insemination is more economical than natural mating. 
• The mating of animals of different sizes becomes possible without any accidental injury. 
• Artificial insemination extends the usefulness of sires of high genetic merit which for physical reasons are unable to copulate normally.

Limitations of artificial insemination
There are few disadvantages of artificial insemination even if it is properly performed. The major limitations are due to lack of trained personnel to provide proper service. Poor breeding efficiency may occur in herds when owners do not watch their animals closely for estrus and the inseminator does not breed them at the proper time. The inseminator may be, if not careful, a means of spreading infections from one herd to another. Herd owners should avoid intensive use of a limited number of sires, which may increase inbreeding in the herd. Increased inbreeding is usually associated with low fertility and decrease in vigor and overall productivity.
Semen and its collection
One of the most important steps in an artificial insemination program is the collection of semen and its proper handling. The following methods are used for the collection of semen from bulls.

1. Artificial vagina method 
2. Massage Method 
3. Electro-ejaculation method

The best procedure and practical method for collecting semen is with an artificial vagina. Various sizes and shapes of artificial vagina are used, but all consists of a heavy rubber cylinder with a rubber lining inside. This enables a clean, complete ejaculate to be collected in the glass tube fitted on the lower end of the artificial vagina.
In the massage method, the operator's hand is inserted about 10 inches into the rectum of the bull. The vesicular glands are picked up with the fingers by carefully feeling the rectal wall. They are then massaged gently causing a slightly turbid fluid to appear which cleanses the path for semen. Then the operator massaages the ampulla of the vas deferens until the sperm containing semen appears. The semen is collected in a test tube placed in front of the sheath.
Electro-ejaculation is the method used for males that refuse to serve the artificial vagina or when injuries makes this impossible. A rectal probe with either a ring or straight electrode is used to provide the necessary electrical stimulation.
After the semen is collected, it is evaluated for quality (live and dead sperm, motility etc) and diluted with suitable extenders (diluent). Then it is stored in liquid form in the refrigerator at 5 Degrees Celsius or in frozen form in liquid nitrogen at below 196 Degrees Celsius.
Insemination procedure.
Detection of heat (estrus) of the female is the first step. When a female is found to be in heat, the inseminating rod is passed through the spiral folds of the cow's cervix by the recto-vaginal technique. Part of the semen is deposited in the uterus, just inside the cervix, and the remainer in the cervix while the rod is withdrawn. Extra care must be exercised in the case of those animals that have been inseminated previously. If pregnancy is suspected in an animal, insemination should not be repeated.
Spermatozoa can survive in the genital tract of a cow or buffalo for a little over 24 hours. The egg has a short survival time, about 6 hours at the most. Optimum fertility is obtained when inseminations are performed 13-18 hours before ovulation. Reasonably good results are obtained even during the period 7-12 hours before ovulation. The cows should be checked twice daily with a teaser bull to detect estrus. As a rule of thumb, cows showing estrus in the morning should be bred the same day in the afternoon, and cows showing estrus in the afternoon should be inseminated the next morning.

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: 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.