After reading this article you will learn about the three important body functional systems of animals which influence them for feeding and management of health:- 1. Digestion 2. Reproduction 3. Lactation.
Digestion includes all the mechanical and enzyme action that takes place in the digestive tract by which food is rendered absorbable. Enzymes are organic catalysts that are generally proteins .There are large differences in the digestive process in different species of animals.
There are anatomical and physiological adaptations according to the requirements of their feeding nature. The digestive process varies accordingly. The maximum difference is seen in the digestion between ruminants (cattle, buffaloes, sheep and goat) and non-ruminants or simple stomached animals (pig, dog).
The digestive tract in the non-ruminant is with a simple stomach. In the carnivorous (meat-eating animals) – cat and dog – the large intestine is small. In omnivorous (all-eating) animals like pigs the caecum and colon are of large size. The various feed nutrients are mostly digested by the action of various enzymes of the digestive tract.
The ruminants (Figure 1) have an expanded part of digestive system (rumen) so as to hold the bulky fibrous feeds and delay their passage through the digestive tract so as to allow microbial fermentation. This enlarged part is represented by the rumen, which is the biggest compartment in their four-celled stomach.
The stomach of the ruminant (Figure 1, right) comprises of four compartments – rumen, reticulum, omasum and abomasum. The rumen can be pictured as a large fermentation vat, which provides an appropriate environment for the continuous culture of a large numbers of bacteria and protozoa.
There is a symbiotic existence between the ruminant and the microorganisms by which both are benefited. The microorganisms help and modify the digestive process in the ruminants to the benefit of those animals.
The fibre splitting enzyme cellulase is produced by the microorganisms. It acts upon cellulose (fibre), which is not amenable to digestion by any of the enzymes secreted by the large animals. Cellulose, pentosans and starch are hydrolysed to mono-saccharides and then fermented to volatile fatty acids (VFA).
The important differences in carbohydrate digestion in ruminants from that of non-ruminants is that:
(i) Cellulose is utilised,
(ii) The digestion is mainly microbial and,
(iii) The end products are VFAs and not glucose.
Bacteria digest food proteins, the end products being ammonia and short chain fatty acids. Simultaneously, a synthetic process also takes place in which not only amino acids, but also non-protein nitrogenous substances like ammonia are made use of to manufacture their body cell proteins.
Considerable proportions of the protein needs of the animals are met through such microbial protein. These microorganisms ultimately pass from the rumen to the lower stomach and intestines, where digestion of the microbial protein takes place in the same way as protein is digested in non-ruminants.
The most important result of the above protein breakdown cum rebuilding process is that the dietary protein and nitrogen are converted into bacterial protein in substantial quantities. In this process, many essential amino acids are synthesized afresh, from the non-essential ones or simple nitrogenous substances.
Thus, even if the animal is fed protein of poor biological value, it is converted into high quality microbial protein. Therefore, in the feeding of ruminants, quality of protein is not of much significance provided the total quantity is met. Besides, the microorganisms have the capacity to incorporate nitrogen present in non-protein nitrogenous (NPN) substances like ammonia and urea, into their cell protein.
This gives us the option to feed part of the nitrogen requirement of ruminants as urea or similar NPN. The other side of the same coin is that when good quality true proteins are fed there is considerable wastage due to ruminal fermentation.
Reproduction is the process through which individuals of own type are produced in order to propagate the population.
Reproductive process involves:
(a) The production of male and female gametes,
(b) Their union or fertilization; and
(c) Development of young ones.
All domestic animals and poultry are bisexual (male and female) in nature and both sexes produce gametes independently of each other.
i. Male Reproductive System:
Males have a pair of testes as primary sex organ and tubular genital tract. The pair of testes (testis = singular) are ovoid shaped solid glands situated extra-abdominally in the skin folds called scrotal sacs. The testes in the male and the ovary in the female also produce internal secretions known as reproduction hormones.
Testosterone produced in testes is responsible for:
(a) Formation of spermatozoa and development of tubular genital tract and accessory sex glands,
(b) Their maturation in epididymis
(c) Growth and development of tubular genital tract and accessory sex glands
(d) Exhibition of male sex features, and
(e) Male sex characters.
The tubular genital tract arising from each testis includes epididymis; ductus deferens, accessory male sex glands (seminal vesicles, prostate, ampulla and bulbourethral glands) and penis (see Figure 2). Testis produces sperms (the male germ cell) that pass through the tubular tract. Ultimately, the sperms are deposited in the female genital tract at the time of copulation.
The process of depositing spermatozoa or semen out of male genital tract is termed as ejaculation. Before ejaculation, the spermatozoa are also mixed with nutrient fluid secreted from various accessory glands. Semen is the term given to secretion of male genital tract and it contains spermatozoa and seminal plasma.
Semen contains spermatozoa in varying number (sperm concentration) in the fluid called seminal plasma. Semen composition is described in terms of volume of ejaculate (i.e. volume of semen discharged per ejaculate) and sperm concentration (i.e. billions of sperms present in one milliliter of semen).
ii. Female Reproduction System:
This comprises of a pair of ovaries (primary sex organ) situated in the pelvic cavity and tubular genital tract (Figure 3). Ovary in cow is an almond sized solid organ that produces eggs or ova (singular = ovum, the female germ cell or egg). Tubular genital tract includes a pair of oviducts (Fallopian tubes), one uterus, cervix and vagina.
Ovaries are the primary sex organ responsible for production of:
(a) Female gamete called ovum, and
(b) Female sex hormones, estrogen and progesterone.
Both these functions begin after onset of puberty (sexual maternity) in female. In order to regulate the reproductive system, the various events should be timed precisely and regulated in such a manner that chance of fertilization is maximum. This coordination of various events totally depends upon release of the above two hormones from the ovaries.
Estrous Cycle in Females:
Reproductive cycle (Figure 4) in females is expressed in a cyclic manner which correspond with development of follicles and corpus luteum on the ovaries and secretion of estrogen and progesterone hormones in a cyclic fashion. Length of estrous cycle is 21 days in cattle/buffaloes, 18 days in sheep, 22 days in mare. The estrous cycle is divided into two phases i.e. follicular phase (4-6 days) and luteal phase (15-17 days).
Follicular phase is dominated by the presence of follicles on the ovaries and further comprises of proestrus (3-4 days) and estrus (1-2 days) in cows. Luteal phase is dominated by the presence of corpus luteum on the ovaries and further comprises of metestrus (2-3 days) and diestrus (12-15 days). These four phases alternates during the entire cycle in a definite sequence in non-pregnant females.
In case, the ovum is fertilized, the diestrus phase continues into pregnancy. Growth and development of follicles and corpus luteum on ovary is regulated by two hormones, called gonadotropins, secreted by the anterior pituitary. First gonadotropin is follicle stimulating hormone (FSH) which brings about follicular development.
The second gonadotropin is leutinizing hormone (LH) which is responsible for shedding of ovum (i.e. ovulation) and development of corpus luteum. The cyclicity of female reproduction is physiologically important in providing optimal conditions for survival of male and female gametes, their fertilization and subsequent development into embryo and fetus. Diagrammatically, estrous cycle stages can be depicted in Figure 4).
After pregnancy, the mother delivers the young one and after that the next estrous cycle starts only after about 2-3 months of rest period called post parturient period.
Out of the four stages of estrous cycle, the estrus phase is the behavioral phase in which signs of heat (estrus) are exhibited. During this phase, the follicle attains maximum growth, undergoes ovulation and corpus luteum (necessary for continuation of pregnancy) development begins.
Behavioural signs of estrus are – bellowing (particular sound), stringy mucus discharge from vagina, restlessness, mounting on other animals, slight rise in body temperature, off feed, reduction in milk.
The estrus stage lasts for 18-24 hours in cows and if mated or inseminated in this stage, chances of fertilization are best. In order to get fruitful results, the farmer or animal owner must look for this stage carefully and allow mating with a good quality fertile bull or go for insemination of animal.
Fertilization, Implantation, Pregnancy and Parturition:
Fertilization is defined as union of male and female gamete to form the zygote fertilised orum/egg. During estrus phase, the cow is mated or inseminated with fertile semen. Ovum and spermatozoa undergo fertilization in the ampulla of oviduct within a few hours of mating.
If no mating occurs, the ovum degenerates and a new ovum is released in the next estrus. For the next ovum, a fresh insemination/mating is necessary for fertilization. With fertilization, the zygote forms that subsequently develops into embryo that grows into foetus. Zygote moves into a uterine horn by 4-5 days and becomes embryo and remains free in the lumen up to 32-35 days.
Implantation is defined as attachment of the embryo to the inner wall of the uterus for taking nutrients from maternal blood. Implantation occurs after about 35 days in cow embryo, which is subsequently termed as fetus. After implantation placenta is formed.
Placenta (different in different species) is a specialized structure for maintaining connectivity between mother and fetus inside the uterus. Placenta also produces hormones like progesterone placental gonadotropins required for maintenance of pregnancy.
Pregnancy is defined as the period during which young one remains in uterus. It starts from the day of fertilization till the day parturition or delivery or birth of the young one. Period of pregnancy (different in different species) is also termed as gestation length. During this period, development of fetus is completed.
Parturition is the act of the act of giving birth after completion of gestation length. Before parturition, the fetus draws all nutrients and oxygen from the maternal blood. After completion of the gestation period, fetus is capable of living an independent life.
Dam or mother approaching parturition exhibits some typical symptoms such as restlessness, enlargement of udder, vulva, sticky discharge from vagina, relaxation of pelvic ligaments, frequent change in posture etc. Parturition occurs in three stages.
First stage is protrusion of water bag during which a membranous bag containing fluid comes out.
Second stage comprises of fetus expulsion (front feet and then head comes first) from the birth canal.
Third stage is a longer phase during which placenta (fetal membranes) are expelled out.
During parturition, proper attention is required in terms of cleanliness and hygiene. Assistance of a qualified veterinarian should be sought in case of any difficulty in parturition or when placenta is not expelled timely. Duration of the various stages of reproduction varies in different species (Table 2).
Structure of the Udder:
The udder is located outside the body wall and is attached to it by means of its skin and connective tissue supports. Blood and nerve supply to the udder is extensive. It comprises of four separate compartment called quarters – two on each side. These are joined closely but are divided by membranes so that there is no direct communication between them (Figure 5).
The secretory portion of the udder consists of countless alveoli or chambers lined with individual cells where milk is produced. Each of these alveoli is drained by a small duct, which leads to larger ducts (Figure 6).
Ducts of increasing size drain clusters of alveoli resembling a bunch of grapes, until some 10 to 20 ducts conduct milk into the gland cistern. The gland cisterns continue into the teat sinus or cistern. At the tip of the teat there is a sphincter tightly closing the outlet of the teat sinus. It is through the teats that milk is removed during milking.
Milk ‘Letdown’ Mechanism:
When milk secretion has continued for a considerable time after milking, the alveoli, ducts and gland and teat cisterns are filled with milk. Milk in the cisterns and larger ducts can be removed readily by milking. Milk in the smaller ducts and alveoli does not flow out easily.
However, the cow and other animals have developed a mechanism for releasing milk from the mammary gland. Stimulation of the central nervous system by something associated with the milking process is necessary to initiate the reaction.
Stimulation of nerve ending in the teats that are sensitive to touch, pressure, or warmth is the usual mechanism. The sucking action of the calf is ideal for this. However, massaging the under or washing with warm water is also equally effective. Stimulation is carried by the nerves to the brain, which is connected with the pituitary gland located at its base.
Then the mechanism is activated for the liberation of a hormone oxytocin. Oxytocin is carried by the blood stream to the under where it acts on the small muscle cells surrounding the alveoli, causing them to contract. The pressure thus created forces the milk out of the alveoli and smaller ducts as fast as it can be removed from the now turgid teat.