- GROWTH AND DEVELOPMENT
Growth is described as an increase in body weight (or mass).
Development is described as a change in body proportions.
Four processes are involved in the final form of an adult animal. These are:
The transformation of mother cells to different cell types. Examples of this are the mother cells which differentiate i.e. change to form the specialised cells of the brain, kidneys, liver, intestines, etc. This process is irreversible so once these specialised organs have been formed the cells cannot change back.
The organisation of cells into tissues, the building of tissues into organs and the development of organs into the whole body.
Is the sum total of those co-ordinated biological and chemical processes that start when the ovum is fertilised and end when the body attains a size and conformation that is characteristic of the species.
Is the co-ordination of the diverse processes which results in an adult with a form or appearance that is characteristic of its species. Development goes on for longer in higher species than in the lower animals.
Although growth itself is a highly complex process, growth curves showing increases in body-weight against age in weeks can be drawn up for each type of animal. Puberty, which is the onset of sexual activity, normally occurs at about 30% of body-weight.
Figure 1: The growth curve
Phase 1, which is the self-accelerating phase, is the period of most rapid growth in the young animal.
Phase 2, the self-inhibitory phase is the period when growth slows down and then stops altogether.
Point of inflection shows the time of maximum growth in Phase 1. It is at this point that puberty is reached. It may be used as a reference point for comparing the physiological age in different species.
- 2. PRE-NATAL GROWTH
This can be divided into three stages;
- Growth of the ovum
- Growth of the embryo
- Growth of the foetus
Growth of the ovum:
This stage lasts from the fertilisation of the egg to the start of cell differentiation and covers the first week or so after fertilisation. Little change takes place in the shape or weight of the ovum.
Growth of the embryo:
Cell differentiation starts and the major tissues and organs begin to develop. This phase lasts from about the 6th day to the 34th day after fertilisation in sheep, and from the 13th day to the 45th day in cattle.
Self-inhibitory: Hinder, restrain or prevent by itself. Distal: situated away from the centre of the body or from the point of attachment. Proximal: situated nearer to the centre of the body or the point of attachment. Growth of the Foetus:
This phase is characterised by a rapid increase in the weight of the foetus and the various organs and tissues show marked differences in size and shape.
± 34th to 140th day in sheep; ± 45th to 282nd day in cattle.
The foetus increases in weight from conception, with rapid increases occurring in the 3rd and last stage of pregnancy. The growth of organs in the foetus follows the functional needs of the foetus. Thus organs necessary in the early stages of life (pregnancy and just after birth) develop first, e.g. the liver, heart, kidney, brain and spinal cord. Organs needed for post- natal life develop later, e.g. abomasum (stomach) not needed until birth. Only 17% of a lamb’s four stomachs exist at 56 days of foetal life and 56% at birth.
The head and shoulders develop before the abdominal area; the distal ends of limbs develop before the proximal ends.
|At 56 days foetal life (lamb)||Head||= 33%||of foetal body weight|
|At Birth||Head||= 12%||of actual weight|
Major components differ in comparative rates of development.
Nervous tissue develops before bone which develops before muscle, etc.
Differential growth occurs in these ‘categories’: i.e. bones of head and shoulders develop before those of the loin region: the axial skeleton develops earlier than the appendicular skeleton.
- 3. POST NATAL GROWTH
A young mammal has relatively large legs and head, and a small body at birth. Nervous tissue and organs are well developed. Bone tissue is more developed than muscle tissue which is more developed than fat.
Organs show growth variation in post-natal life as in pre-natal growth:
Table 1: Mean Weights of Tissue of the Carcass of a Sheep Foetus as a Percentage of the Carcass Weight.
|COMPONENT||Foetal Age in Days|
|Brain and Spinal Cord||7.5%||4.2%||3.3%||2.4%|
The most vital organs, (brain, eyes, lungs, heart, kidneys, oesophagus, abomasum, and small intestine) are relatively well-developed at birth, but have a slower growth than such organs as the rectum and rumen, which become functional with feeding.
The body regions also change in proportions in post-natal life. Body depth increases in relation to length; head and legs shorten in relation to the body. Body tissues also show differential growth after birth: bone growth precedes muscle which precedes fat. Differential growth rate occurs within major tissues; i.e. bones of the spinal cord and distal end of limbs develop before bones in the proximal end. The attached muscles also show differential growth.
Major growth changes are complete in early post-natal life. The patterns of bone and muscle growth appear to follow the animal’s requirements; e.g. long, strong legs at birth, etc.
Abdominal fat develops first, then inter-muscular fat, subcutaneous fat and intramuscular fat. The distribution of fat is genetically controlled, dairy breeds having more abdominal fat than beef breeds.
Distinctions between different fat locations may also exist between functions. Different functions may be found in one location – e.g. in a pig, the subcutaneous fat layer is divided, the top layer acting primarily as an insulation and the lower layer as an energy reserve.
Changes in the Carase Composition with Age and Live Weight
There are three important general characteristics of growth:
- As live weight increases, bone increases at a constant low rate
- As live weight increases, muscle increases rapidly
- As live weight increases, fat increases slowly at first, and then rapidly finish the animal These three characteristics may be used to estimate the tissue composition of the body.
FACTORS WHICH AFFECT CHANGES IN BODY SIZE AND COMPOSITION
- Number in the litter: Normally individuals from large litters are lighter than those of small litters.
- Size of dam: The offspring at birth is of a size determined by the size of the dam and not that of both parents. Within a breed this is not as clear-cut and length of gestation period appears to be more important than the size of the dam on the offspring’s size and weight.
- Age of Dam: A younger dam will produce smaller offspring than will a mature dam. Birth weights decline as the end of the reproductive life approaches.
- Sex: Males are generally larger and heavier at birth than females.
- Plane of Nutrition: This can have a marked influence on birth weight. Low nutrition up to 3 months of pregnancy has no effect. In the last two months of pregnancy birth weight can be affected by low nutrition. The various organs and tissues are affected differently by low pre- natal nutrition. In the case of very poor nutrition before birth, the size of nearly all the major organs in the foetus can be significantly reduced.
POST NATAL CHANGES
- Sex: Female sheep mature earlier than do male sheep. Females attain a more advanced stage of development earlier than males do. Males attain a greater size and degree of development at maturity.
- Castration: Reduces sex differences. Males castrated early do not develop secondary characteristics. Their bones grow in length but not breadth. In pigs, gilts (i.e. young sows) mature later than do castrated males. There is better development of muscle tissue in gilts than in hogs at bacon weights.
- Nutrition: A high level of nutrition causes the animal to reach a given body composition at a younger age and a lighter weight than normal. A low level of nutrition extends the time for these changes to take place.
- Generally the effects of nutrition in modifying body composition depend on the level of nutrition and the stage of development of the animal. A high level of nutrition at an early age, when growth potential is high, produces better results than the same level in an older animal. Low level of nutrition early in life retards muscle and bone growth, whilst low level nutrition later retards fat deposition. If undernourishment is excessive, deposition of bone growth continues (even when body weight is decreasing). Recovery may occur but if under-nutrition is severe enough, the animal may never recover a normal adult form. In bones, growth in length is less susceptible than thickness – thus under conditions of low nutrition bones may not increase in thickness. This change is irreversible.
Figure 2: Nutrition and the Days to Slaughter (Sheep)
High-high: High level of nutrition from birth to 100kg live weight.
High-low: High level of nutrition for 16 weeks, then low level. Low-high: Low level of nutrition for 16 weeks, then high level. Low-low: Low level of nutrition throughout.
Low nutrition in early phases reduces early developing tissues (muscle and bone). Low nutrition in late phases reduces fat deposits.
EARLINESS IN MATURITY
An early-maturing animal is one in which the changes which occur in body proportions and constituent tissues during pre-natal and post-natal life, take place relatively quickly. In a late-maturing animal, these changes take place over an extended period of time.
Early maturity is associated with adult size. Smaller breeds generally mature earlier than larger ones. The larger breed grows at a slower physiological time-rate than the smaller breed.
In the meat trade, once an animal has reached the required market weight, it is said to be ‘finished’. Due to differences in time in reaching maturity, different breeds have different ‘finished’ live weights. Below this weight, there is too much bone and muscle to carry and too much fat.
- 4. COMPENSATORY GROWTH
If an animal whose growth has been retarded is allowed a normal nutrition level, its growth rate will be much faster than an animal of the same age and type, whose nutritional levels were normal from the start. This is called compensatory growth.
At a low level of nutrition, physiological ageing proceeds at a slower rate. Once adequately fed, the animal grows at its appropriate physiological age rather than actual age. The general effect of this on mild under-nutrition is to allow the animal to attain its normal size. Over- compensation may cause the animal to exceed normal adult size. This may be associated with excessive fat deposition. Six factors which determine the extent of compensatory growth are:
- The nature of the restricted diet;
- The degree of severity of under-nutrition;
- The duration of the period of under-nutrition;
- The stage of development of the body at the commencement of under-nutrition;
- The relative rate of maturity of the species; and
- The pattern of re-feeding (or re-alimentation).
Recovery from both carbohydrate and protein restriction is usually complete. The stage of growth at which under-nutrition commences is of prime importance in determining the animal’s recovery. Under-nutrition at the point of inflexion is likely to result in the maximum inhibition. The organs that pass their inflexion points at very early ages (even pre-natal) are only slightly affected by under – nutrition. Later maturing tissues are affected more so by a period of restriction (e.g. muscles).
The recovery of an animal after under-nutrition is accompanied by:
- A longer time to reach mature weight; and
- Increase in weight gain during the adequate feeding periods (particularly early on).
The length of the growing period is restricted, and may take twice as long that of fully-fed animals. Prolonged or excessive under-nutrition will cause permanent stunting and modification of the adult form. It appears that late pre-natal and early post-natal are the critical periods.
- 5. GROWTH HORMONES
Growth is a diffuse process involving many hormones. Specific hormones involved are the growth hormone of the anterior pituitary, hormones of the thyroid, the gonads and the adrenal cortex.
This is the most important of growth regulators and is essential in mammals. It produces stimulating hormones to control other endocrine glands. It also produces somatotrophin, or growth hormone. Removal of this gland stops growth. Reintroduction of this tissue or its hormones restarts growth.
These growth hormones may act on protein metabolism (growth is normally accompanied by an increase in body protein) by increasing the rate of protein synthesis, or decreasing the rate of protein breakdown.
Thyroxin, produced by this gland is primarily concerned with the regulation of energy metabolism. If this gland is removed in young animals, growth is retarded. Low thyroid activity (hypothyroidism) is associated with low metabolic rate, poor feeding, low blood sugar and-liver glycogen, and low nitrogen retention. Fat deposition tends to increase.
An overactive thyroid (hyperthyroidism) increases metabolic rate, the breakdown of tissues (catabolism) exceeds the rate of tissue building (anabolism) and weight loss results.
Growth is profoundly affected by the hormones of the adrenal cortex. This gland produces many hormones collectively known as cortin. There are three important ones:
- Deoxycorticosterone: causes sodium chloride and water retention. It promotes potassium excretion and supports growth in the animal with no adrenal glands.
These are growth inhibitors involved in profound tissue breakdown and accelerated protein catabolism. The oxidation of protein to sugar is also inhibited. This may result in an excessive excretion of nitrogen and sugar in the urine.
Sex Hormones – Androgens and Oestrogens
These primarily control the development of size and form, the characteristic of the sex. They are essential for the development of accessory sex organs and secondary sex characteristics. These hormones are steroids.
These increase appetite and growth in castrated dogs and induce increased nitrogen retention (acting anabolically). They stimulate the growth of the accessory sex organs most effectively, but also act this way to some extent on kidney mass. Testosterone is its most potent product. This hormone increases muscle growth and increases muscular capacity in the heart.
If given in considerable doses to male animals, it profoundly depresses their growth, but if administered carefully, in small doses, will lead to an increase in growth. Different growth rates between the sexes are apparent all through life.
Female rats which are allowed to breed grow more rapidly than non-pregnant litter mates. Females with no ovaries grow faster than virgin females, but both stop growing at the same age. Oestrogen secreted by the ovaries in virgin rats may inhibit growth. After castration, this inhibitory action is removed.
Progesterone may cause this increased rate of growth in pregnant rats by inhibiting oestrogen,
although it is thought that progesterone plays a more active role. Rats bred late in life do not gain as much advantage from the progesterone as rats bred early or at the normal time. Thus they do not attain their potential.