Animal Growth and Development Process

November 13, 2023 Agric4Profits

Obtaining such products as meat, milk, and eggs would not have been possible if the animals were not in the proper physiological state. Reaching the proper state is determined by the process of growth.

In animal production, growth is perhaps the most significant factor that determines its efficiency. Several mechanisms are involved in growth but not all of these are fully understood.

Nevertheless, a good understanding of the animal growth and development process is necessary in order to exploit the benefits of animal production.

What are Animal Growth and Development?

There is no single definition for growth. Growth may simply be defined as an increase in size. It could be an increase in the weight of an animal or any part of the animal as it approaches mature body size.

Growth is an increase in body height, length, girth, and weight that occurs when a healthy young animal is given adequate food, water, and shelter. It is a fundamental characteristic of all living organisms.

Growth can be more specifically defined as a normal process of increase in size, produced by the accretion of tissues similar in constitution to that of the original tissue or organ. This increase in size is accomplished by any of these processes:

Hypertrophy – an enlargement of cells

Hyperplasia – an increase in the number of cells

Accretion – an increase in the quantity of non-cellular material

Growth can be considered in two aspects of animal production. Firstly, true growth, which comprises an increase in the structural tissues such as muscle, bone, and vital organs. Secondly, fattening, which comprises an increase in the deposition of fat or adipose tissue (where fat is stored).

Growth usually involves an increase in size and development. Growth, in terms of increase in size, can occur without development but development cannot occur without growth.

Development is a gradual progression from a lower to a higher stage of complexity as well as gradual expansion in size. This increase in complexity involves differentiation and specialization for increased functionality.

Differentiation is the process by which cells and organs become unique and acquire completely individual characteristics such as simple cells of the embryo diversifying into muscle cells or brain cells or cells of different organs.

Differentiation also involves morphogenesis, which involves the organization of various dividing cells into specific organs. Differentiated cells undergo maturation, which is when they become fully developed.

Fully mature tissues have attained their highest stage of complexity. The end of growth and development is senescence when individual tissues and organs are no longer maintained in their mature form but begin to degenerate without complete replacement or repair (Forrest et al., 1975).

The growth of animals commercially exploited in animal production can be divided into two phases, prenatal and postnatal. Animals had started growing before being born, and thereafter continue to grow until slaughter or the period when products are harvested.

1. Prenatal Growth

Growth in the prenatal period involves many distinct phases that ultimately end in birth. It begins when a fertilized ovum divides rapidly to form a cluster of undifferentiated cells called a morula, which differentiates to form a blastocyst.

The blastocyst has three layers of cells (called germ cells), which are ectoderm, mesoderm, and endoderm. It is from these germ cell layers that all the parts of the body are formed.

Ectoderm: skin and hair; neural tube, which forms the brain and spinal cord

Mesoderm: somites, which form osteoblasts and mesenchyme; osteoblasts form skeleton; mesenchyme forms myoblasts and fibroblasts; myoblasts form muscles, and fibroblasts form connective tissue.

Endoderm: digestive tract, liver, pancreas, lungs, and bladder All these tissues and organs are formed before birth.

Birth weight may be an indicator of how well the prenatal growth period has been. Birth weight could be affected by sex, maternal age, nutrition, breed, and gestation length.

The longer the gestation length, the greater the birth weight; males are carried longer than females and therefore, generally have higher birth weights.

Severely restricted maternal nutrition affects the weight and vigor of the newborn, and the older the mother the greater the birth weight. Early maturing breeds have a lighter or lower birth weight than late maturing breeds.

2. Postnatal Growth

Immediately after birth is a period of slow growth followed by a rapid increase in mature body size after which the growth rate slows down.

The postnatal growth of all animals follows a sigmoid curve pattern when weight is plotted on a graph against time. This curve can be divided into three growth phases, lag phase, log phase, diminishing growth phase, and stationary phase (Figure 1).

Animal Growth and Development Process — Figure: Animal growth curve

During the lag phase (the period immediately after birth), the rate of growth is very slow. The rate of growth of muscles, bones, and vital organs, then increases rapidly during the exponential phase.

After some time the growth rate of these tissues slowly diminishes due to less efficient nutrient utilization, and fattening begins to accelerate. As the animal reaches mature size, growth begins to retard. This phase constitutes the stationary phase.

After birth, the structure and shape of an animal change as size increases due to different growth rates among various tissues of the body, bone, muscles, and fat. Each of these tissues reaches a peak production rate at different stages of the animal’s life.

The nervous tissue is well developed before the animal is born so as to initiate processes necessary to support life. The first peak of growth occurs with the bone tissue, followed by the muscular tissue required to cover and attach to the bone.

Afterward, fat tissue predominates. For example, the proportion of fat in the body of the immature animal is less than that in the mature animal, as well as, the proportion of skin and hide.

Postnatal growth may be affected by many factors, which can be categorized as endogenous (within) and exogenous (outside) factors.

Endogenous: Genetics or breed; sex; and hormones.

Exogenous: Level of nutrition; health; and environment.

For example, cattle raised for meat (beef cattle) have a higher muscle-to-bone ratio than cattle raised for milk (dairy cattle); and some animal breeds reach maturity earlier (early maturing) than others (late maturing).

Sex influences carcass composition because males grow bigger and use feed more efficiently than females or castrate (animals whose sex organs have been removed, either male or female).

In addition, the lack of adequate somatotropin (growth hormone, responsible for bone elongation and uptake of nutrients by cells) results in a severe restriction in growth. Hormones are under genetic control.

Animals given a low level or low plane of nutrition at the exponential phase and switched to a higher plane at the latter growth stages have higher proportions of fat at maturity than those with a higher plane at the exponential phase switched to a lower plane subsequently.

Various diseases affect the physiological state of animals and unhealthy animals will not grow well. The commonest indication of a disease state is anorexia (loss of appetite). Animals perform satisfactorily within an ideal temperature range of 15^oC – 25^oC.

A lower temperature (outside the ideal) does not affect an animal or its growth as much as a higher temperature because a very high temperature discourages the animal from eating.

Growth is usually measured as a change in weight (gain or loss) over time. The time period could be in days, weeks, months, and years. When the gain is measured per day, it is referred to as average daily gain, which is the rate of weight increase each day of the time period.

Growth rate = Final weight – Initial weight / Time period

Growth can also be measured by using graphs by plotting weight against time.

Finally, growth can be measured by the use of growth equations (models). These equations describe/predict the relationship between weight and time and are usually complex to resolve. Some popular models are the Gompertz model, Robertson model, von Bertalanffy model, and Taylor model.

In conclusion, a good knowledge of growth and development and various factors affecting different stages in the life-cycle of production animals helps in the development of proper strategies for satisfactory and efficient production.