Concepts in Growth and Development


"Quality Management" and "Conception to Consumption" are now trendy themes for conferences and annual meetings in food animal agriculture, but seldom do we find any real attempt to reach the stated objectives. Think of a comparable situation in a manufacturing industry - such as car manufacturing - to grasp the magnitude of the problem. To understand the complete industrial sequence, you must start with the geology of iron ore, work through the physics of metallurgy and the mathematics of mechanical engineering, and conclude with the psychology of customer preferences. In our own industry, there are many individuals who are knowledgable about one sector, such as animal production, slaughtering, or retailing, but very few who have a grasp of the total system. As butchers, we may deplore farmers who foolishly provide us with carcasses that are too fat or too heavy, but do we understand the issue from the other viewpoint?

What Is Growth?

It is difficult to give a perfect definition of meat animal growth because many of the changes involved are reversible. If an animal increases its live weight by drinking water, do the resulting increments to live weight really constitute growth? Yet, meat often contains 80% water, and it must come from somewhere. If an animal increases its body weight by accumulating fat between and within its muscles, most people might accept that these are true growth increments. Yet, these fat depots readily might be lost if the animal is placed on reduced feed. Likewise, even the myofibrillar proteins of lean meat may be used as an energy reserve in fasting animals, although the growth of the vital organs and nervous system is usually considered to be practically irreversible.

Roughly speaking, growth is an increase in body height, length, girth and weight that occurs when a healthy young meat animal is given adequate food, water and shelter. Live weight is the most important and commonly measured of these features and, if recorded at regular intervals, it may be used to plot a simple growth curve. For the practical purpose of determining feed conversion efficiency, the same data may be expressed as a curve for rate of gain. When live weight data are examined for the small differences in growth that may be important when large numbers of animals are being raised, even the measurement of live weight becomes a problem, because of weight changes from defaecation, urination and mud on the hide. Thus, in experimental conditions it is preferable to use measurements of body weight that have been mathematically reassembled from the weights of emptied viscera and clean hides obtained after slaughter.

Measuring Growth

However, absolute measurements of body composition can only be made after an animal has been killed and then, of course, further growth is impossible. There are several solutions to this problem, but none of them is entirely satisfactory. Firstly, the animals may be kept alive and allowed to grow, and the meat yield measured with a non-destructive method of limited accuracy. If nondestructive measurements of growth are made from animals kept alive and remeasured at intervals, the data require careful statistical analysis since each measurement on the same animal is partially related to preceding and succeeding measurements. As a second solution, a homogeneous group of animals may be individually slaughtered in a sequence that enables the growth attainment of living animals to be visualized. The problem here is that any differences between the animals are superimposed on the visualized growth sequence. For example, a slow-growing animal slaughtered in sequence after a fast-growing animal may lead to a dip in the reconstituted growth curve. A third solution is often used in the comparison of breeds or diets: the animals are grown to a set point, slaughtered, and then compared. But then the problem is to identify an equivalent slaughter point at which comparisons may be made. Animals may be slaughtered when they reach a set age, degree of fat deposition, or live weight, but these may differ between breeds or sexes, and different slaughter end points may give different results.

Although studies of carcass composition have an important place in agricultural research, they are not the best means of investigating the process of growth itself. Linear measurements are largely a reflection of skeletal development rather than muscular development, and it is difficult to make accurate skeletal measurements on large animals when they are still alive. With poultry, however, the measurement of linear dimensions may be useful for studying growth and as a basis for genetic selection. Curvilinear measurements made round the girth of the hind-limb of a meat animal are a reasonable index of muscle development, but these are difficult to standardize.

Surface to Volume Ratio

Imagine a simple geometrical structure such as a sphere: if it increases its height, its surface area will increase in proportion to the square of height, while its volume will increase in proportion to the cube of height. If isolated animal cells are grown in suspension in a constantly replenished medium, they form balls of cells. These grow up to several millimetres across, until they reach the limit to which radial diffusion and surface to volume ratio may supply nutrients and oxygen, and the limit at which waste products may still be removed from the center of the ball. If these cells were primitive life forms early in geological time, little further evolutionary increase in size would be possible until they had solved these problems. One solution is gastrulation, to increase the surface to volume ratio by becoming cup-shaped (like pushing a finger into a hollow, soft ball). Another solution is to develop a blood vascular system to enhance the transport of nutrients, oxygen and waste materials. Even for a ball of cells growing in a laboratory dish, however, there are often subtle factors that limit or regulate growth. Biophysical studies with microelectrodes have revealed mechanisms by which cells communicate with each other and "agree" on which cells should divide.

As meat animals grow from birth to slaughter weight, they do not maintain a constant shape. In general, however, even a small increase in linear dimensions causes a proportionately greater increase in body weight because body weight is a function of volume. In newborn farm animals, the rate of heat loss from the body may be a serious problem. In small animals, the surface area for heat loss is relatively high, while the volume of muscle capable of generating heat is relatively small. Since animals maintain an approximately constant body temperature during growth, there must be subtle interactions between body size and the mechanisms which regulate body temperature. Basal metabolism (energy use in a resting animal) is proportional to body weight raised to the power of 0.75, rather than to the power of 0.66, which would be expected on simple geometrical grounds.

Concept of Development

Among meat animals, it is difficult to find a situation in which an animal may grow to any marked extent without also exhibiting development of one type or another. Whereas growth may be considered as an increase in height, length, girth or weight, development may be considered as a change in composition, structure or ability, although neither definition is completely satisfactory.

In functional systems such as the locomotory system, development may occur in direct response to growth in live weight. For example, as an animal grows heavier, it may need to use more of its muscle mass to oppose gravity, and there may be changes in the physiological properties of the diverted muscle mass. Similar functional changes occur in the digestive system, bones and other body systems. After growth has ceased in the adult, developmental changes continue as the animal passes through maturity to senescence. Developmental changes are directed towards the attainment of a mature composition, structure or ability. However, the retrogressive changes that occur later in life and which are associated with a decline in composition, structure and ability, are also developmental changes (senescence). Development and senescence are merely the early and late stages of ontogeny, the progress of an individual animal through its life cycle.

Although some aspects of growth such as fat deposition appear to be reversible, this is rarely true of developmental processes. As an animal is growing, a vast number of developmental changes are taking place concurrently. These changes are not usually undone if an animal simply loses weight. For example, a reversible accumulation of the triglyceride stored in fat cells may be accompanied by an increase in the number and size of fat cells, but the loss of a moderate amount of triglyceride may occur by all cells releasing a share of their triglyceride. Thus, after "degrowth" appears to have occurred, the animal has not reverted to the developmental state (adipose cell number) it had before the period of growth started.