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PERSPECTIVE ON GROWTH AND DEVEIOPMENT* H. B. HEDRICK University of Missouri, Columbia, Missouri

The production of meat and all other food is almxt entirely dependent upon the growth process. The growth phemmenon is the heart of the livestock and meat industry both frm the standpoint of animal growth and the production of most of the raw materials which animals convert to meat and other usable by-products. A basic understmding of growth has potential for direct application to problems that confront world fmd production. The need for increased food production and availability were never greater than at the present time. When food is scarce, its availability becomes a highly emotional issue and has great influence on world economic and political issues. Thus, the growth of animals and plants is an underlying factor that influences many major world problems.

Many people of the world advocate that foregoing meat would free grain for the hungry. The truth of the matter is that animals cannot be ignored if one is to consider seriously the food-population-environment problem. Animls are providers of meat and by-products for the human population. A very important factor in favor of animal agriculture is man's dependence on an almost endless list Df by-products obtained from animals which are used for food, clothing and health aids. Some pharma- ceuticals originally Dbtained from meat animals can be manufactured, but meat animals remain the best source for many of these products. For example, insulin, needed by many diabetics to regulate their blood sugar, is obtained from the pancreas of meat animals. Those who advocate foregoing meat fail to realize, either intentionally or through lack of knowledge, man's dependence upon animals for numerous by-products. The meat packing industry has long been noted for its efficiency in the processing and utilization of by-products.

A large portion of the world's land is unsuited for intensive agriculture. Consequently, a ruminant livestock industry is essential to providing food for an increasingly hungry world. In the future these animals must become increasingly greater converters of plant resmrces which are not usable directly by man. The possibility of increased allocations of available feed grains to human food poses problems for future swine and poultry production. To meet this challenge alternative sources of protein and energy for use by these animals must be sought. An important research challenge to animal agriculture is to increase the efficiency of all animals to convert feed grains, roughages and other materials to meat and by-products.

* Presented at the 28th Annual Reciprocal Meat Conference of the American Meat Science Association. 41 To meet these challenges, an understanding of the growth process is essential. Not all aspects of growth are completely understood, particularly the mechanisms involved in initiation of growth, regulation of growth rate and termination at maturity. However, much that is known about animal growth remains to be applied at the production level.

The attainment of scientific infornration concerning growth, which abounds at the present time, can trace its ancestry to earliest recorded history. For example, as early as the fifth century B.C., Greek physicians developed a method for the study of growth which is employed by scientists today. A hen was set upon a number of eggs and each day one of the eggs was opened and the changes that took place were observed. This miraculous transformation of the white and yellow mass of a hen's egg into a respectable befeathered chick all in 21 days must have been as exciting to the early Greeks as it is to today's school child.

Early work concerned with the body composition and growth of meat animals dates back to the often cited study of beef animal composition of Lawes and Gilbert (1859). During this same general time period German workers were studying the development of adipose tissue (cited by Bell, 1909). In the U .S ., concerted research work on growth and development began in the early part of the present century.

Among the earliest investigations conducted in the U.S. on growth and developnent was the comprehensive one initiated at the University of Missouri in 1907 as a part of the "Use of Food" investigations. H. J. Waters received the credit for the general plan of the experiment which he outlined as "an experiment to determine: (a) if an immature animal can use its stored fat to protect growth when sparsely nourished, and to what extent the body fat may be relied upon to supplement a limited ration to insure the continuation of the process of growth; (b) what changes occur in the composition of the body of immature animals when fed for a considerable time on a maintenance ration, and also what changes occur when such animals are kept on a ration above maintenance, but not in sufficient quantity to supply the maximum growth of which the animal is capable.

The investigation initiated by Waters and other investigations conducted at Missouri yielded numerous publications detailing body composition of beef animals during growth and development, and energetic efficiencies related to meat and milk production. During the past 50 years, U .S . Agricultural Experiment Stations, medical schools, research organizations, and various research groups from other countries, princi- pally in Europe, have turned out a deluge of publications concerned with growth and development. When Samuel Brody published his book "Bioenergetics and Growth" in 1945 he cited over 2000 references most of which were related to growth and development and energetic efficiencies of animals. The flow of information is likely to continue in the future because there is still much to be learned about this important phenomenon. 42

In general, what do we know about growth and development of meat animals and what aspects of these processes should we be concerned with in the future? The variation that exists among animals is due to heredity and environment. Heredity provides the necessary potential for growth and development and the environment will maximize or minimize this potential.

Other hypotheses of grmth regulation have been suggested which possibly would not be included under the heredity-environment means of growth regulation. An extensive search of the literature will reveal that hypotheses abound as to how growth is regulated in tissues and organs. These hypotheses include inhibitors; stimulators; combinations of inhibitors and stimulators; organs and tissues controlling their own growth; a central control center for controlling growth of the entire body; functional demand determinlng growth of an organ or tissue; etc. The establishment of a cmplete explanation of how growth is regulated remains a task for the future. When this is accomplished we should have an answer for the cause and cure for cancer and for dystrophic conditions which plague mankind, in addition to being able to control growth and development of meat animals.

The importance of heredity in meat animal production is paramount. Robert Bakewell's recommendation was to breed the best to the best because like begets like. Your contribution to the future of mankind you carry in your "genes" is an old pun to illustrate this point. This is quite timely since l'jeans" are common attire for both boys and girls. Animals within a given breed and among breeds may vary in growth rate, body composition, growth efficiency, and reproductive performance when placed In a suitable environment and given an adequate diet because of variations in hereditary constitution. Those animals that have preferred characteristics should be identified and their numbers increased while those with less desirable characteristics should be decreased in numbers. Because of diverse environmental conditions which exist in the U.S. and the world, there is no one type of animal best suited for all conditions.

Although heredity dictates maximum growth and development possible, nutrition governs the rate of normal growth and the extent to which development is attained. The utilization of ingested nutrients is partitioned among various tissues and organs according to their meta- bolic rate and physiological importance. Maintenance and function of vital physiological systems take precedence over muscle growth and fat deposition. The order of precedence is first tissues comprising vital organs and physiological processes, second bone, third muscle, and lastly fat deposition. When the nutritive supply is plentiful all tissues of the body receive sufficient nutrients for maintenance and for normal growth and fattening. However, if the nutrient supply is limited, the tissues are affected in reverse order of physiological importance. Severe restrictions of nutrients will result in body tissues of less importance being utilized to maintain those of more vital importance. The various tissues and organs of the body of a growing animal which have been retarded in development by a restricted environment may exhibit remarkable compensatory growth when changed to a favorable environment. It is 43 posslble to control the rate at which different tissues and ~rtsof the body grow and develop by: altering the nutritional level of the animal and selecting the time at which the nutritional level is altered. The stage of postnatal growth over which the nutritional treatment is imposed will affect the nature of the response. The efficiency of meat animals to convert feed consumed into meat is generally related to the level of feed intake, rate of gain and body weight.

A balanced diet consisting of water, protein, energy, minerals and vitamins is necessary for animals to attain their maximum inherent growth and development. The dietary requirements differ among species and vary during the various stages of growth and development. Although a vast amount of research has been conducted to determine the dietary requirements of meat animals and more is known about the dietary requirements for meat animals than for man, much is not known about this important aspect of meat an-1 production. For example, least-cost pr3tein requirements, unidentified growth factors, and trace mineral requirements.

The function of various tissues and organs plays an important role in determining size or extent of growth. Brody (1945) concisely stated that "the organism changes geometrically so as to remain the same physiologically ." This statement implies an interrelationship between size and function. It is also recognized that hypertrophy and atrophy of tissues and organs are generally the result of use and disuse, respectively.

During the prenatal phase of life various organs, systems, and tissues grow and develop at different rates. Continuous changes in chemical composition and conformation of the fetus occur prenatzlly. Considerable informtion is available on the genesis of the various tissues but the exact nature of the events occurring during the genesis of the wrious tissues is not known. The effects of intra-uterine environment on the ultimate potential for growth and development of an anhlhave not been fully elucidated. The composition and particularly the protein content of the developing fetus is more dynamic than during the postnatal period of growth and development.

Nitrogen metabolism is one physiological process most intimately linked with the phenomenon of growth. This includes the synthesis and anabolism of protein and its interactions with other constituents of the body to form and maintain specific cells and tissues. For example, muscle growth involves synthesis of specific protein monomers, alignment of these into structured elements peculiar to this tissue, and development of fibers according to muscle type and function. Since the advent of the electron microscope, various researchers have characterized the ultra- structural features of muscle and other tissues.

The importance of nutritional, neural and hormonal influences on growth of muscle and other, tissues are well recognized, but how these factors are translated into the specific protein synthetic response in the tissues is not understood. During growth and the entire life of an 44 animal, muscle and all other cellular constituents are involved in continuous processes of anabolism and catabolism. The rate at which these processes occur is variable for different tissues and constituents of various tissues. It appears that the various parameters of protein synthesis change in a congruous manner during the process of normal growth. The rate limiting step or steps in protein s7Jnthesi.s remains to be identified and the quantitative significance of the factors which affect the rate of protein synthesis remains to be established. The catabolic and anabolic processes are known to be influenced by m8ny factors such as age of animal, stress conditions, nutritional level, certain hormones, etc. Thus, one can truly say that the essence of life is change. Papers presented at the Symposium on Protein Synthesis and Muscle Growth held during the 65th Annual Meeting of the American Society of Animal Science and published in the May issue of the Journal of Animal Science cover the topic in considerable1974 detail.

The accretion of fat in the body during postnatal growth and development is the result of animals being fed for extended periods of time at levels in excess of their maintenance requirement. Quantita- tively, fat is the most variable constituent in the body and in muscle. Growth of muscle is not dependent upon an increase in fat content, but generally is accompanied by an increase in fat content in the form of intramuscular and intracellular lipids. Breed of animal, sex condition, caloric intake, and physiological age are some of the main factors that influence fat deposition in muscle and in the body. "he principle factor considered in beef, lamb and pork grade standards is the amount and distribution of fat.

Traditionally it has been thought that ariimals must be fattened rather excessively to provide acceptable quality meat. This practice of excessively fattening animals is not necessary to have meat products of acceptable quality. Slaughter of animals at the optimum stage of gr9wth and development, proper processing and cooking are more important factors that influence eating characteristics than the addition of excess quantities of fat. Excessive fattening of animals is a waste of feed grains and roughages resulting in enormous quantities of waste fat being trimmed from carcasses, wholesale cuts and retail cuts. A major challenge for the livestock industry in the future is the reduction of the excess fat production. Perhaps this can be done by management systems which divert more nutrients to protein synthesis and less to fat synthesis. Specifically, there is an optimum stage or point on the growth curve where animals should be slaughtered for optimum meat production in relation to nutrients consumed. This point on the growth curve varies among animals of the same species in terms of body weight and chrono- logical age. In the future more attention needs to be given to this established fact, particularly if we are going to be concerned with energy conservation from the standpoint of m,imizing the efficiency of meat animals to convert feed grains and roughages into edible meat. Some of our live animal management practices could be revised to help accmplish this goal. The male bovine, for example, is a more efficient converter of nutrients into muscle than is the castrate. It is rather 45 ironical that we castrate the male bovine and then feed or implant hormone-like compounds for essentially the same effect as the intact testes.

Normal prenatal and postnatal skeletal growth is accomplished by cartilaginous and osseous formation and removal at selective sites. Cartilage formation and rate of ossification varies among species and among individuals within species. Throughout life, bones undergo continuous remodeling. These processes are influenced by sex associated hormones, nutritional level 2nd dietary mineral intake. Termination of growth does not occur simultaneously in allbones, e.g., the sacral vertabrae reach maturity before the lumbar, and the lumbar before the thoracic vertebrae.

During prenatal and postnatal growth changes occur in body form and composition. In order for an animal to maintain homeostasis the body must change in form during growth or increase in weight. For example, length of body and legs are proportionately greater in an immature ankilthan in a mature animal. As an animal grows, there is a decline in growth rate in terms of depth and thickness of the body. The proportions of body components, carcass, hide, organs, stomach, intestines, etc., change during growth. The proportim of muscle, fat and bone in the carcass changes during growth. An increase in proportion of fat results in 8 concurrent decline in proportim of both muscle and bone. The rate and extent of change is variable among animals of the same species and is influenced by factors such as level of nutrition, breed, sex condition, environmental conditions, health, etc. The chemical composition of muscle, bone and fat changes during growth. The concentration of intracellular proteins increases, water decreases, lipid usually increases, and minerals increase duririg nom1 gmwth. The amount of connective tissue in muscle increases more while muscle cells are increasing in number (prenatal growth) but becomes proportionately less while cells are increasing In size (postnatal growth) due to dilution. The tDtal amount of connective tissue in muscle varies among muscles. Connective tissue fibers (collagen) increase in strength during growth due to chemical cross-linkages within the collager, macro-molecule. Carbohydrate is present in muscle and liver, mainly in the form of glycogen. The glycogen content of these tissues is highly varizble among different species, among different muscles of the same animel and with different physiological states. The structural fxm and composition of an animal at any particular time is only temporary.

Historically, nearly every proposed change that could conceivably have influenced the characteristics of meat or meat products has received some opposition either from some segment of the industry or from the consumers. For example, when the recent proposal for changes in beef grade standards was issued several individuals and groups opposed the proposal and claimed that a reduction in a degree 3r less of marbling in some carcasses would result in essentially an unacceptable product. The majority of the research on meat quality refutes such 46 claims. However, in this instance, experimental data were intentionally ignored or not considered due to lack of information. Perhaps we as meat scientists are not doing our job in conveying the facts about meat to the industry and to the consumer. We need to do a better job of disseminating the facts ourselves and not sit back and let the perpetual uninformed orator or journalist do it for us.

The challenge for meat and animal scientists in the future is to effectively promote more efficient meat animal production in order to better serve the needs for the well-being of mankind. To do this, meat and animal scientists need to work more closely as a team with geneticists, nutritionists, economists, technologists, politicians, and social scientists. Much could be accomplished by more effective use of present knowledge concerning meat anal production and utilization, and especially growth and developnent. A great deal is known about growth and developnent and as in other fields, additions to this know- ledge have been piecemeal. Many pieces of the growth and development puzzle have been assembled in their proper places. The complex puzzle will be completed and hopefully the era for completion is near at hand.

Selected References

Ashmore, C. R. 1974. Phenotypic Expression of muscle fiber types and some implications to meat quality. J. Anim. Sci. 38:1158.

Bailey, Milton E. and S. E. Zobrisky. 1968. Changes in proteins during growth and development of animals. Body composition in animals and man. National Academy of Sciences. Publ. No. 1598.

Beecher, G. R. 1974. Some practical applications of protein synthesis and muscle growth. J. Anim. Sci. 38:1071.

Bell, E. T. 1909. I. On the occurrence of fat in the epithelium, cartilage, and muscle fibers of the ox. 11. On the histogenesis of the adipose tissue of the ox. The her. J. of Anatomy 9:kOl.

Bergen, 'Werner G. 1974. Protein Synthesis in animal models. J. him. Sci . 38 :1079. Brody, Samuel. 195. Bioenergetics and Growth. Reinholdt Publishing Corporation.

Forrest, John C., Elton D. Aberle, Harold B. Hedrick, Max D. Judge and Robert A. Merkel. 1975. Principles of Meat Science. W. H. Freeman and Company.

GOSS, Richard J. 1972. Regulation of Organ and Tissue Growth. Academic Press.

Hafez, E.S.E. and I. A. Dyer. 1969. Anha1 Growth and Nutrition. Lea and Febiger. 47 Hedrick, H. B. 1967. Bovine growth and composition. Mo. Agr. Exp. Sta. Res. Bul. 928, N.C. Regional Publ. No. 181.

Hedrick, H. B. 1972. Beef cattle type and body composition for maximum efficiency. J. him. Sci. 34:1103. Lawes, J. B. and J. H. Gilbert. 1859. Experimental inquiry into the composition of t'ne animals fed and slaughtered as human food. Rothamstead Experimental Reports. 493.

Strohman, R. C., B. Paterson, R. Fluck and A. Przybyla. 1974. Cell fusion and terminal differentiation of myogenic cells in culture. J. Anim. Sci. 38:1103.

Stromer, Marvin H., Darrel E. Goll, Ronald B. Young, Richard M. Robson and Frederick C. Parrish, Jr. 1974. Ultrastructural features of skeletal muscle differentiation and developmelzt. J. Anim. Sci. 38:1111. Swatknd, H. J. and R. G. Cassens . 1974. The role of innervation in muscle development and function. J. him. Sci. 38:1092.

Swick, Robert M. and Harriet Song. 1974. Turnover rates of various muscle proteins. J. Anim. Sci. 38:ll5O. Thompson, William C . and Stuart M. Heywood. 1974. Basic aspects of protein synthesis in muscle. J. him. Sei. 38:lO5O.

Trenkle, Allen. 1974. Hormonal and nutritional interrelationships and their effects on skeletal muscle. J. him. Sci. 38:1142. Trowbridge, P. F., C. R. Moulton and L. D. Haigh. 1916. Effect of limited food on growth of bEef animals. Mo . Agr . Exp. Sta . Res. Bul. 28. West, R. L. 1974. Red to white fiber ratios as an index of double muscling in beef cattle. J, Anim. Sei. 38~165.

Young, Vernon C . 1974. Regulation of protein synthesis and skeletal muscle growth. J. him. Sci. 38:1054.