Growth in Meat-Producing Animals and Poultry
26.4.1 Growth of Organs
In general, organs and tissues grow at different rates relative to each other as well as to the whole body. Even though their growth rate is genetically determined, several other factors, viz., nutrients supply, endocrine and growth factors also control their growth.
Generally, growth pattern of all tissues and organs follows the sigmoid or S-shaped growth curves.26.4.2 Isometric Growth
Any organ or part of the body that grows at a same rate as that of the general body growth is known as isometric growth.
26.4.3 Allometric Growth
Any organ or body part that grows at a different rate than the general body growth is known as allometric growth.
The growth of tissues or organs in the body follows both isometric and allometric growth patterns during various stages of life. During the prenatal growth period, some systems developed more rapidly than others depending on the functional needs of the body and the developmental stage of the animal. For example, during prenatal phase nervous system developed at maximum rate followed by the circulatory system, connective tissues, bones, muscle, and finally the adipose tissues. However, during the postnatal growth period, initially loins grow rapidly followed by the pelvis, thorax, head, and finally the legs.
In general, growth and development of meat animals consist both increase in total body cells and differentiation of these cells. Due to differentiation, some tissues, organs, or parts of the body grow at different rates and mature at different time from that of the whole body which results in a change in shape and size of the meat-producing animals. For example, the head grows faster in early life and proportionally larger, while other parts like limbs make rapid growth later and form a larger proportion of the body weight. The shape, size, and composition of animals vary continuously during the growth period.
The major body tissues show differential growth patterns in the order as follows: the skeleton, muscle, and fat. Rate of growth of different tissues in animals is as follows: nervous system followed by bone then fat. Fat is the last tissue to mature in the body. There is a difference in maturation exists within the fat tissues based on their location of deposition. Orders of maturation of various fat tissues in animals are as follows: perinephric fat, intermuscular fat followed by subcutaneous fat, and intramuscular fat.Different sequence of maturity of tissues or organs has major implications for the growth of meat animals with desirable carcass quality. For lean meat production, animals may be killed before much fat deposition has been taking place. There is a variation in the composition of animals as it grows due to the differential growth rate of different parts of the body. Muscle, bone, and fat are the major tissues of carcass and they show differential growth rate during the development of meat animals. Differential growth of these tissues determines carcass composition and quality. Age and weight at slaughter are also some of the important factors that influence the carcass composition. As age and weight increase, muscle and bone percentage decrease and fat percentage increases.
26.4.4 Muscle Growth
Muscle is the major body tissue by weight basis that increases in size after birth by an increase in the size of myoblast cells. Muscle fibers are formed by the fusion of single nucleated cells called myoblasts which in turn form multinucleated myotubes. These are immature muscle fibers. They elongate into mature fibers. Only a few cells have retained their replication potential called satellite cells. The muscle fibers constitute more than three-fourth of the muscle mass and also its size and number determine the muscle mass of the animal. Muscle fibers number are genetically determined and fixed at the time of birth. Hence, the postnatal muscle fiber weight is increased due to increase in its diameter and length.
The size of the muscle fiber is affected by both age and nutrition. During early stages of growth, muscle fiber diameter increases significantly then progressively slow down till mature size is reached. Animals having larger number of muscle fiber at birth grow faster than animals with smaller numbers. Once the muscle fiber reaches its maximum growth further weight gain must be due to fat deposition. Therefore, for those animals that attained maturity, the increase in muscle weight increases without an increase in muscle fiber size could be partially clarified on the basis of intramuscular fat deposition.26.4.5 Protein Deposition
Maintenance of protein concentration in the body involves controlled regulation of protein turnover. The protein deposition in farm animals depends on the balance between the rates of protein synthesis and degradation in tissues. This process is affected by various factors such as nutritional status, tissue types, environmental conditions, and general physiological status. Growth rate have been correlated with different rate of protein synthesis. Protein synthesis in the body is an energyconsuming process and a considerable amount of energy is used to form the activated amino acids to be linked together. Protein-containing tissues are being continuously turned over. For one unit of net protein deposition, about 5 units of protein are being synthesized. About 15-33% of energy needed for maintenance is being diverted towards the maintenance of the existing protein and synthesis of new protein. Protein deposition in animals has negatively correlated with growth and body weight gain. Various factors regulate protein deposition in animals. Endocrine status of an animal is a major factor that governs the growth rate and the relative rates of protein and fat deposition. Normally the rate of protein deposition is positively linked with the rate of protein degradation due to the consequence of the need for remodeling of the tissues as it grows.
The balance between the synthesis and degradation of protein gives fine control to the overall rate of protein deposition. The rate of protein deposition also depends on the protein quality, energy content of the diet, and protein intake by the animals. Dietary protein and energy intake increases protein deposition since it requires energy and also dietary protein can contribute to the energy demands of the animal. Supplementation of additional energy resulted in an increase in protein synthesis and decrease in protein degradation that ultimately leads to enhanced net protein deposition. Additional protein supplementation with constant energy caused an increase in both protein synthesis and protein degradation which resulted in a smaller net increment of protein deposition. Protein metabolism is influenced by the hormonal status of the animal. Growth hormone concentration positively correlates to protein deposition. In contrast, catabolic hormones corticosteroid decreases the protein synthesis and deposition in muscle tissues and increases the degradation of muscle protein. The concentration of insulin favors the protein synthesis and amino acids uptake by the cells lead to protein deposition. Male hormone androgen tends to stimulate protein deposition in animals. Estrogen tends to stimulate protein synthesis and deposition indirectly by stimulating the secretion of growth hormones or thyroid hormones.26.4.6 Fat Deposition
Fat cells are called adipocytes. Its production and deposition are normal during growth and development of the meat animals. The rate of fat deposition has shown great variation in different parts of the body. Their size differs in different fat depots and different stages of growth. Fat tissues are widely distributed throughout the body and vary between species. Based on their deposition in adult meat animals it is classified as
(a) Subcutaneous (between the skin and muscle) fat
(b) Intermuscular fat (between muscles)
(c) Perinephric fat (surrounding the kidney)
(d) Intramuscular fat (within the muscle/between the muscle fiber) also called marbling
(e) Omental fat (surrounding the stomach)
(f) Mesenteric fat (surrounding the mesentery of intestines)
Carcass quality is determined by the amount and distribution of fat.
Intramuscular fat in the carcass is desirable since it gives flavor and juiciness to the meat whereas fat accumulation in the body cavity and surrounding the organs decreases the carcass quality.26.4.7 Growth in Birds
In general, growth is a complicated process that is regulated by various genes, hormones, nutrients, and environmental factors. Growth in avian species is divided into five phases such as embryonic, post-hatch, prepubertal, pubertal, and adult. Time required for an egg to hatch differs among the species. Fertilized chicken egg takes 21 days for hatching.
However, the sequence of developmental process is very similar to all species of birds. Growth patterns are typically s-shaped like other livestock species. After chicks are hatched, they required supply of water and protein-rich feed, and most importantly warm environment. Visible growth changes in growing chicks will be evident during the 5-6 weeks of age. During this period the teenage (prepubertal) female is referred as pullet whereas male is called as cockerel. Gender differences are more evident between the weeks 7 and 15. During the prepubertal stage birds require adequate amount of protein, vitamins, minerals, and electrolytes for optimal growth and production. Around 15-18th weeks, birds start to lay eggs. Once the pullet started laying egg, female birds become hen and male birds become rooster. During the egg-laying stage from 18 to 72 weeks, birds requires essential amino acids, vitamins, and minerals, particularly at right ratios of calcium and phosphorous for improved growth performance, good quality egg, and strong bones. Growth rates vary among the species and also within the species depending on the feed availability and surrounding environment. Bigger size birds grow more slowly than smaller birds. Around 72 weeks of age, molting may begin. During molting stage, protein supplement is very much required for the regrowth of feathers.
26.5