Growth Manipulation
Enhancement of growth rate is possible by the use of various growth promotors, feed additives, and growth factors which will reduce the production cost of the animals.
26.5.1 Growth Promotors
Growth promoters are substances that are used in feed as additives/supplements or injections as a medicine to increase animal production by increasing body weight and product output.
There are different types of growth promotors used in the animal and poultry industry to augment growth and production. These can be classified based on their mode of action and by the nature of the agents. These growth promotors are divided into gut flora modifiers (antibiotics, antibacterial, probiotics, prebiotics, nutraceuticals, organic acids), nutrient modifiers (enzymes, vitamins, and minerals), and physiological regulators (hormones) that have been widely used to promote health and production of animals.Antibiotics are given at sub-therapeutic dosage to stabilize the gut microbes for enhancing growth performance, feed conversion, and prevention of some disease occurrence. Frequently used as growth stimulants in domestic animals include bacitracin, penicillin, flavomycin, chlortetracycline, oxyteracycline, doxycycline, colistin sulfate, avilamycin, erythromycin, aureomycin, tiamulin, Iincomycin, furazolidone, enrofloxacin, and neomycin sulfate.
26.5.2 Hormones
Hormonal growth promotors are widely used in meatproducing animals for better performance in growth and enhancing feed conversion efficiency. Natural (estradiol, progesterone, and testosterone) as well as synthetic hormones (melengestrol acetate, trenbolone acetate, and zeranol) are used as implants and feed additives. The main aim of the use of these growth promotors are (1) to improve meat quality; (2) to enhance feed conversion efficiency; and (3) to increase milk production. Hormonal growth promotors increase growth by stimulating appetite, improving feed efficiency, and increase rate of weight gain.
Major hormones that regulate growth and development during prenatal and postnatal life are growth hormone, Insulin-like growth factors (IGFs), insulin, thyroid hormones, glucocorticoids, prolactin, and gonadal steroids (androgens and estrogens). Generally, the role of gonadal hormones in growth and development has been studied by evaluating the effect of hormone deficiency after endocrine tissue ablation as well as effect of excess hormones administered to animals in vivo. Fetus secretes several hormones and its functional display based on the receptors present in the target tissue of fetus. Fetal hormones may affect its own growth and also regulates maternal function to assure suitable environmental conditions for its continued growth and development till the parturition. These hormones also stimulate mammary gland growth (mammogenesis), lactogenesis, and galactopoieses either directly or indirectly by interacting with maternal endocrine system for its survival during the postnatal period.
26.5.2.1 Growth Hormone (GH)
GH is the most important hormone affecting the growth and development. It is biologically active in some species and their young ones are born with opened eyes and stand up immediately after birth. These young ones are known as precocial young ones, e.g., cow, mare, sheep, and goats. Growth hormone physiologically inactive in some species and their immature young ones are born with their eyes closed, e.g., dog, cat, and laboratory animals. Exogenous administration of GH has been reported to enhance the growth rate and feed conversion efficiency of calves, pigs, and lambs; however, it is not producing any effects on fetal growth during prenatal period since a smaller number of GH receptors in fetal life and increases rapidly after delivery. In dairy animals, administration of GH reportedly increases milk production whereas in pigs and lambs carcass composition shifted towards lean meat quality (less fat and more protein).
26.5.2.2 Insulin-Like Growth Factors (IGF)
The insulin-like growth factors system (IGF-I and IGF-II) plays a major role in regulation of fetal and placental growth during the pregnancy period.
IGF-I stimulates fetal growth when adequate nutrients are available and IGF-I synthesis is sensitive to undernutrition. IGF-II regulates placental growth and nutrient transfer. IGF-I and II are expressed in fetal tissues from early pregnancy (pre-implantation) to before birth. IGF-I primary growth factor supports fetal growth whereas IGF-I plays role in the later stage of pregnancy. After birth, IGF-I plays a predominant role and its production becomes GH-dependant to ensure appropriate postnatal growth in the new nutritional environmental conditions. Deficiency of IGF-I, II, or undernutrition retards fetal growth.26.5.2.3 Thyroid Hormones
The thyroid hormones (T3 and T4) are traceable in fetal circulation from early gestation and play important role in fetal growth and development. These hormones are important for morphogenesis, metabolic, tissue accretion, and differentiation in the fetus in all species including humans. These hormones’ bioavailability in fetal plasma and tissues is regulated developmentally and also varies with species, pregnancy stage, nutrient availability, and uterine environment. Its deficiency during prenatal life retards the intrauterine growth, is associated with developmental abnormalities of some individual tissues, and compromises its adaptation to the external environment during postnatal life.
26.5.2.4 Insulin
Insulin is an important hormone that regulates fetal glucose metabolism and is essential for optimal growth, development, and metabolism of fetus. Insulin stimulates glucose uptake in insulin-sensitive tissues and also the conversion of glucose into glycogen in the fetal liver. It also has an effect on protein metabolism for optimal fetal growth.
26.5.2.5 Gonadal Hormones
The gonadal hormones of the fetus are necessary for sexual differentiation. In male, testosterone produced by the fetal testes is necessary for the testicular descent into the scrotum.
26.5.2.6 Glucocorticoid
Cortisol, a major stress hormone secreted from fetal adrenal gland that prepares the fetus for delivery and also supports the maturation of organs (lungs, thyroid gland, and GI tract) and metabolic pathways (glucose metabolic pathway in liver) during the transition from intrauterine to extrauterine life.
After birth, a high concentration of cortisol in the newborn animal due to birth-related stress has a high impact on the adequate adaptation of the animal to extra-uterine life. Cortisol enhances the maturation of thyroid axis and somatotropic axis around birth leading to increased conversion of T4 to T3 and prenatal fetal growth (growth independent of GH) to postnatal growth, respectively.26.5.3 Recombinant Gene Transfer Technology—Advantages and Limitations
Farm animals provide high-quality protein in terms of meat, milk, and egg for human consumption. The production performance of animals mainly depends on the genes, environment, and their interactions. Apart from that they are utilized for various research purposes which help us to improve our understanding of the basic and applied physiology of animals. Recent advances in genetic engineering technologies open a new era for scientific and industrial applications in the field of whole genome sequencing and genetic manipulation for improving meat, milk, and egg production through recombinant DNA (rDNA) transfer technology. Recombinant DNA is a DNA molecule produced through laboratory methods from DNA materials derived from two or more sources to produce a hybrid. Gene transfer technology: it is defined as an introduction or transfer of genes, i.e., DNA sequences into an organism by using genetic engineering technologies to obtain desirable traits. The complex process of animal growth is regulated by the endocrine system which also mediates the effects of nutritional, environmental, and genetic factors in animals (NRC 1988). The main hormones that affect growth in animals are growth hormone, thyroid hormone, glucocorticoids, prolactin, and gonadal hormones. Among these hormones, growth hormone (also known as somatotropin) is a protein hormone secreted from the pituitary gland that stimulates growth. It is generally considered as the most important hormone governing the growth and development of skeletal muscle and bone, lipolysis, and milk production.
Many studies reported that recombinant bovine growth hormone (bGH) is physiologically active and possess the same properties as native pituitary bGH. Based on the functions of growth hormones, it is a drug of interest in animal production field since it has been proven as the efficient growth promotor in ruminants and pigs through better feed conversion, and also a production enhancer to improve lactation performance in dairy cows. Recombinant growth hormones (rGHs) are synthesized in large quantities in several forms corresponding to various animal species (bovine, equine, porcine, fish, etc.) and widely used across the country to stimulate milk production and as a general growth promoter in meat producing animals.Advantage
1. Production of transgenic animals that are of great value to basic research and medicine.
2. Synthesis of proteins, peptides, vaccine, amino acids, and enzymes of interest.
3. Improving disease resistance.
4. Enhancing the production of meat, milk, and egg with higher nutrient content.
Disadvantages
1. Concern about biosafety of proteins or transgenic animals or by-products generated by rDNA technology.
2. Gene transfer affects the functionality and stability of existing gene in animals.
26.5.4 Probiotics
Probiotics are free-dried cultures of friendly bacteria such as Lactobacillus sp. or Bifida bacteria and yeasts. Live microbial dietary supplements are widely produced and used for harmonizing the microbial population in the gut by the production of different compounds, competitive elimination and displacement of pathogens from enterocytes, maintenance of gut pH and thereby enhancing the health and immune status of animals.
26.5.5 Prebiotics
Prebiotics are defined as non-digestible food ingredient/sup- plement that selectively stimulates the growth of some or all of the non-pathogenic favorable organisms (bacteria) in the gut of the host. Prebiotics enhance the immune system thereby improving health and production.
The commonly used prebiotic includes oligosaccharides of galactose, fructose or mannose, lactulose, inulin, galacto-oligosaccharides (GOS), mannan oligosaccharides (MOS), and fructooligosaccharides (FOS). Prebiotics shows their beneficial action on the host by selectively feeding the harmless microbes at the cost of the harmful ones.26.5.6 Organic Acids
Organic acids have been commonly used in commercial compound feeds which include lactic, formic, acetic, propionic, tartaric, fumaric, and citric acids. Due to their antimicrobial effect, they have been proven effective in maintaining the growth performance of animals. It also lowers the pH which leads to the reduction of many pathogenic organisms in the gut.
26.5.7 Exogenous Enzymes
Exogenous enzymes are commonly added in feeds as additives which are highly specific proteins, promoting a particular biochemical process (hydrolysis). These include non-starch polysaccharides degrading enzymes (glucanase, cellulase, arabinose, xylanase, arabino-xylanase), proteases, and phytase. These enzymes have the potential to aid many digestive processes either by hydrolyzing the nutrients or making them more available or by hydrolyzing the anti- nutritional factors like lectins, gels, phytates, or polyphenols.
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