A hormone is defined as a regulatory substance produced in an organism and transported in tissue fluids such as blood or sap to stimulate specific cells or tissue into action. The endocrine system encompasses a group of tissues that release hormones into circulation for travel to and action on distant targets. Endocrine tissue is typically a ductless gland (e.g., pituitary, thyroid) that releases its hormones into capillaries that run through the tissue. These glands are richly supplied with blood.
New hormones continue to be discovered. Some act only on a single tissue, whereas others have effects on virtually all cells of the body. The effects of hormones on their targets are varied—from the enhancement of nutrient uptake to altering cell division and differentiation, among many others.
Hormone manipulation – Synchronization of Oestrus
Let’s take sheep as an example. This fairly simple method is commonly used to induce heat in ewes that are not in heat. Briefly, it works as follows:
Progesterone is a progestogen sex hormone involved in the menstrual cycle, pregnancy, and embryogenesis. It belongs to a group of steroid hormones called progestogens and is the major progestogen in the body.
Progesterone is a hormone that maintains gestation (pregnancy). Secretion of progesterone from the corpus luteum (a yellow body that develops in the ovary after the ovum is released) prevents the female from coming into heat again before gestation is over. When progesterone is administered artificially, it fools the female’s body into thinking it is pregnant and the animal will not ovulate or come into oestrus. With the removal of the progesterone source, the body will return to normal functions and within a set period (approximately 36 – 48 hours), enter oestrus. In this way, a large group of females can come into oestrus at a specific type (known as synchronisation of oestrus cycles).
There are several types of synthetic progesterone sources:
- Intravaginal sponges are the most commonly used method. Ewes, for example, come into oestrus from 24 – 48 hours after the sponges have been removed.
- Synthetic progestogen implants are implanted subcutaneously (usually in the ear) after which the females will come into oestrus after a specific time.
- Melengesterol acetate (MGA), which can be mixed into feed, is commonly used to prevent oestrus in heifers that are in feedlots.
It is very important to note that the use of these synthetic hormones can be harmful to the animal and also to humans and should only be used by trained reproductive experts, such as veterinarians, embryologists and appropriately trained reproductive physiologists.
Growth Promoters
Growth promoters are substances that are added to feeds as a supplement or injection to improve feed utilisation (i.e., the feed conversion efficiency), growth (i.e., average daily gain) and carcass quality or milk production of farm animals. The changes in performance result in an economic benefit to livestock producers and impact the relative price competitiveness of the animal product produced as compared to other sources of the same product.
Other benefits of the use of growth promoters in livestock production include:
- The prevention and treatment of animal diseases.
- Improvement of reproductive performance.
- Improvement of the environment.
Livestock waste is one of the major sources of greenhouse gases, as the abnormal fermentation of gastrointestinal tract contents can produce lots of methane, ammonia, and carbon dioxide, as well as foul-smelling gases (nitrogen and phosphorus), which contribute to environmental pollution, water eutrophication and ecological imbalance. Some antimicrobials in feed could inhibit abnormal fermentation and consequently, reduce the emission of greenhouse gases (mainly methane). Ionophorous antibiotics have been widely used as feed additives in ruminants due to their favourable effects on rumen fermentation and the reduction of methane emissions from livestock.
In addition, growth promoters contribute to the inhibition of biogas production – when bacteria digest organic matter (biomass) in the absence of oxygen. As a result of the wide use of antimicrobial drugs in intensive animal production for growth promotion and prevention or treatment of disease, a large proportion of ingested drugs are excreted in manure and end up with livestock waste water. Excreted antibiotics in the environment may partially inhibit methanogenesis in anaerobic waste storage facilities, and thus, decrease the rate at which bacteria metabolise animal waste products. Antimicrobials that have inhibitory effects on methane production during the anaerobic digestion of livestock waste include amoxicillin, aureomycin, oxytetracycline, thiamphenicol, florfenicol, sulfadimethoxine, and tylosin.
Benefits of growth promoters
A primary goal of animal producers worldwide is to increase the efficiency of the conversion of feed into high-quality food products for humans while minimising risk to consumers. However, the benefits and risks of growth promoters continue to be complex and debatable issues. One of the public health concerns is the use of antibiotics on animals increases the possibility of antibiotic-resistant bacteria developing in animal products. For example, the use of penicillin on animals increases the risk of penicillin-resistant bacteria in animal products. Antibiotic-resistant bacteria, in turn, could make humans who consume those animal products less responsive to future antibiotic treatment.
Another concern is that humans are predisposed to consume whatever chemical(s) the animal has consumed or been exposed to by consuming animal products (meat, milk, eggs, and the like). These chemicals may include veterinary drugs, insecticides used on the animal, herbicides and fertilisers used on pastures, and chemical additives used in animal feed. Some of these chemical substances are poisonous, especially pesticides and herbicides, and some are disagreeable in other ways to be used on animals whose products are consumed by humans.
Long-term use of growth-enhancing technologies has proven that the compounds are a safe and effective way to improve animal performance. The compounds are rapidly metabolised and excreted from the animal, assuring no risk of potential residues in the edible tissues. The safe use of growth-enhancing compounds benefits the consumer. First, consumers benefit from the reduced production cost associated with the use of this technology in livestock production. Second, consumers benefit from the improved animal product options produced form livestock reared with growth-enhancing technologies.
In other words, despite public health concerns, livestock growth promotion products, when used consistently according to their label instructions, are safe for the animal, the consumer, and the environment and deliver significant economic benefits to the livestock producer and the consumer.
Types of Growth Promoters:
- Feed additives
- Hormonal implants
- Growth hormone (Somatotropins)
- Repartitioning agents (Beta ()-agonists)
- Probiotics
Feed Additives
A feed additive is a substance added to feed to satisfy a specific need of the animal. The additive may provide a needed nutrient or increase an animal’s resistance to disease. Many feed additives are available such as antibiotics, organic acids and exogenous enzymes. These compounds are added to the milking and fattening diet of farm animals to improve their growth performance, nutritional parameters and carcass traits.
Antibiotics used as feed additives, develop their activity in the digestive tract of animals by suppressing the undesired competitive microorganisms that use nutrients and produce undesirable or toxic substances resulting in an optimal environment for the intestinal mucosa, which allows efficient nutrient absorption. Therefore, growing animals will get maximum benefits from antibiotic growth promoters (AGPs) regarding improved, nutrient utilisation, feed conversion ratio and growth rat.
Antibiotic growth promoters that have been proved to have a low resistance capacity at authorised utilisation levels include:
- Monensin.
- Flavomycine.
- Virginiamycine.
Hormonal Implants
Implanting hormonal growth promoters for better performance in growth and improvement of feed efficiency may enhance growth during the suckling, growing and finishing stages of meat production. They are implanted under the skin (usually behind the ear) of the animal in the form of depot capsules, where they release a specific dose of hormones over a fixed period.
Depot capsule implantation into the skin behind the ear
Types of hormones most widely used in livestock production in the form of implants include:
- Natural hormones, (estradiol, testosterone and progesterone)
Estradiol is responsible for female characteristics, testosterone is responsible for male characteristics, and progesterone is responsible for maintaining pregnancy. Estradiol promotes growth by stimulating appetite and improving feed conversion efficiency (FCE). Testosterone or testosterone propionate, alone or in combination with other hormonally active substances, is used primarily to improve the rate of weight gain and feed efficiency by the anabolic action of androgens. It is well established that progesterone not only serves as the precursor of all the major steroid hormones (androgens, oestrogens, corticosteroids) in the gonads and adrenals, but it also converts into one or more metabolites that ultimately improve the growth rate of the animal.
- Synthetic hormones (trenbolone acetate and zeranol).
These hormones mimic the biological activity of the natural hormones – trenbolone acetate mimics the action of testosterone and zeranol mimics estradiol.
Growth hormones
Growth Hormone (GH) increases weight gain by stimulating metabolism and protein accretion and reducing fat deposition. For example, Bovine Somatotropin (BST) is a bovine growth hormone produced by the pituitary gland of the cow. This hormone is a protein, like insulin, not a steroid hormone, such as sex hormones or cortisone. During lactation, BST mobilises body fat for use as energy and diverts feed energy towards milk production rather than tissue synthesis. BST increases efficiency in milk production by 10 – 15 %. Though the use of BST is primarily concentrated on increasing milk product its effects on beef cattle are increased growth rates, improved feed conversion and lean carcass production while decreasing carcass fat. However, the effect on the eating quality of the meat associated with reduced carcass fat is reduced acceptability because of lower scores on tenderness.
Repartitioning agents (β-Agonists)
An agonist is a substance which initiates a physiological response when combined with a receptor. β-adrenergic agonists enhance growth efficiency by stimulation of beta-adrenergic receptors on cell surfaces. They act as repartitioning agents (a repartitioning agent is an agent which will direct substrates away from adipose tissue depots and towards protein accretion in muscle) to modify carcass composition by altering nutrient partitioning to lower fat deposition by up to 40 % and increase muscle protein content up to 40 %. Increased protein accretion leads to increased muscle protein synthesis.
A wide range of compounds has been investigated as β-agonists including:
- Cimaterol.
- Clenbuterol.
- Fenoterol.
- Isoprenaline.
- Mabuterol.
- Ractopamine.
- Salbutamol.
- Terbutaline.
- Zilpaterol.
One of the major problems that arise when β-agonists are used as growth promoters during chronic exposure is that when the product is removed, it leads to an increase in fat deposition and a reduction in muscle mass. Therefore, the most effective use of a repartitioning agent is in the finishing period in the 1 – 2 months before slaughter.
Probiotics
Probiotics are good replacers of antibiotics produced from freeze-dried cultures of ‘friendly bacteria’ and some yeasts, which when introduced in the gut should stick to the gut wall and inhibit hostile bacteria. Probiotics are mono or mixed cultures of living microorganisms, which induce beneficial effects on the host by improving the properties of the indigenous microflora.
Several microorganisms have been considered probiotics including fungi particularly mushrooms and yeast, bacteria and mixed cultures comprising various microbes. Bacteria are more commonly reported as probiotics than fungi. The most common microorganisms used as probiotics include the genera Lactobacillus and Bifidobacteria. Other bacteria that have been used, to a lesser extent in poultry and animal probiotics include Bacillus, Enterococcus, Streptococcus, Lactococcus, Pediococcus and Selenomona scerevisiae.
MSD Animal Health endocrines and growth promoters.
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MSD Animal Health Endocrines and Growth Promoters |
Description |
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Ralgro® Cattle Implants
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Hormone-free growth stimulant for improved body mass Composition: · Each pellet contains 12 mg zeranol (73,8 % m/m). · Each implant contains 36 mg zeranol (3 pellets). |
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Revalor® G
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A slow-release anabolic agent containing trenbolone Composition: · Each dose is equivalent to 40 mg trenbolone acetate · 2 small yellow pellets, each containing 20 mg of |
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Revalor® H
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A growth promotant with a slow release of trenbolone Composition: · Each implant contains 200 mg trenbolone acetate and |
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Revalor® S
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A growth promotant with a slow release of trenbolone Composition: · Each implant contains 140 mg of trenbolone acetate |
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Revalor® XS
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A growth promotant with a slow release of active Composition: · Each implant contains 200 mg of trenbolone acetate and 40 mg of 17ß-oestradiol. |
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MSD has several other endocrines and growth promoters; however, they are all scheduled products that require veterinary consultation. |
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