GROWTH and DEVELOPMENT AGRICULTURAL SCIENCE Animal Science § GROWTH – is the increase in size and weight as the animal gets older. § DEVELOPMENT – is the change in proportion of various parts of the body. Can also be described as the stages of growth of an animal. STAGES OF GROWTH: 1. Prenatal – involves Embryonic and Foetal growth. 2. Postnatal – involves Puberty and Maturity. PRENATAL GROWTH – Factors affecting size at birth: 1. Number of young born. 2. Size of the dam. 3. Age of the dam. 4. Sex of the litter members. 5. Level of nutrition. POSTNATAL GROWTH – Factors affecting growth: 1. Size of the animal at birth. 2. Sex of the animal. 3. Breed 4. Environment 5. Genetics 6. Nutrition Hormone Growth Promotants. § A group of veterinary drugs that mimic the hormones that influence the growth of cattle. § They may be natural or synthetic in compound. § They improve the feed conversion rate of cattle. § Are used to increase growth rates and muscle development. § They are small implants given in the ear which slowly dissolve and release a hormone into the bloodstream. § HGP’s can contain female hormones – oestradiol or progesterone. § Also contain male hormones – testosterone or trenbolone acetate. § Sometimes they will be a combination. § Implants containing trenbolone acetate are generally referred to as ‘aggressive’ implants because they increase growth rates. § Delay fat composition. § Do not improve meat quality. § Repeat usage has shown a reduction in meat tenderness and marbling. BREEDING SYSTEMS Random: § Animals are allowed to mate at will. § A large genetic pool. § Produces great variation. § Results are unpredictable. Inbreeding: § Mating of close relatives – father/daughter, mother/son, full and half siblings. § Increased uniformity. § Greater chance of genetic diseases – homozygous recessives. Linebreeding: § Bloodlines based on a single common ancestor (stud breeders). § Useful in passing on the genes of an outstanding animal over several generations. Outbreeding: § Mating of unrelated animals of the same breed. § Most commonly used system for commercial sheep and cattle herds. Crossbreeding: § Mating of unrelated animals of different breeds. § Offspring are more productive than either of the parents (heterosis/hybrid vigour). Diseases of Livestock The affects of disease: 1. Death 2. Weakening and weight loss. 3. Stunted growth. 4. Lower production. 5. Infertility resulting in fewer offspring. 6. Reduced sale price. Infection and Disease Infection by an organism depends on: § A source of the pathogen. § Transferral of the pathogen. § Invasion of the host – overcoming the host’s barriers to infection. § Establishment of the pathogen within the host. The extent of damage will depend on the ability of the organism to grow or produce toxins. Disease Control 1. Eradication involves the complete removal of the disease by a process of testing and slaughter. § Very expensive § Difficult to achieve 100% success. § Used to eliminate brucellosis and tuberculosis from Australian cattle herds in the 1980’s. 2. Vaccination stimulates the body to produce antibodies against disease- causing organisms. § Vaccination is a cheap and effective method of preventing disease. § Used for both bacterial and viral diseases. § Vaccines contain either – dead organisms, live organisms (weakened), toxins. § Examples contain 7 in 1 for cattle – protects against 5 clostridial diseases and 2 types of leptospirosis. 3. Chemical control – is used for internal and external parasites. § Quick and effective. § Reasonably cheap – debateable. § Disadvantage is that parasites and pathogens are continually developing resistance. § Many chemicals are toxic to humans. § Chemical residues can remain in the meat and milk.
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