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The growth-promoting effect of sub-theraputic levels of antibiotics in animal feed was first described in the late 1940s when chickens fed fermentation waste from an antibiotic (tetracycline) plant grew more rapidly than controls (Dibner and Richards, 2005). Since 1950, a wide range of in-feed antibiotics (IFAs) has been available to the broiler industry. In the 1950s, the bulk of the antibiotic agents from the pharmaceutical industry were derived from 'Streptomyces spp'. Many antibiotic substances have been shown to improve FCR and animal growth and reduce morbidity and mortality due to clinical arid subclinical diseases (Rosen, 1995). In a holo-analysis of several studies, Rosen (1995) reported that in 72 % of 12,153 studies from 55 countries, conducted in the preceding 50 years, antibiotics improved growth and feed efficiency by 2-3 %. Ewing and Cole (1994) suggested that average growth improvement of IFA is between 4-8 % and feed utilization can be improved by 2-5 %. |
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