Effect of gentamicin on early mortality and later performance of broiler and Leghorn chickens

Field and floor-pen trials demonstrated that subcutaneous injection of 0.2 mg gentamicin at one day old reduced bacteria-caused deaths in White Leghorn and broiler-type chicks. Growth, feed efficiency, and condemnation data from broiler trials suggested an advantage for treated chickens over unmedicated controls. Ninety-nine percent of clinical isolates were susceptible to gentamicin (10 mug) when tested by the modified Kirby-Bauer method. In vitro susceptibilities of these isolates to neomycin, nitrofurazone, chloramphenicol, and kanamycin varied from 90 to 94%. About half of the isolates were susceptible to dihydrostreptomycin or tetracycline in vitro, while less than 10% were susceptible to penicillin or clindamycin. In vitro tests to determine the lowest concentration of antibiotics required to inhibit the growth of 31 bacterial isolates demonstrated that less gentamicin is required than neomycin, spectinomycin, or oxytetracycline.