Abstract
Four battery trials, each with 280 broiler chicks, were run to compare the relative efficacy (increased growth rate and improved feed conversions) of four antibiotics under several intermittent versus continuous feed regimens. Two antibiotics that have been in use for many years (procaine penicillin and zinc bacitracin) and two newly introduced (lincomycin and moenomycin) were each evaluated at 10 mg./kg., except for lincomycin, which was fed at 4 mg./kg. The periods of supplementation included 0 to 2 weeks (Trial 1), 0 to 4 weeks (Trial 2), 4 to 8 weeks (Trial 3) and 6 to 8 weeks (Trial 4) for all four antibiotics with one antibiotic also fed from 0 to 8 weeks in each trial, using a different one in each trial. All four antibiotics stimulated growth and improved feed conversion during the respective periods of supplementation, except when penicillin and zinc bacitracin were fed for only the last 2 weeks (Trial 4). Analysis of the data from the four trials failed to show any statistical difference (p > .05) among the four antibiotics.Combining of the growth and feed conversion data for the four antibiotics under each feeding regimen showed that continuous feeding from 0 to 8 weeks and feeding from 4 to 8 weeks (Trial 3) were the most effective, based on 8-week performance data. These responses were not significantly different (p > .05) from each other but were significantly better (p < .05) than the 8-week responses obtained when the antibiotics were fed from 0 to 2 weeks (Trial 1), 0 to 4 weeks (Trial 2), 6 to 8 weeks (Trial 4) and to nonsupplemented controls. While the latter three feeding regimens did improve 8-week weights, compared to controls, the increases were not statistically significant (p > .05). The four antibiotics appear to exert their positive effects essentially during the periods of supplementation with the feeding for the last 2 weeks being of insufficient duration to maximize efficacy.
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