The response of cage-reared cockerels to dietary medication with growth promoters. I. Size and consistency of the response.

Day-old cockerels were used to examine the growth responses obtained when 50 ppm procaine penicillin, 25 ppm zinc bacitracin, or 20 ppm nitrovin were administered in a wheat-based diet. The birds were fed ad libitum from one day of age a plain or supplemented diet in a series of 10, 21, and 56-day tests conducted between 1971 and 1974. Growth responses, measured by improvement in the live weight gain and feed conversion, were obtained with each treatment, consistently over each annual period. The penicillin and zinc bacitracin gave maximal differences in live weight gain compared with the controls before 14 days of age and, possibly, before 7 days of age, whereas the maximal response to treatment with nitrovin occurred after 14 days of age.