Observations on the growth‐promoting effects of procaine penicillin and zinc bacitracin on chicks in different environments

The growth‐promoting effects of procaine penicillin and zinc bacitracin in the diet of chicks have been compared in a series of trials, during which the general level of hygiene of the premises was varied. ‘Dirty’ conditions were deliberately produced by the introduction of droppings from older birds, and good conditions were re‐established by cleaning and fumigation. the growth response to the two antibiotics was comparable whatever the condition of the premises. the average weight achieved after cleaning and fumigation was superior to that in the less clean premises. the order of response to either antibiotic showed that there had been no diminution of effect over the sixteen years during which antibiotics had been in continuous use in the same premises.