Abstract
INTRODUCTIONIN WORK conducted in this laboratory a variation in response to unidentified factor supplements has been noted over a yearly period. The response is usually smaller in the fall, at the time that pullet breeders are used, and gradually increases to the period of June or early July. After this time the response of chicks from these hens to unidentified factor supplements usually decreases. Similar experiences have been reported by Waibel, Morrison and Norris (1955), Barnett and Bird (1956), and Morrison (1956). It has been thought that this response is due in part to seasonal changes in the microbial population in the experimental laboratories.Many studies have been conducted which demonstrate or attempt to explain the influence of environmental microbial populations on chick growth rate in the presence of growth promoting substances. The work of Coates et al. (1951) and others, for instance, indicates that the response to antibiotics …
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