Data of White Leghorn pullets consuming feed ad libitum were used to study multiphasic growth during rearing and early lay. Two mathematical functions were used. Body weight gain and growth of the chemical components DM, CP, crude fat (Cfat), and ash (Ash) were described as a function of age, using a multiphasic growth function. To describe relative growth between CP and DM, Cfat and DM, and Ash and DM, a multiphasic allometric function was applied. Body weight gain from hatching to 24 wk of age was estimated most accurately by a tetraphasic growth function. The first two growth spurts represented 82% of mature BW. Growth in these two phases seemed to be related to the development of bones, muscles, and essential metabolic organs. Body growth was further characterized by a third phase at 19 wk of age, which was related to growth of the reproductive tract. The fourth phase consisted mainly of body fat deposition.

Both multiphasic functions showed that the nature of component growth was diphasic, with a transition area between the two phases at around 11 wk of age. On the basis of the duration of growth and of allometric growth coefficients, growth of CP, Cfat, and Ash in the first phase seemed to be functionally related to each other and consisted of muscle growth, intermuscular fat deposition, and skeletal growth, respectively. After 11 wk of age, protein deposition is presumably a function of sexual maturity, i.e., the development of the reproductive tract; whereas fat growth is mainly abdominal fat deposition. The observed relationships indicate that the use of multiphasic functions might be helpful in the interpretation of changes in body growth and development of pullets.