ONSET OF LAY RELATED TO MULTIPHASIC GROWTH AND BODY-COMPOSITION IN WHITE LEGHORN PULLETS PROVIDED AD-LIBITUM AND RESTRICTED DIETS

Growth of the body as a whole and of parts of the reproductive tract o f White Leghorn pullets that ate ad libitum and restricted diets were analyzed by a multiphasic growth function. Parameter estimates were re lated to onset of lay. Chemical body compositions at defined stages of pubertal growth were related to the development of the reproductive o rgans. Data of pullets that had been restricted in one of two rearing periods (0 to 6 and 7 to 18 wk of age) were used. In each period, pull ets were fed a low-lysine diet or a daily restricted amount of feed, o n a pair-gain basis. In all mathematical fits, a well distinguishable ''pubertal body growth spurt'' at around 19 wk of age was found. About 40 to 70% of total growth within this phase consisted of growth of th e reproductive tract (the ovary covered 19 to 35%, the oviduct 15 to 2 3%, and the uterus 7 to 11% of total growth). Age at maximum gain of t he pubertal body growth spurt was 20.1 wk for lysine-restricted pullet s and 19.6 wk for feed-restricted pullets. This difference reflected d ifferences in onset of lay (50% rate of lay) between lysine- and feed- restricted birds (22.3 and 21.6 wk, respectively). An interval of 14 t o 15 d existed between maximum gain of the pubertal body growth spurt and onset of lay. It was suggested that the occurrence of the pubertal body growth spurt can serve as a predictor for subsequent onset of la y. Body composition at defined moments during the pubertal body growth spurt was calculated by interpolation from chemical analyses of Weeks 15, 18, 20, 22, and 24. Crude fat content in the body at start of the pubertal body growth spurt varied considerably between the feeding re gimens (112 to 179 g). The fat-free body (consisting of CP, ash, and w ater) showed less variation between the treatments at this stage (807 to 870 g). It is concluded that a particular amount of fat-free tissue is critical in pullet development and may be required before sexual o rgan development starts.