Multiphasic growth in the layer pullet : effects of nutrient restrictions during rearing

Traditionally, layer pullets are reared on a least-cost basis. Feeding regimens, that allow pullets to consume a restricted amount of nutrients, have been designed to control body growth towards a 'target' weight and age. It was questioned whether the pattern of body growth in relation to the rate of development of particular organs during rearing interferes with the productive potential of the young hen. For the present study, it was hypothesized that the supply of nutrients for some organs may be critical at certain ages, as a result of their individual growth patterns. In this thesis, growth and development of the pullet body and its constituents has been studied by means of multiphasic growth functions. It was found that body weight at end of rearing ('target weight') is less important than type (which nutrient?) and phase (what age?) of restriction in determining egg performance. Furthermore, a pronounced growth spurt in the body growth curve was distinguished at around 19 wk of age ('the maturity growth spurt'), that appeared to be related to both the development of the reproductive organs and the onset of lay. The assessment of this growth spurt in a flock may help the producer in taking nutritional decisions. A certain amount of fat-free tissue in the body is suggested to be critical for the initiation of sexual growth. Fat growth at early rearing seemed to be functionally related to growth of the fat-free body: pullets on a low-lysine diet did not increase their fat-to-protein ratio at that stage of development. Fat growth at late rearing is stored as an energy buffer (abdominal fat pad). The composition of the fat-free body was not affected by dietary treatment. It was concluded that the fatfree body of pullets is a better measure of physiological age than body weight. Effects of nutrient restrictions on growth of body constituents should be presented relative to the fat-free body. Multiphasic analyses of pullet growth quantified some growth relationships between body components which had not been revealed if a simple monophasic growth approach had been used.