Diphasic allometric growth of some skeletal bones and the digestive tract in White Leghorn pullets consuming ad libitum and restricted diets

Wageningen Institute of Animal Sciences, Department of Animal Nutrition, Wageningen Agricultural University, The Netherlands. rene.kwakkel@alg.vv.wau.nl

Growth data of some bones (shank, tibia, and keel) and the digestive tract in White Leghorn pullets, which consumed ad libitum and restricted diets, were analyzed by mono- and diphasic allometric functions. Fat-free plucked empty body mass (FFEBM) or a functional entity was used as the independent variable in the equations. Pullets had been fed a low-lysine diet or a daily restricted amount of an adequate diet, from 0 to 6 or 7 to 18 wk of age. An additional group of pullets consumed ad libitum a control starter and grower diet. Relative growth of the skeletal bones and parts of the digestive tract, vs FFEBM, was described most accurately by a diphasic model. For each constituent, allometric slopes of the first growth phase (beta1) vs FFEBM were smaller than 1 (beta1 varied from 0.39 to 0.43 for shank and tibia, from 0.48 to 0.73 for the keel, from 0.89 to 0.98 for the total digestive tract, and from 0.80 to 0.84 for the gizzard, separately). These results suggest that each of the assessed organs matured earlier than the FFEBM. Except for the keel, which grew relatively faster than FFEBM if an early nutrient restriction had been applied, beta1 of all other assessed structures was similar for all treatments. If the allometric slope of the second growth phase (beta2) was estimated to be not different from zero, then the breakpoint between both phases was defined as the moment of attainment of maturity for the respective constituent. The attainment of maturity of the different body structures confirmed the classical growth sequence studies of the Hammond School. No differences in mature weights for the assessed organs between the feeding regimens were observed. The results indicated that in studies in which a nutritional deficiency had been applied, the weights of several body structures, most often expressed in terms of weight per 100 g BW, are strongly related to the decrease in growth rate of the fat-free body. It was concluded that most of the reported effects on growth retardation as a result of nutrient restrictions are primarily a consequence of the mobilization of fat per se.

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