Responses of metabolized energy, body protein gain and body energy gain of growing chicks to planes of energy intake and dietary composition of energy-yielding nutrients were studied. Ten diets were formulated to cover the entire range of composition (0-100%) of protein, fat and carbohydrate on the basis of gross energy (GE) and three sets of 10 groups of chicks were force-fed these diets at planes of 1380, 1076 and 773 kcal GE/chick per 10 d. The response surfaces of metabolized energy at each plane of Ge intake were convex, and the upper two surfaces contacted in the field of 0% GE of dietary protein (PGE). The response surface of body energy gain was almost parallel to that of metabolized energy at each plane of GE intake, but the slopes of the body energy gain upward to the peak from the foot on the 100% PGE field were steep compared with those of the metabolized energy. On the other hand, the response surfaces of body protein gain at any plane of GE intake were also convex, and sloped down from the field of about 60% PGE to the higher PGE field and to the 0% PGE field. Regardless of the plane of GE intake, the body energy gain reached a plateau at about 25% PGE and the body protein gain at about 60% PGE. That body protein gain depends on both protein intake and plane of GE intake was clearly shown. On the basis of these results we calculated partial efficiency of metabolized energy utilization, and discussed validity of the classical definition of a 'balanced ration.'
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