Responses of metabolized energy, body protein gain and body energy gain of growing chicks to dietary composition of energy-yielding nutrients were studied. Twenty-seven groups of White Leghorn male chicks were force-fed purified isocaloric diets that covered the entire range of composition on a gross energy basis (0 to 100%) of protein, fat and carbohydrate for 10 d. Prediction equations of response surfaces for metabolized energy, body protein gain and energy gain were formulated by multiple regression techniques. The response surface of metabolized energy had a convex configuration and showed that the contribution rate of each energy-yielding nutrient to energy metabolizability varied with the levels of the three nutrients in the diet. Feeding protein-free diets especially diminished the metabolizability of carbohydrate and rat. The response surface of body energy gain was roughly parallel to that of metabolized energy, but the former was highly dependent on the calorie ratio of dietary carbohydrate to fat and reached a plateau at about 20% protein gross energy. Body protein gain increased almost linearly with an increase in protein in a low protein field, reaching a plateau at about 55% protein gross energy and was not very much affected by the calorie ratio of dietary carbohydrate to fat. In terms of energy-yielding nutrients, these results have provided an insight into the 'balanced ration' for maximal body energy and gain and body protein gain.
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