A commercial line (CL) and genetically-selected lean (LL) and fat (FL) lines were fed a purified diet containing either 2% corn oil or 2% corn oil plus 7% animal-vegetable fat. Both diets contained 23% crude protein and 3150 kcal nitrogen-corrected apparent metabolizable energy (AMEn)/kg. Apparent fat digestibility (AFD), apparent metabolizable energy (AME), AMEn, true metabolizable energy (TME), and nitrogen-corrected TME (TMEn) were determined on birds fed ad lib or restricted to 25 g for 36 hr. The birds on the ad lib program were than killed for net energy of production (NEp) determination. Apparent fat digestibility increased (P less than .05) from 1 to 3 weeks of age for both diets. This resulted in significantly lower AME, AMEn, and TMEn at 1 week than at 3 weeks. True metabolizable energy was not affected (P greater than .05) by age. The AMEn and TMEn at 1 kg body weight were less precise on the restricted than on the ad lib program. The AME, AMEn, TME, and TMEn determined by ad lib feeding at 1 kg body weight were not affected by line (P greater than .05) by line or diet. However, AME and AMEn from the same program were affected by line and diet. Nitrogen-correction of ME was only necessary for TME on the restricted program. Net energy of production was only different (P less than .05) among lines. Because of its greater precision, the AME assay with ad lib-fed birds is the preferred assay, at least at a young age.
