Relationships Between Estimates of Bioavailable Energy Made with Adult Cockerels and Chicks: Effects of Feed Intake and Nitrogen Retention

Data from three experiments are used to illustrate the relationships between estimates of apparent and true metabolizable energy (AME, TME) in feedingstuffs and the effects of a nitrogen correction on the estimates (AMEn, TMEn). In the first experiment, adult cockerels were precision-fed, by intubation, daily for 10 days; feed inputs were less than the maintenance requirements. The second experiment was a conventional TME bioassay with adult cockerels in which there were single inputs of a basal diet diluted with various amounts of cellulose. In the third experiment, broiler chicks were offered the cellulose-diluted diets of Experiment 2 ad libitum for 78 hr. Inputs and excretions of energy and nitrogen were measured in each experiment. Variation in feed intake affected estimates of bioavailable energy (BE) and their standard errors, but the effects decreased as intake increased. The AME and AMEn values tended to increase curvilinearly with intake, because they failed to take account of metabolic plus endogenous energy losses. The TME decreased curvilinearly but in a less pronounced manner; this curvature was due to the difference between the excreta energy output of fasted birds and the intercept of the regression of excreta energy on feed input. Cellulose dilution had no significant effect (P>.05) on the TME or TMEn estimates for the basal diet, but by altering intakes, dilution did affect AME and AMEn estimates. For adult cockerels, nitrogen retention, measured by balance (RNb), was negative; consequently, AMEn>AME and TMEn TME. In Experiments 1 and 2, nitrogen correction reduced the variation of BE estimates, but in Experiment 3 the reverse was observed because of the range of intakes for each diet. The data raise the possibility that the factor used in the nitrogen correction may not be a constant.