A preliminary investigation of the utilization of true metabolizable energy by chicks.
The change in carcass composition associated with intake of true metabolizable energy (TME) by young broiler chicks was investigated. The experiment comprised 17 treatments, 16 of which formed a 2 X 4 X 2 factorial design with six replications; the experimental unit was a pen of four male chicks. The factorial treatments consisted of two basal diets (lipogenic and proteogenic ), four levels of cellulose dilution (to cause a range of basal diet intakes), and two feeding periods (10 to 17 and 10 to 24 days of age). The birds were fed ad libitum, feed intake was recorded, and final carcass composition was measured. The 17th treatment comprised 24 pens of chicks killed at the start of the experiment to estimate initial carcass composition. The gross energy of carcass fat and protein was estimated by chemical and physical analysis and by regression analysis, the latter yielded estimates of 24.5 and 39.5 MJ/kg for protein and fat, respectively. Basal diet intake was reduced by more than 50% at the highest level of cellulose dilution and increased by about 60% when the feeding period was extended from 17 to 24 days. Pen weight gain and carcass composition were affected by basal diet intake. The energy retained in the carcass (MJ/W X 75 kg/day) (RE') increased in a linear manner with the intake of TME (MJ/W X 75 kg/day) ( ITME ), the slope of the line being greater for the lipogenic than for the proteogenic diet. Projected back to zero ITME ', the regression lines for pens in positive energy balance gave an estimate of .420 MJ loss, After 7 days on experiment, the energy retained as fat (RE'f) and as protein (RE'p) increased approximately linearly with ITME ', but after 14 days some curvature was apparent, the deposition of protein increasing more rapidly at the lower levels of energy input, and the deposition of fat increasing more rapidly at the higher levels of energy input. The RE'p was only slightly higher on the proteogenic diet than on the lipogenic diet, at the same level of ITME ', but the effect of basal diet on RE'f was considerable. For both diets at RE' = 0 there was gain in body protein accompanied by loss of body fat.