The traditional method for estimating the ME
m requirement is to use the linear relationship between the RE and MEI by extrapolating to the MEI at zero energy retention (i.e., the intercept on the X-axis) [
7,
9]. Furthermore, when the MEI is equal to zero, the intercept on the Y-axis of this equation represents the FHP (i.e., NE
m) [
7]. According to this method, the estimated ME
m values were 594 kJ/kg of BW
0.75/d in the ICM and 618 kJ/kg of BW
0.75/d in the CSM. The ME
m values determined herein are similar to the value (602 kJ/kg of BW
0.75/d) determined from broiler breeder pullets (4 wks of age) by Sakomura et al [
8] at 22°C, and were in the ranges of values estimated by Nieto et al [
25] for male broiler chickens (519 to 628 kJ/kg of BW
0.75/d). The NE
m data for broilers calculated from linear regression are limited. The NE
m values of 386 kJ/kg of BW
0.75/d in the ICM and 404 kJ/kg of BW
0.75/d in the CSM were in agreement with the values of 395 and 387 kJ/kg of BW
0.75/d for two breeds of laying hens measured by the same method [
7].
The logarithmic relationship between the HP and MEI is usually used to calculate the NE
m as being the HP at zero MEI [
5,
26]. Similarly, the ME
m can also be calculated by extrapolating the HP being equal to the MEI. In the current study, the estimated ME
m values obtained by logarithmic regression were 607 kJ/kg of BW
0.75/d in the ICM and 619 kJ/kg of BW
0.75/d in the CSM, which were nearly equal to the respective value calculated by linear regression. The NE
m data for broilers calculated form logarithmic regression are also lacking. The NE
m value obtained from logarithmic regression was 448 kJ/kg of BW
0.75/d in the ICM and 462 kJ/kg of BW
0.75/d in the CSM, which were greater than the NE
m values calculated by linear regression in this study. The NE
m values of 497.48, 457.31, and 387.02 kJ/kg BW
0.75/d for broiler breeder pullets (4 wks of age) at 15°C, 22°C, and 30°C, and 418.57, 334.09, and 289.32 kJ/kg BW
0.75/d for laying hens (2 wks of age) at 12°C, 22°C, and 31°C were determined with logarithmic regression between the HP and MEI by Sakomura et al [
8,
11]. Moreover, the NE
m can also be estimated by direct measurements of the FHP in fasting animals [
27]. O’Neill and Jackson [
10] founded that the FHP varied between 404 and 464 kJ/kg BW
0.75/d for hens and between 223 and 349 kJ/kg BW
0.75/d for the cockerels. Furthermore, Noblet et al [
2] suggested that the present FHP values measured in modern lines of broilers should be expressed as per kg of BW
0.70, and the FHP values in 0.5 to 3.0 kg broilers ranged between 410 and 460 kJ/kg BW
0.70/d. These results suggest that the estimates of NE
m are affected by types (breed, age, sex, etc.) of animals, the experimental environment, and measurement methods. Within one animal species, the constant FHP can be obtained by being expressed as per unit of metabolic BW after the exponent of metabolic BW being calculated for an animal over a large BW. Noblet et al [
2] indicated that the FHP was linearly related to the BW
0.70. In our previous study, the exponent of metabolic BW was 0.74 for AA broilers weighing 0.94 to 2.75 kg, and the FHP per kg of BW
0.74 were constant for broilers in this BW range [
28]. Therefore, the respective NE
m values of different types (breed, sex, etc.) of animals should be determined in standardized condition to calculate the NE content of a feed ingredient. The NE value of a poultry diet should express the energy cost of production (growth, egg, etc.) and NE
m. The K
m values of 73.8% from the ICM and 75.0% from the CSM calculated by logarithmic regression were higher than the values of 65.0% from the ICM and 65.4% from the CSM obtained by linear regression, which was caused by the lower NE
m values determined by linear regression. The K
m values determined in the present experiment from logarithmic regression are similar to those estimated by Sakomura et al [
8,
9] for broiler breeder pullets (75%, 76%, and 72% at 15°C, 22°C, and 30°C, respectively) and for broiler chickens (76%, 80%, and 76% at 13°C, 23°C, and 32°C, respectively), in which the NE
m values were calculated by logarithmic regression between the HP and MEI and the ME
m values were calculated by the linear relationship between the RE and MEI, respectively. Balnave [
29] indicated that the variability in the efficiency for maintenance ranged between 66% and 78%. This variability in efficiencies of energy utilization for maintenance could be related with the composition of the diets [
25].