### INTRODUCTION

_{m}) and production (NE

_{p}). Therefore, the determination of the NE value of a feed will be influenced by NE

_{m}evaluation. Fasting heat production (FHP), which represents the basal metabolic rate of animals, is usually used as a surrogate for NE

_{m}[2]. However, the determined FHP value may be affected by types (breed, age, sex, etc.) of animals, the length of the fasting period [3], and previous feeding conditions with a lower FHP at lower feed intake [4]. An alternative method to estimate NE

_{m}is to feed animals at several levels of feed intake to build the logarithmic regression between heat production (HP) and metabolizable energy intake (MEI) [5]. Then, the NE

_{m}can be calculated by extrapolating the HP to zero MEI from the logarithmic regression [6]. Furthermore, the metabolizable energy for maintenance (ME

_{m}) can be calculated by extrapolating the HP being equal to MEI. However, the traditional method for ME

_{m}is to use the linear relationship between retained energy (RE) and MEI [7]. The FHP (i.e., NE

_{m}) can also be obtained from this linear regression. Moreover, the linear regression was used to calculate the ME

_{m}and the logarithmic regression was used to calculate NE

_{m}in some research, respectively [8,9]. However, the comparison of using logarithmic regression and linear regression to calculate both ME

_{m}and NE

_{m}are lacking. Furthermore, evaluations of the NE

_{m}in laying hens and broiler breeder pullets have been reported [8,10,11]. Similar research is scarce in broiler chickens.

_{m}in broilers using the CSM and ICM. The effect of regression model selection on ME

_{m}and NE

_{m}values was also compared.

### MATERIALS AND METHODS

### Equipment

^{3}were used in this study based on a design similar to that of Van Milgen et al [12]. Briefly, the respiration chamber was air conditioned to maintain a constant temperature and humidity using an air conditioner and a heater. Gas was extracted continuously from the respiration chamber by a vacuum pump. Gas concentrations in each chamber were measured at 3-min intervals by an analyzer. The O

_{2}was measured with a zirconium oxide sensor (Model 65-4-20; The Advanced Micro Instruments, Huntington Beach, CA, USA), whereas CO

_{2}was measured with a nichtdispersiver infrarot sensor (AGM 10; Sensors Europe GmbH, Erkrath, Germany) in the analyzer. The analyzer had a range of measurement of 0 to 25% for O

_{2}and 0 to 2.5% for CO

_{2}.

### Experimental procedures

*ad libitum*feed intake by the Arbor Acres Broiler Commercial Management Guide each day. In the ICM, 96 male AA broilers aged d 15 were used with 4 birds per replicate and 6 replicates in each feed intake treatment. In the CSM, 116 male AA birds aged d 15 were used. Among these 116 birds, 20 were selected for the initial data and 96 were assigned to 4 feed intake treatments with 6 replicate cages and 4 birds each. All bird management was consistent with the recommendations of the Arbor Acres Broiler Commercial Management Guide.

### Indirect calorimetry method

*ad libitum*access to diet and water. After 8 h of fasting being enforced by the withdrawal of feed [13], birds in each of 4 respiration chambers were weighed and were fed their respective level of feed intake at d 15. The amounts of O

_{2}consumption and CO

_{2}production were determined from d 15 to 21 to calculate the HP, using the Brouwer [14] equation without correction for urinary nitrogen excretion. The respiration quotient (RQ) was determined as the volume of CO

_{2}produced, divided by the volume of O

_{2}consumed. Water was offered

*ad libitum*at all times. Measurement was suspended for 2 h each day to replenish feed and to collect excreta. The collected excreta were pooled for each chamber over 5 d, stored in a freezer, dried, and ground to pass through a 0.5-mm screen. On d 21, birds were weighed to determine their weight gain and feed:gain.

### Comparative slaughter method

### Chemical analysis

### Calculations

_{f}) and protein (RE

_{p}) were calculated as follows:

^{0.75}/d.

^{0.75}), and b is constant.

### Statistical analyses

_{2}consumption, CO

_{2}production, and RQ data measured by the ICM and RE

_{f}and RE

_{p}by the CSM were analyzed by one-way analysis of variance (ANOVA) of SPSS 19.0 (2010, SPSS Inc., Chicago, IL, USA). All other data were analyzed by two-way ANOVA as a 2×4 factorial arrangement of treatments using the general linear model procedure of SPSS to test the main effects of the evaluation method, the feed intake, and their interaction. Differences among treatment means were determined using a Duncan’s means comparison when the significance of the factor was p<0.05.

### RESULTS

### Broiler growth performance

*ad libitum*feed intake. Birds fed 25% of

*ad libitum*feed intake had negative BW gain and feed:gain.

### Energy value of the diet and energy balance of broilers

*ad libitum*feed intake was higher (p<0.01) than that in birds fed 100% of

*ad libitum*feed intake. The dietary NE and NE:ME in the treatment of 100% of

*ad libitum*feed intake were lower (p<0.01) than these values in the other three treatments. The MEI, HP, and RE of broilers decreased (p<0.01) as the feed intake decreased. No evaluation method×feed intake interaction was observed on these parameters.

_{2}consumption and CO

_{2}production measured by ICM decreased (p<0.01) as the feed intake decreased. The RQ decreased (p<0.01) from 0.97 to 0.73 with the level of feed intake reducing from 100% to 25%. Similarly, energy retained as fat and protein measured by the CSM decreased (p<0.01) as the feed intake decreased.

### Energy requirement for maintenance

_{m}, NE

_{m}, and K

_{m}are shown in Table 5. From the linear regression equations (Equations 1 and 3), the ME

_{m}, which was calculated by extrapolating the MEI to zero energy retention, was 594 kJ/kg of BW

^{0.75}/d in the ICM and 618 kJ/kg of BW

^{0.75}/d in the CSM, and the NE

_{m}calculated as the intercept on the Y-axis of the linear regression equation was 386 kJ/kg of BW

^{0.75}/d in the ICM and 404 kJ/kg of BW

^{0.75}/d in the CSM. The K

_{m}, calculated as the ratio between NE

_{m}and ME

_{m}, was 65.0% in the ICM and 65.4% in the CSM. From the logarithmic regression equations (Equations 2 and 4), the calculated ME

_{m}was 607 kJ/kg of BW

^{0.75}/d in the ICM and 619 kJ/kg of BW

^{0.75}/d in the CSM, and the NE

_{m}calculated by extrapolating the HP to zero MEI was 448 kJ/kg of BW

^{0.75}/d in the ICM and 462 kJ/kg of BW

^{0.75}/d in the CSM. The K

_{m}was 73.8% in the ICM and 75.0% in the CSM.

### DISCUSSION

### Broiler growth performance

*ad libitum*feed intake had negative BW gain and feed:gain. It has been suggested that when the MEI is below the maintenance requirement, the energy used by broilers will not only be supplied by their diet, but also by their body reserves [6].

### Energy value of the diet and energy balance of broilers

*ad libitum*, which is in agreement with the observation reported by Hill and Anderson [17]. The increased dietary ME content at the lowest feed intake level might be ascribed to the change in the metabolic and endogenous energy losses of broilers. The determined feed NE increased as the feed intake decreased. The dietary NE value is usually calculated as the sum of the FHP (i.e., NE

_{m}) and RE [18]. Therefore, the increased NE value in a lower feed intake may be supported by previous observations that feed intake affects the FHP [4]. The dietary NE:ME in 100% of the

*ad libitum*feed intake group was 67.7%, which was lower than the mean NE:ME value (76.4%) observed by Carré and Juin [19] and 70.5% observed by Yang et al [20]. This could be associated with the differences in the diet composition and types (breed, age, sex, etc.) of poultry. The NE:ME of diet increased as the feed intake decreased, which means that the proportion of ME transformed into heat increment decreased as the feed intake decreased. This may be another reason for the higher NE value in the lower feed intake group.

_{f}), activity HP (HP

_{a}), plateau fasting HP (FHP

_{p}), and RE. Therefore, the reduction of the HP

_{f}and RE that accompanies a reduction in the MEI may result in the reduction of the HP. Energy retained as protein was positive and as fat was negative in 25% of the

*ad libitum*feed intake group measured by the CSM, which means that broilers can deposit protein by expending body lipid at a lower MEI [22,23]. On the another hand, the RQ for broilers receiving 25% of

*ad libitum*feed intake measured by the ICM dropped to 0.73, which also indicated that broilers utilized body fat deposits to maintain energy metabolism [24].

### Energy requirement for maintenance

_{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].

_{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].

_{m}and NE

_{m}estimated from the linear relationship between the RE and MEI were 594 and 386 kJ/kg of BW

^{0.75}/d in the ICM, and those in the CSM were 618 and 404 kJ/kg of BW

^{0.75}/d. The ME

_{m}and NE

_{m}estimated by logarithmic regression between the HP and MEI were 607 and 448 kJ/kg of BW

^{0.75}/d in the ICM, and those in the CSM were 619 and 462 kJ/kg of BW

^{0.75}/d. These results provide references for the determination of NE values of broiler feed ingredients.