Exploring the net energy for maintenance and the allometric exponent of the metabolic body weight in different categories of poultry: A meta-analysis approach

The basal metabolic rate in poultry, expressed as net energy requirements for maintenance (NEm), is often characterized by measuring fasting heat production (FHP) under standardized conditions. This study applies meta-analysis techniques to 206 individual observations from 23 peer-reviewed papers to evaluated the allometric relationship between FHP and body weight (BW), described by the model: <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mtext>FHP (kJ/d)</mml:mtext> <mml:mo>=</mml:mo> <mml:mi>a</mml:mi> <mml:mo>×</mml:mo> <mml:msup><mml:mtext>BW (kg)</mml:mtext> <mml:mi>b</mml:mi></mml:msup> </mml:math> , where FHP is the fasting heat production, BW is the body weight, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>a</mml:mi></mml:mrow> </mml:math> is the NEm requirement per unit of metabolic body weight (MBW), and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>b</mml:mi></mml:mrow> </mml:math> is the allometric exponent. The objective of this study was to estimate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>b</mml:mi></mml:mrow> </mml:math> and determine the appropriate NEm for growing broiler chickens, laying hens, and roosters. We evaluated three non-linear models: Model 1, category-specific exponents; Model 2, common exponent across categories; and Model 3, exponent fixed at the theoretically value of 0.75. Model 1 showed distinct <i>b</i> of 0.73, 0.65, and 0.58 for growing broilers, laying hens, and roosters, respectively, with corresponding NEm values of 450, 352, and 385 kJ per kg of MBW. Model 2, which estimated a common exponent (<i>b</i> = 0.64), improved model fit (root mean square prediction error (RMSPE) = 173 kJ/d, <i>R</i> ² = 0.76). Model 3, using the standardized <i>b</i> = 0.75, yielded NEm of 478 kJ/kg BW^0.75 for growing chickens and 320 kJ/kg BW^0.75 for adult birds (RMSPE = 184 kJ/d, <i>R</i> ² = 0.78), with a model performance comparable to Model 1 (RMSPE = 187 kJ/d, <i>R</i> ² = 0.72). Error decomposition further indicated that Model 3 presented a lower error due to regression (0.66%), a lower intercept (80 kJ/d) and a higher slope (0.68) in the observed-predicted regression, demonstrating its robust predictive ability. In conclusion, while category-specific exponents offer a precise FHP characterization, adopting a wide-know exponent of 0.75 is a practical alternative, provided the NEm is adjusted for each category. These findings provided a framework for estimating NEm recommending values of 480 kJ/kg BW^0.75 for growing chickens and 320 kJ/kg BW^0.75 for adult birds, to support accurate energy requirements models and cross-study comparison in poultry nutrition.