Classic net energy (CNE), calculated as apparent metabolizable energy (AME) minus the heat increment (HI), has shown limited ability to predict protein accretion (PAC) while performing better for fat gain (FG). This study evaluated three modifications intended to improve the predictive capacity of CNE for PAC and FG and compared them with productive energy (PE), defined for broilers as net energy for gain (NEg) + net energy for maintenance (NEm), both directly measured. Five energy systems were assessed: (1) NE-1, the classical NE estimate using HI calculated from calorimetry total heat production minus a reference fasting heat production (FHPr); (2) NE-2, using the measured fasting heat production (FHPm); (3) NE-3, using directly determined NEg but still FHPr; (4) NE-4, derived from AME efficiency to produce BW gain; and (5) PE. In this study, PE was determined as Arkansas Net Energy (NEg measured with dual-energy X-ray absorptiometry, DEXA; NEm measured with calorimetry chambers using FHPm). Eight experiments were conducted, each starting with 240 day-old chicks allocated to 12 pens and fed one of 12 diets differing in digestible amino acids (or dCP), digestible starch, digestible fat, and non-starch polysaccharides. All experiments began simultaneously, and each 7-day feeding period was initiated one week apart. A digestibility study determined AME and AMEn. Recorded variables included BW gain (BWG), PAC (g/bird per day), FG (g/bird per day), NEg (kcal/kg; DEXA), FHPm, AME/BWG efficiency, NE-1 through NE-4, and PE. Data were analyzed using a mixed model with 12 treatments, 8 replications, and experiment as a random effect. PE strongly predicted PAC (P = 0.006; adj. R2 = 0.98) and FG (P < 0.001). NE-1 (P>0.05) and NE-2 (P = 0.031) predicted FG. NE-1 (P>0.20) and NE-2 (P=0.15) did not predict PAC. NE-3 improved the P-values for PAC prediction relative to NE-1 and NE-2, but it still failed to significantly predict PAC (P>0.05). NE-4 likewise did not predict PAC (P>0.05). In conclusion, none of the refined NE systems based on metabolizable energy matched the predictive strength of PE for protein accretion in broilers. Energy systems derived from metabolizable energy fail to predict PAC efficiently.

Martinez, D., C. Ponce-de-Leon, O. Aguilar and C. Coon. 2026. Three evaluated approaches to refine the classical net energy system fail to match the predictive strength of productive energy in broilers.2026 International Poultry Scientific Forum, abstract 380P.