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Efficient and sustainable poultry production requires accurate estimation of productive (retained) energetic values of feed ingredients and complete diets. Currently, metabolizable energy corrected for zero nitrogen retention (MEn) is the most commonly used energy value for ingredients and diets for the broiler industry. There is an ongoing debate between researchers on whether the industry would benefit from a net energy (NE) system over the ME system. The purpose of the NEg value for feed is to characterize the quantity of the energy in the feed that is retained in the body and not released as heat. The objective of this study was two-fold: 1) to evaluate the accuracy of a novel mathematical modelling methodology for energy partitioning to determine HP, RE, and NEg from ME intake compared with the CST and 2) to estimate diet-specific HIF by comparing the slope of the linear regression of HP on ME intake of 2 energetically different diets.
The experiment was conducted as randomized block design of a 2 × 6 factorial arrangement of treatments with 50 broilers in 4 pens (blocks) fed with an isonitrogenous low-ME (3,111 kcal/kg ME) or a high-ME (3,383 kcal/kg ME) grower diet from day 14 and were provided ad libitum feeding or received 50, 60, 70, 80, 90% of ad libitum from day 19 with a precision feeding (PF) station. The PF system recorded individual BW and feed intake on a per visit basis. Shank temperature measurements were taken from all birds on day 22, 28, 35, and 42 with a handheld infrared camera. At day 45, all birds were killed. The abdominal fat pad, filled gastrointestinal tract (GIT), breast muscle, heart, legs, and liver weight were recorded during dissection. Carcass samples were analyzed in duplicate for determination of total carcass dry matter, crude protein, lipid, and ash using standard chemical analysis procedures. Two methods were used to determine HP and RE in this study, the CST and a mathematical model explaining energy intake as a function of BW and gain.
BW at day 45 did not differ between birds fed with the high-ME diet and the low-ME diet. As the diets were isonitrogenous, the high-ME diet had a higher ME:CP ratio than the low-ME diet (13.70 kcal/g vs. 12.35 kcal/g). Therefore, birds fed with the high-ME diet could have overconsumed ME to meet their CP requirement. BW-corrected breast muscle (P = 0.028) and liver weight (P = 0.002) were higher in birds fed with the low-ME diet than those in birds fed with the high-ME diet. BW-corrected fat pad weight was higher in birds fed with the high-ME diet (P = 0.014). The nonlinear mixed model underestimated HP by 13.4% and overestimated RE by 22.8% compared with the CST. The model was not able to distinguish between net energy for gain values of the diets (1,448 ± 18.5 kcal/kg vs. 1,493 ± 18.0 kcal/kg for the low-ME and high-ME diet, respectively), whereas the CST found a 148 kcal/kg difference between the low-ME and high-ME diets (1,101 ± 22.5 kcal/kg vs. 1,249 ± 22.0 kcal/kg, respectively). The estimates of the net energy for gain values of the 2 diets decreased with increasing feed restriction. The heat increment of feeding did not differ between birds fed with the low- or high-ME diet (26% of MEI).
The nonlinear model provided a noninvasive real-time method to measure HP and RE in broilers. However, the model was not able to distinguish the NEg values of the 2 diets. Estimates of the NEg values increased when feed intake was reduced. The HIF could be determined with the modeling methodology and was in the range of values in the literature. Additional measurements on heat dissipation, physical activity, and immune status indicated that the energetic content of the diet and feed restriction affect some parameters (shank temperature, feeding station visits) but not others (leukocyte counts, H:L ratio, and immune cell function). Further research is needed to understand dietary factors affecting ME available for productive processes, including more comprehensive analysis on the energy expenditure on activity and immunity.