The net energy (NE) system describes the useful energy for growth better than the metabolizable energy (ME) system. Therefore, the NE system should demonstrate a more predictable animal response when a wide range of ingredients are used, but this fact needs to be demonstrated in practice. The objective of this thesis was to evaluate the NE system in diets containing a diverse set of ingredients. Two experiments were conducted: the first experiment consisted of comparing growth performance, carcass characteristics and the efficiency of dietary energy of 2,054 pigs housed in pens in a commercial research barn. Pens were assigned to one of 5 different feeding regimes. A corn-soybean meal control diet served as the basis to establish baseline levels of ME and NE for both programs. Two treatments added DDGS to the control diet formulated using the ME or the NE system and another set of diets that added both DDGS and corn germ meal and was formulated using the ME or the NE system. Diets formulated with the ME and the NE system maintained overall whole body growth performance. However, carcass parameters (except carcass G:F and lean percentage) declined with the addition of co-products especially in diets formulated with the ME system. The intake of NE decreased in the same fashion that carcass gain did, suggesting a relationship between energy intake and energy retention. Additionally, NE per kg of carcass was similar among diets suggesting that NE is better at explaining carcass results. The second experiment compared the apparent total tract digestibility (ATTD) of energy and of nutrients and the nitrogen retention (NR) of 40 gilts. The 5 dietary treatments included a control corn soybean meal-based diet, a diet similar to the control diet but containing 6% each of corn distillers dried grains with soubles (DDGS), corn germ meal and wheat middlings with NE constant relative to the control diet, or allowed to decline. A last set of diets contained 12% each of the same co-products and NE held constant or allowed to decline. The diet digestion containing increasing levels of co-products and formulated to a constant NE concentration resulted in the expected equivalence of DE, ME and NE concentration. However, NR declined on all co-product diets. In conclusion, adopting the NE system does not imply a decline of productive parameters compared to the ME system, because in no-instance was the ME system superior to the NE system.