Abstract: Our current net energy (NE) systems were developed based on work conducted around 40 years ago. At that time, it was normal that pigs deposited no more than 40% of the protein they were fed. However, modern genetic lines are much leaner than older genetics and they deposit more protein than earlier. It is, therefore, not uncommon that protein deposition in modern genotypes exceeds 60% of dietary protein intake. This means that more of the amino acids in the protein-rich ingredients are used for protein deposition and less are being deaminated with a subsequent reduced energetic cost of deamination associated with feeding protein-rich ingredients. As a consequence, the theoretical energy value of protein rich ingredients such as soybean meal (SBM) is greater when fed to modern pigs than to older genotypes that were used when our current NE systems were developed. Indeed, in several recent experiments it was demonstrated that the ME and NE of SBM is greater than indicated in current feed ingredient tables (Table 1). A greater NE than predicted has also been demonstrated for field peas, whereas corn does not seem to have increased energy value compared with previous estimates (Table 2), which is consistent with the hypothesis that it is the increased protein efficiency of modern pigs that is the reason for the greater NE value in SBM. As a consequence of the greater NE value of SBM, less added fat needs to be used to reach a certain NE level in the diet, which will reduce overall diet costs. However, because of the greater genetic capacity for protein deposition in modern pigs, there is a need to upgrade the NE system and to develop new equations to predict NE in diets and ingredients based on the protein deposition capacity of modern pigs. It can also be argued that because commercially fed pigs always are group housed and allowed ad libitum intake of feed, pigs fed to generate prediction equations for NE also need to be group housed and allowed ad libitum intake of feed because both animal behavior and level of feed intake may affect the NE value of diets.
In conclusion, because of the increased ability of modern genotypes of pigs to retain dietary protein in the body, there is reduced deamination of amino acids and therefore reduced energy loss in modern pigs than in older genotypes, which results in greater NE of protein rich ingredients including SBM. There is, therefore, a need for developing a new NE system that preferably uses group housed pigs that are allowed ad libitum access to feed.
