The chicken had a better or poorer feed efficiency. The results depended on which alternative feed ingredient was chosen. The performance was almost always different and, in most cases, the performance was poorer. The reason is because the chicken does not respond to the gross energy content. The chicken responds to the metabolizable energy content of the ingredient.
Metabolizable energy is roughly the equivalent of the digestible energy content of the ingredient.
Research has shown that if other nutrients are correctly balanced, corn can be replaced by other ingredients based on their relative metabolizable energy content and produce the same predictable performance. Metabolizable energy, whether ME, TME, AME, or another suitable measure, is the best evaluation of the useable energy in a feed ingredient. Commercial nutritionists use metabolizable energy values in formulating feeds because they know ME values are important and they know ME values work. The use of digestible amino acid values is the same basic idea and is just as important. There are 13 essential amino acids.