Here is another fascinating question I had to answer in the broiler grower conference in which I took part recently: Is it possible to completely replace soybean meal with amino acids?
How do I reply to this question – as a scientist or as an applied nutrition consultant? Let's see both answers.
In theory, broilers – and all farm animals – do not absorb proteins they consume. They digest them down to individual amino acids which, in turn, they absorb to make their own proteins. So, assuming all essential and non-essential amino acids are present at sufficient concentrations, then it may be possible to replace the protein fraction of soybean meal with free amino acids. I say the protein fraction only, because soybean is not just protein, but that is another conversation.
Then, there is the issue of different speed in amino acid absorption versus that of glucose (energy) from starch. Free amino acids are absorbed faster (no need to be broken down through the digestion process), whereas glucose needs to be released from starch by enzymes (this takes time). In the meantime, however, absorbed amino acids, not finding enough energy to build up proteins, have nowhere to go or are stored and they get de-aminated (excreted). Again, this is the theory, but broilers being nibblers, they eat all day long and they keep a full stomach, so they should have enough energy to cover the first wave of absorbed amino acids. But that assumes all broilers have ample access to feed, which may not always be the case. So, many qualifications before one takes the theory into practice.
The issues described above have no clear answer, as soybean meal is not just amino acids, and I do not believe there would be any cost savings in the end. So, why bother? Simply because there is not enough local soybean meal, and it is less costly to transport free amino acids than soybean meal. But, again, research has shown that very low-protein diets actually do not support equal performance as a more balanced diet that contains soybean meal (or an equivalent protein source, such as rapeseed meal) and some free amino acids.
Then, the question becomes, how low can we go in protein (or soybean meal, actually) when we have available so many free amino acids in readily available form and at interesting prices? Well, that would be another conversation.