I recently received an email from a client in Canada. He had read some reports that stated there should be a specific ratio between lysine and protein -- something like 7.5:1, which is very close to the lysine concentration in lean tissue. His question was how to achieve this level without overdoing it with crude protein or lowering the total lysine in the diet.
Here’s my answer based on what I learned at graduate school and to my knowledge still holds true, as it is based on undisputable digestive physiology principles:
1. Protein in feeds and lean tissue is composed of amino acids. Some of them cannot be recycled from others and as such they must be consumed through feed. These are the “essential” amino acids, among which lysine is usually the most deficient in feeds (relatively to requirements) for pigs, and methionine for poultry. Hence, these two are called first limiting amino acids in such feeds.
2. Protein in the gut is digested. That is, it is broken apart into individual amino acids that subsequently are absorbed, enter the blood stream, and are used for maintenance and productive purposes. The inefficiency of this breakdown is what determines “digestibility.”
3. Pigs (and poultry) do not require protein (that is, crude protein) in their diets. Research has shown they can thrive on a diet that is composed of all individual amino acids – no matter how expensive such a diet might be!
4. Pigs and poultry require digestible amino acids. These requirements are well established. The same is true for the digestibility of these amino acids in feeds.
5. As long as amino acids are provided to required levels, then crude protein can be lowered. There is point where this can no longer happen, and this point is determined by the cost and availability of essential amino acids. Thus, we say, with the four feed-grade amino acids, we can safely lower crude protein by four percentage points. Going beyond that, it will make another amino acid limiting and this will limit growth and productivity.
In brief, as long as monogastric (or non-ruminant) animals receive adequate levels of amino acids, one needs to pay but little attention to crude protein.