Protein is not a nutrient. Animals require amino acids and not protein
per se. The quality of protein supply is determined by its potential
to cover the physiological requirements in terms of amino acids for
maintenance and performance (growth, reproduction, production of
milk and eggs). The quality of protein required is different depending
on the animal species, age, genotype and sex as well as on the
performance level. It follows therefore that there are two important
factors with respect to protein quality, a) the amino acid profile which
is the ratio of essential amino acids in the protein and b) their digestibility
(see 3.7). Ingredients that contain excellent quality proteins are
dried skimmed milk powder and complete egg protein. Other protein
sources lack certain essential amino acids and thus are unable to
closely match the requirement of the animal in terms of their amino
acid composition.
Figure 6 demonstrates that soybean meal alone, with the exception
of sulphur amino acids (methionine, cystine), can supply all essential
amino acids necessary to satisfy the requirements of a pig (30 – 50 kg
liveweight). However corn has an amino acid profile which meets the
requirements for sulphur amino acids.
A protein quality must always refer to a specific situation with respect
to protein use. In practice, standards of requirement for amino acids
are usually given for age and performance levels for all of the major
livestock species. With the aid of these standards the quality of a feed
protein can be estimated as shown in the example in Figure 7.
For a particular feed ingredient or mixture of ingredients supplying
protein, the amino acid which is in shortest supply compared with the
requirement of the animal, is defined as the first-limiting amino acid.
The first limiting amino acid limits the value of protein.
In the previous example the amino acid lysine is the first limiting amino
acid in corn protein and methionine or the sum of sulphur containing
amino acids in soybean protein.
Unfortunately, the total content of amino acids in a feed protein, which
is determined by chemical analysis, is not fully digestible (see 3.7). A
measure of the total amino acid content of a feed protein in comparison
with a set of requirements can therefore only give a first approximation
with respect to the quality.
The ruminant animals derive their amino acids supply jointly from
dietary protein which escapes rumen degradation (by-pass protein)
and microbial protein synthesized in the rumen. The dietary protein
is extensively degraded in the rumen and is mainly used by rumen
bacteria for their own protein synthesis. The microbial protein that
reaches the intestine presents the most appropriate protein quality
for ruminants since it has a similar amino acid profile as the animal
requirements. The amino acid composition of the by-pass fraction
usually is not in line with animal requirements therefore amino acids
need to be supplied.