Hello, everyone. My name is Su A Lee from the Stein Monogastric Nutrition Laboratory at
the University of Illinois. I’ll be discussing effects of dietary amino acids
concentration on digestibility of amino acids in soybean meal and soy protein concentrate
fed to growing pigs. This work was presented in 2020 ASAS Midwest meeting.

Let’s start with take-home messages before we go over to the presentation. First, values
for the standardized ileal digestibility (SID) of amino acids are not influenced if
dietary crude protein is less than 26%. Second, values for the SID of amino acids in feed
ingredients may be determined below, at, or slightly above requirements without impacting
results.

Because of microbial fermentation in the hindgut of pigs and almost all amino acids are
absorbed in the small intestines, digestibility of amino acids should be determined at the
end of ileum for an accurate estimation of amino acid absorption. However, when we collect
the digesta from pigs, both diet and basal endogenous origin amino acids are included.
Therefore, by correcting the apparent ileal digestibility (AID) for the basal endogenous
losses of amino acids, standardized ileal digestibility (SID) values can be calculated,
and these values are additive in mixed diets fed to pigs.

One of the reasons values for the SID of amino acids are additive is that the values are
not affected by level of amino acids in the diet. However, diets used in the previous data
did not contain extreme levels of protein that is very low or very high compared with the
requirements of pigs. And it has not been known that the level of protein or amino acids
below, at, or above requirement affects the SID of amino acids.

Therefore, the hypothesis of this experiment was that the SID of amino acids is not
influenced by dietary crude protein even if dietary amino acids are below, at, or well
above the requirement of pigs.

One source of soybean meal was prepared. There were 5 experimental diets. The first 4
diets included 10, 30, 50, and 70% soybean meal to contain about 5 to 30% crude protein in
diets. To determine the basal endogenous losses of amino acids, an N-free diet was also
formulated. Linear and quadratic effects of adding soybean meal to the diets were tested.

A total of 30 pigs with approximately 31 kg of body weight were used and fed ad libitum.
Pigs were randomly allotted to the experimental diets and there were 6 replicates per
diet. Ileal digesta were collected for 2 days after 4 days of adaptation. 

Jumping to the results, let me set up the slide first. The horizontal axis represents
inclusion rates of soybean meal in diets: 10, 30, 50, and 70%. The vertical axis
represents the AID of crude protein and amino acids in soybean meal in percent. As soybean
meal in diets or dietary protein levels increased, the AID of crude protein, lysine,
methionine, threonine, tryptophan, and valine quadratically increased. Other amino acids
showed the similar trend. This increase in the AID of amino acids can be explained by a
greater proportion of endogenous nitrogen and amino acids in the ileal digesta if protein
level is low. Therefore, the proportion of endogenous nitrogen and amino acids in the
ileal digesta is reduced as dietary crude protein increases, which results in an increase
in the calculated value for AID.

Now, we are looking at the results for the SID values and same are set up for this slide
as before except that the vertical axis represents the SID of amino acids in soybean meal
in percent. Even if soybean meal or protein levels increased in diets, the SID of amino
acids were not affected. This was what we expected because we have learned that values for
the SID of amino acids are not affected by dietary protein levels. 

Then what about other amino acids? Results from this experiment showed that the SID of
crude protein, histidine, lysine, leucine, phenylalanine, and threonine linearly or
quadratically decrease as soybean meal increased in diets. By adding soybean meal into the
diet, not only protein is increased, but also other nutrients are increased. 

For example, oligosaccharides such as stachyose and raffinose levels are also increased by
increasing inclusion levels of soybean meal. Concentrations of stachyose and raffinose
were approximately 5 and 1.5%, respectively. To eliminate effects of oligosaccharides from
soybean meal, Experiment 2 was conducted with soy protein concentrate, which was believed
to contain less oligosaccharides than soybean meal. This was because the low molecular
weight sugars are removed by alcohol extraction during production of soy protein
concentrate. Analysis data showed that concentrations of stachyose and raffinose in soy
protein concentrate were approximately 3 and 0.4%, respectively.

Let’s move on to Experiment 2. A total of 6 diets were prepared. First 5 diets included 10
to 50% of soy protein concentrate to contain from about 7 to 32% dietary crude protein.
The last diet was an N-free diet that was used to determine the basal endogenous losses of
amino acids to calculate the SID values from the AID values. As in Experiment 1, linear
and quadratic effects of increasing soy protein concentrate in diets on digestibility of
amino acids were tested using contrast coefficients.

We utilized 24 growing pigs and allotted them to 6 diets using a quadruplicated 6 x 2
Latin square design with 2 periods. Pigs were fed at 3 times maintenance requirement for
energy. Digesta samples were collected for 2 days after 5 days of adaptation.

Moving on to the results, let me set up the slide first. The horizontal axis represents
the inclusion rate of soy protein concentrate in diets and the vertical axis represents
the AID of crude protein and amino acids. The AID of crude protein and most amino acids
quadratically increased as soy protein concentrate increased in diets. This result is the
same as Experiment 1 and the reason the AID values were increased can be explained by the
different contribution of the basal endogenous losses of amino acids in diets containing
different levels of amino acids.

Moving to the next slide, where the SID of amino acids in soy protein concentration is
presented in the vertical axis, the results showed that values for the SID of arginine,
isoleucine, leucine, methionine, phenylalanine, tryptophan, and valine were not affected
by dietary protein levels. And we can see that there were more amino acids that are not
affected by dietary protein level in Experiment 2 compared with Experiment 1. 

However, there were a few amino acids that were affected by increasing soy protein
concentrate in diets. The SID of crude protein, histidine, lysine, and threonine
quadratically or linearly decreased as protein levels in diets increased. Results from
both Experiment 1 and 2 indicated that the SID of some amino acids is affected by dietary
protein.

Going back to the results from Experiment 1, where the SID values were decreased by
increasing the inclusion rate of soybean meal, we re-analyzed the data without the last
diet. This was because most decreases were found in the last diet. Without the last diet,
where 70% soybean meal was included in the diet to contain about 30% of crude protein, no
linearity was found. This means that if diet contains less than 22% of crude protein, the
SID values are not affected by inclusion level of soybean meal.

Looking back to the results from Experiment 2, where the SID values were decreased by
increasing the inclusion rate of soy protein concentrate, because there was a big drop of
the SID of amino acids in the last diet, we also re-analyzed the data without the last
diet. Without the last diet, no linearity was found. This means that if diet contains less
than 26.6% crude protein, the SID values are not affected by inclusion level of soy
protein concentrate.

Let’s move into conclusions. Results from the 2 experiments indicated that the SID of
amino acids are reduced if dietary crude protein concentration approaches 30%, but more
amino acids were negatively affected by dietary crude protein level if soybean meal was
used than if soy protein concentrate is used. The SID of amino acids seems not to be
affected if the dietary level is less than 26%, which is well above the requirement and
this means that pigs have a high capacity for digesting proteins. The implication of this
observation is that the direct procedure may be used to determine the SID of amino acids
in a wide range of feed ingredients. Specifically, these data indicate that the direct
procedure may be used to determine the SID of amino acids in cereal grains and other
low-protein ingredients that are often used in diets fed to pigs. 

In the first experiment, we started with soybean meal, which contains a lot of
antinutritional factors including oligosaccharides, fiber, trypsin inhibitor, and so on.
Therefore, by adding more soybean meal into the diets, not only protein but also other
nutrients increase in the diets.  Therefore, the effect of oligosaccharides was eliminated
in the second experiment by using soy protein concentrate. Next experiments shall be
conducted with soy protein isolate, which does not contain any oligosaccharides and
trypsin inhibitor, and with casein and crystalline amino acids that does not contain any
anti-nutritional factors.

I would like to acknowledge everyone from Dr. Stein’s lab. And if you want to learn about
our research that we are conducting in the Stein Monogastric Nutrition Laboratory, please
visit our website, or you can search “Stein” and “pig” on google. Thank you for listening.