Slide 1
Hello. I am Hans H. Stein, and I'm an Associate Professor in the 
Department of Animal Sciences at the University of Illinois. I'm here 
today to talk to you about a concept called the standardized total tract 
digestibility of phosphorus by pigs. This is a relatively new concept 
that we have developed right here at the University of Illinois. We do 
believe that if you formulate diets based on the standardized total 
tract digestibility of phosphorus, then you will get a more accurate 
diet formulation, which will result in reduced feed costs and also in 
reduced excretions of phosphorus from the pigs. So I'll try to explain 
some of the details of this new concept. I do want to point out that we 
first prepared this presentation for the International Phytase 
Symposium, which was conducted in Washington, D.C. in September 2010.

Slide 2
Traditionally, we have evaluated phosphorus in feed ingredients by 
measuring what we call the relative bioavailability of phosphorus. This 
concept involves measuring the bioavailability of phosphorus in a 
standard -- in this case the standard is depicted by a green color -- 
and then there is one or two or several test ingredients involved. A 
basal diet is formulated and then two or three levels of phosphorus from 
the standard and from each of the test ingredients is added to the basal 
diet. That will give a response, which can be bone ash or bone breaking 
strength or bone phosphorus concentration, at each level of phosphorus 
inclusion. After having measured the response to the different levels of 
phosphorus in the diets, the response criteria is regressed on 
phosphorus intake for each feed ingredient and regression lines are 
developed. By dividing the slopes of the regression line for each test 
ingredient by the slope for the regression line for the standard, the 
relative bioavailability of phosphorus in each test ingredient is 
calculated. This procedure has been used for the last 40 years in the 
U.S. swine industry, and it has worked very well in particular when 
different sources of phosphorus are compared. So, it's easy to calculate 
the bioavailability of phosphorus in one source compared to another. 
However, all values depend on the standard and because the standard is 
not the same in all experiments, values from one experiment cannot be 
compared to values from a different experiment. That creates problems in 
diet formulations, because in diet formulations we use multiple 
ingredients and the values for relative bioavailability are not additive 
in mixed diets. There is, therefore, a need for developing a new 
concept.

Slide 3
Instead of measuring relative bioavailability, the digestibility of 
phosphorus can also be measured. It is not difficult to measure 
digestibility of phosphorus in feed ingredients; however, there are 
several questions that need to be answered when digestibility is 
measured. One of these questions is: Should we measure the ileal or the 
total tract digestibility of phosphorus; what is most accurate? Another 
question is: What is the effect of the phosphorus level in the diet on 
phosphorus digestibility? And a third question is: Should we use values 
for apparent or for standardized or for true digestibility to get the 
most accurate values that can be used in practical feed formulation? 
I'll talk about all three of these issues and I'll also give some 
examples of how to calculate digestibility.

Slide 4
We first talk about the apparent ileal digestibility versus apparent 
total tract digestibility. Here are some data from a paper that was 
published in the Journal of Animal Science by Bohlke et al. several 
years ago. And in this case, there were three feed ingredients used: a 
low-phytate corn, a normal corn, and soybean meal. Both the apparent 
total tract digestibility and the apparent ileal digestibility were 
measured. And it's easy to see here that for all three of these 
ingredients, there was no difference between values for apparent total 
tract digestibility and values for apparent ileal digestibility because 
all the values are very close to each other. This indicates that both 
values for ileal digestibility and total tract digestibility can be 
used, because they are virtually the same. However, it is easier and 
less expensive to measure values for total tract digestibility than to 
measure values for ileal digestibility, and therefore we suggest using 
values for total tract digestibility.

Slide 5
Here's the second experiment in which values for duodenal digestibility, 
ileal digestibility, and total tract digestibility were compared. And in 
this case, it's easy to see that the duodenal digestibility is very low 
-- about 10% for all three diets that were used here -- whereas values 
for ileal digestibility and values for total tract digestibility are 
much greater, and they are very close to each other for all three diets. 
That means that, as you saw before, total tract digestibility gives 
values that are similar to ileal digestibility. In this case, the three 
diets that were used were the control diet consisting of corn and 
soybean meal, and then there was that same diet with 500 units of 
phytase or 1000 units of phytase. And it is clear from these data that 
the duodenal digestibility is not affected by the presence of phytase in 
the diets. However, both the ileal and the total tract digestibility 
values are much greater when either 500 or 1000 units of phytase is 
added to the diet. So, from this it is also concluded that phytase 
increases the digestibility of phosphorus in a corn-soybean meal diet.

Slide 6
However, not all feed ingredients will see an increase in phosphorus 
digestibility when phytase is used in the diet. And here are some data 
from an experiment by Almeida and Stein that was published in 2010 in 
the Journal of Animal Science. In this case, we used corn, soybean meal, 
and distillers dried grains with solubles, also called DDGS, and we fed 
each ingredient either without phytase (which is the red bars) or with 
phytase (which is the blue bars). For corn and soybean meal, we observed 
a significant increase in the apparent total tract digestibility of 
phosphorus when we added phytase to these ingredients -- the same way as 
you saw in the previous experiment. However, when we used DDGS, there 
was no increase in phosphorus digestibility when we used phytase. We are 
not certain why that is, but we have repeated this and other experiments 
and we are pretty convinced that there's no effect of phytase when we 
add it to DDGS. And the reason may be that the digestibility of 
phosphorus in DDGS is much greater than in corn and soybean meal to 
begin with and therefore there is less room for improvement when we add 
phytase to these diets.

Slide 7
The final question we need to answer is: Should we use values for 
apparent total tract digestibility, standardized total tract 
digestibility, or true total tract digestibility of phosphorus? We can 
calculate values for apparent total tract digestibility by simply 
subtracting the output of phosphorus from the intake of phosphorus for a 
pig fed a given diet. That value will represent the apparent total tract 
digestibility. If you want to calculate the true total tract 
digestibility, we have to subtract the total endogenous losses of 
phosphorus from the output of phosphorus before we subtract from the 
intake. So, this approach requires that we measure not only the intake 
and the output of phosphorus from the pigs, but also the total 
phosphorus losses from the pigs. The third option is to measure 
standardized total tract digestibility values of phosphorus, and in this 
case we also have to make a correction for endogenous losses. But we 
don't measure the total endogenous losses but only what we call the 
basal endogenous losses of phosphorus and we subtract these basal 
endogenous losses from the phosphorus output from the pigs, and then we 
subtract that value from the intake of the pigs. From this, it appears 
that values for true total tract digestibility of phosphorus as well as 
values for standardized total tract digestibility of phosphorus are 
corrected for endogenous losses of phosphorus. But in the case of true 
total tract digestibility of phosphorus, we correct for the total 
endogenous losses, whereas for standardized total tract digestibility of 
phosphorus, we only correct for the basal endogenous losses.

Slide 8
To talk a little bit more about the endogenous losses, it's important to 
point out that endogenous losses of phosphorus are losses that occur 
from the pig no matter which diet it is fed. The total endogenous loss 
of phosphorus consists of a basal loss and of a specific loss. The basal 
loss is the loss of phosphorus that a pig will have just from normal 
cell turnover no matter which diet the pig is eating. And the basal 
losses are not influenced by the type of diet that is fed to the pig. 
The specific endogenous losses, on the other hand, are influenced by the 
type of diet, and the greater the concentrations of fiber and the 
greater the concentrations of antinutritional factors in the diet is, 
the greater are the specific endogenous losses. When we add the specific 
endogenous losses and the basal endogenous losses we get the total 
endogenous losses. There's been, over the last few years, six 
experiments in which the total endogenous losses have been estimated in 
growing pigs. The six experiments are listed here, and it appears that 
there is quite a bit of variation in the estimates of total endogenous 
losses from a low of 8 mg/kg dry matter intake, and all the way up to 
670 mg/kg of dry matter intake. The reason for this great variation 
among different experiments is that the procedure used to estimate total 
endogenous losses is not very accurate, and that results in big 
variations among estimates. So the biggest problem we have with total 
endogenous losses is that we don't have a good procedure that can 
accurately estimate these total endogenous losses, and that makes it 
difficult to calculate the true total tract digestibility of phosphorus 
in diets fed to pigs.

Slide 9
The other option we have is to simply measure the basal endogenous 
losses of phosphorus and then subtract that from the output of 
phosphorus, which will allow us to calculate the standardized total 
tract digestibility of phosphorus. Over the last few years, eight 
different experiments have been conducted to estimate the basal 
endogenous losses. In all of these experiments, these losses were 
measured simply by feeding a diet to the pig that contained no 
phosphorus and then all the output of phosphorus from the pig was 
assumed to originate from endogenous sources such as cell turnovers and 
mucin and microbial phosphorus. The average basal endogenous loss of 
phosphorus from these eight experiments is 188 mg/kg dry matter intake. 
And it appears from this slide that there's not nearly as much variation 
in these numbers as there were in the numbers for total endogenous 
losses. Based on this observation, we suggest that values for 
standardized total tract digestibility of phosphorus are measured 
because we can relatively accurately estimate the basal endogenous 
losses of phosphorus, and these values are not very variable among 
different experiments.

Slide 10
To illustrate the importance of measuring standardized total tract 
digestibility of phosphorus, we conducted an experiment with three 
different types of whey powder. We had traditional whey powder, and we 
had whey permeat called Perlac 850, and we had another whey permeat 
called Variolac 960. You see on this slide that the concentration of 
phosphorus in the whey powder and the Perlac 850 was 0.63 and 0.57% 
respectively; however Variolac 960 contained only 0.10% phosphorus.

Slide 11
We measured the apparent total tract digestibility of phosphorus in 
these three sources, and it appears that values of 84.3 and 86.1%, 
respectively, were obtained for whey powder and for Perlac 850. However 
the apparent total tract digestibility of phosphorus in Variolac 960 was 
only 55.9%, which was significantly less than in the two other sources.

Slide 12
However, when we calculated the standardized total tract digestibility 
of phosphorus in these three sources of whey powder, we saw that values 
of 91.2, 93.1, and 91.8% were obtained for the three sources. And these 
values were not different. What this tells us is that while values for 
apparent total tract digestibility of phosphorus in Variolac 960 was 
significantly less than in the other two sources of phosphorus, this was 
not the case when we converted the values to standardized total tract 
digestibility of phosphorus. And the reason for that is that the 
endogenous losses from the pigs represent a much greater proportion of 
the total phosphorus output in Variolac 960 compared with Perlac 850 and 
whey powder. And the reason for that is that the total concentration of 
phosphorus in Variolac 960 was much less than the concentration in the 
other two sources of whey powder. So, from this, it appears that if we 
have a feed ingredient with a low concentration of phosphorus, then that 
feed ingredient will have a calculated value for apparent total tract 
digestibility that is relatively low because of a greater contribution 
of endogenous losses in the output from the pig. However, this mistake 
is eliminated if we calculate values for standardized total tract 
digestibility of phosphorus, and that is one reason why we believe 
standardized total tract digestibility values are more accurate than 
apparent total tract digestibility values.

Slide 13
Here's another experiment that illustrates the same thing. This is a 
manuscript that was published in the Journal of Nutrition by Dr. Fan and 
his colleagues back in 2001. And what Dr. Fan did was, he had four 
different diets with different levels of soybean meal -- 13.6, 27.3, 
40.8, or 54.6%. In all of these diets, soybean meal provided all the 
phosphorus. So, as the concentration of soybean meal increased in the 
diets, the phosphorus measured as g/kg dry matter also increased. Dr. 
Fan then measured the apparent total tract digestibility of phosphorus 
in these four diets, and he got values of 18.8, 37.6, 38.5, and 45.2% 
for the four diets. And these values were significantly different, and 
they increased as more soybean meal was included in the diet. However, 
if we convert those values for standardized total tract digestibility of 
phosphorus, using an assumed value for the basal endogenous losses of 
200 mg/kg dry matter intake, then we can see we get values of 43.4, 
48.4, 44.7, and 48.1%, and all these values are relatively similar. So, 
again, as was the case in the previous experiment, we see here that if 
we use values for the standardized total tract digestibility, then we 
eliminate the mistakes that we make when we have values for apparent 
total tract digestibility calculated in diets with a low concentration 
of phosphorus.

Slide 14
So, in conclusion from this part, we can say that values for the basal 
endogenous losses are much less variable than values for total 
endogenous losses of phosphorus. And that makes it possible to calculate 
values for the standardized total tract digestibility of phosphorus in 
different feed ingredients. And these values are believed to be additive 
in mixed diets and they are believed to be more accurate than if we used 
values for apparent total tract digestibility.

Slide 15
To illustrate the importance of using standardized total tract 
digestibility values, we measured the digestibility of phosphorus and 
the effect of phytase in distillers dried grains with solubles and other 
corn co-products.

Slide 16
If we look at the concentration of phosphorus in corn, DDGS, 
high-protein DDG, and corn germ, then we'll see that corn contains only 
about 0.26% total phosphorus, DDGS contains about 0.60 to 0.70% 
phosphorus, high-protein DDG contains 0.35% phosphorus, whereas corn 
germ contains more than 1% phosphorus.

Slide 17
We measured the apparent total tract digestibility of phosphorus in 
these four ingredients. We quickly note that the digestibility of 
phosphorus in DDGS and in high-protein DDG is much greater than in corn 
and in corn germ. The reason for this increased digestibility in DDGS 
and HP-DDG, we believe, is because these two ingredients have gone 
through fermentation in the ethanol plant, and therefore they have less 
phytate-bound phosphorus compared with corn and corn germ, which have 
not gone through fermentation.

Slide 18
If we add phytase to these ingredients, then we see an increase in the 
digestibility of phosphorus in corn and in corn germ, and we now get 
values that are close 55 to 60% for these two ingredients. The 
digestibility of phosphorus in DDGS and in high-protein DDG also 
increases, but not quite as much as for the other ingredients.

Slide 19
If we look at the standardized total tract digestibility, we'll see 
values that are greater than the values for the apparent total tract 
digestibility, but again, values for DDGS and high-protein DDG are 
greater than for corn and corn germ.

Slide 20
But when we add phytase to these ingredients, then the differences among 
ingredients are less. It is worth noting here that for high-protein DDG, 
we get a standardized total tract digestibility of phosphorus that is 
almost 90%. That means that almost all the phosphorus in this ingredient 
is absorbed by the pig. Same thing is the case for DDGS, where we have a 
digestibility of almost 80%.

Slide 21
We used these data to conduct a few experiments in which we looked at 
the effects of phytase and DDGS in diets fed to weanling pigs. This work 
was conducted by Mr. Ferdinando Almeida, who is a graduate student in 
the Department of Animal Sciences at the University of Illinois.

Slide 22
In these experiments, we used four different diets. All diets were based 
on corn and soybean meal, but two of the diets contained no phytase, 
while the other two diets contained 500 units of microbial phytase. Two 
diets were formulated based on only corn and soybean meal, while the 
other two diets -- in this case Diets 3 and 4 -- were formulated by 
adding 20% DDGS to these corn-soybean meal diets. All diets were 
formulated to contain 0.32% standardized total tract digestible 
phosphorus. So to get to that level, we had to add 1.15% dicalcium 
phosphate in the corn-soybean meal diet that contained no phytase. 
However, when we added phytase to the corn-soybean meal diet, we needed 
only 0.35% dicalcium phosphate to get to the 0.32% standardized total 
tract digestible phosphorus in the diet. When we added DDGS to the 
corn-soybean meal diet, we needed 0.65% dicalcium phosphate if we had no 
phytase in the diet. However, if we added phytase along with the DDGS to 
the corn-soybean meal diet, then we need no dicalcium phosphate because 
we already had 0.32% standardized total tract digestible phosphorus in 
this diet. We then fed all four diets to weanling pigs from 11 to 15 kg.

Slide 23
We then fed all four diets to weanling pigs from about 11 kg and until 
21 kg. We note here that the average daily gain was not influenced by 
diet composition, although there was a tendency for an increased average 
daily gain when we added DDGS to the diets. Likewise, for the average 
daily feed intake, there was no effect of phytase on feed intake, but 
there was a tendency for an increased average daily feed intake when we 
added DDGS to the diet. Gain:feed was not influenced by any of the 
diets, although there was a tendency for a reduction in gain:feed when 
we added phytase to the corn-soybean meal diet, but that was not the 
case when we added it to the DDGS-containing diets. So overall, the 
final body weight of the pigs was not different among these four diets, 
but again there was a tendency for greater body weight of pigs fed the 
DDGS-containing diets. We also measured phosphorus retention and 
phosphorus excretion from the pigs fed these diets. It appears that 
phosphorus retention was 2.05 g/day and 2.07 g/day when pigs were fed 
the corn-soybean meal diets without or with phytase, and these values 
were not different. However, when we fed the diets containing DDGS, 
retention increased to 2.35 and 2.33 g/day respectively, and these 
values were greater than the values for the corn-soybean meal diets. 
However, we see from these slides that for both types of diets, there is 
no difference between the diets without phytase and the diets with 
phytase, which indicates that the digestibility values that we had 
measured in the ingredients that contained phytase is accurate. We also 
measured the excretion of phosphorus from the pigs, and we can see that 
we reduced the excretions from 1.68 g/day to 0.82 g/day when added 
phytase to the corn-soybean meal diet. When we fed the corn-soybean 
meal-DDGS diet without phytase, we had phosphorus excretion of 1.43 
g/day; however, we were able to reduce this value to 0.82 g/day when we 
added phytase to the diet. Now, it's important here to remember that we 
reduced dicalcium phosphate in the diet in Diet 4, that contained corn, 
soybean meal, and DDGS and 500 units of phytase, contained no dicalcium 
phosphate. And we'll see here that the performance of pigs and the 
retention of phosphorus is exactly the same or better than for the other 
pigs. That means that it is possible to feed pigs without any use of 
dicalcium phosphate in the diet.

Slide 24
To look further into the effects of phytase on the digestibility of 
phosphorus in corn co-products, we conducted a few more experiments.

Slide 25
In this case, we added three different levels of phytase to a diet in 
which corn provided all the phosphorus. We had a diet without any 
phytase, we had a diet with 500 units of phytase, we had a diet with 
1000 units of phytase, and we had a diet with 1500 units of phytase. And 
we will see here, again, that we increased the digestibility of 
phosphorus in corn quite significantly when we added 500 units of 
phytase to this diet; however, if we added more than 500 units of 
phytase to corn, we did not see a significant increase in phosphorus 
digestibility. So, it appears from this that for corn, it is enough to 
add 500 unit of phytase to the diet.

Slide 26
We did the same thing for DDGS. However, as we have seen before, we did 
not see a significant increase in the digestibility of phosphorus in 
DDGS when we added phytase to this diet. And, we see again that the 
digestibility even without phytase in DDGS is quite high -- in this 
case, about 76% -- and this value is not significantly increased when we 
added phytase to DDGS.

Slide 27
For high-protein DDG, we did see a small increase when we added 500 
units of phytase to the diet, but no further increase when we added 1000 
or 1500 units of phytase. And again, as we saw with DDGS, the 
standardized total tract digestibility of phosphorus in high-protein DDG 
even without any phytase is about 75%, which is much greater than in 
corn.

Slide 28
And finally, we also added 500, 1000, or 1500 units of phytase to corn 
germ, and in this case, we saw a significant increase when we added 
phytase to the diet. And there's a linear effect of phytase here, 
indicated that as we add more phytase, we get an increase in 
digestibility. The digestibility we observed in the diet without any 
phytase is very similar to the digestibility we observed for corn, and 
the digestibility values we observed when we added phytase are also 
similar to the values for corn. However, all these values are less than 
the values we have observed when we use DDGS or high-protein DDG. 
Combined, these results indicate that when we use corn and corn germ and 
other ingredients that have not been fermented or used in ethanol 
production, we can get a significant increase by adding 500 units of 
phytase to these ingredients. However, addition of more than 500 units 
of phytase does not seem to increase the digestibility nearly as much. 
On the other hand, when we use high-protein DDG or DDGS, we do not see 
an effect of adding phytase to adding phytase to these diets.

Slide 29
I hope you have learned a few things about using standardized total 
tract digestibility values in formulation of diets for swine. We do 
believe that diets are most accurately formulated if we use these 
values. As we have seen in some of the research we have conducted, it is 
possible to formulate diets, even to weanling pigs, without the use of 
any dicalcium phosphate. And we can reduce excretion of phosphorus quite 
dramatically by using this formulation technique. We have also seen that 
it is probably enough to use 500 units of phytase and there does not 
seem to be a universal improvement in digestibility if we use greater 
levels of phytase. However, the use of phytase in combination with 
distillers dried grains with solubles or other fermented feed 
ingredients will increase the digestibility of phosphorus quite 
dramatically in diets fed to pigs. With that, I want to thank you for 
your attention, but I do want to point out that you can visit our 
website and learn more about standardized total tract digestibility of 
phosphorus or many other issues in swine nutrition. Our website is found 
at nutrition.ansci.illinois.edu.