Hi, everyone. My name is Su A Lee from the Stein Monogastric Nutrition Laboratory at the
University of Illinois. I will be discussing digestibility of calcium and phosphorus in
feed ingredients fed to gestating sows and growing pigs. Most data were from my PhD
experiments, and this work was presented as a Young Scholars Presentation in 2020 ASAS
Midwest Meeting.

Let’s start with take home messages before we go over to the presentation. First, calcium
and phosphorus digestibility is much less in gestating sows compared with growing pigs.
Second, calcium and phosphorus digestibility and retention are greater in late gestation
than in earlier periods of gestation. Third, phytase increased digestibility of both
phosphorus and calcium in growing pigs. Fourth, phytase effects on calcium and phosphorus
are ingredient-specific. Lastly, phytase effects are less predictable in gestating sows
than growing pigs. Let’s see how we have come up with these take home messages.

This was published in 2016. The horizontal axis represents dietary calcium that is
expected concentration of calcium when formulating diets, and the vertical axis represents
the actual calcium levels in diets. The authors drew a black regression line that
corresponds to these dots. To understand this better, let’s draw a red line where y equals
x. We can see that the black line is above the red line, which means that analyzed calcium
is greater than expected calcium. Therefore, we learned from this graph that the actual
calcium in diets are greater than what we meant it to be.

One of the most possible reasons is that calcium sources, such as calcium carbonate, are
relatively inexpensive compared with other feed ingredients, meaning that there will be no
harm if we put more of a cheaper feed ingredient when formulating diets. Another possible
reason is that a lot of feed additives, or sometimes feed ingredients, contain calcium
carbonate as a carrier or a flow enhancer.

What would happen if calcium in diets is too high? The first thing that we should worry
about is phosphorus digestibility. A previous study in 2011 demonstrated that as dietary
calcium increases, shown in the x axis in this graph, digestibility of phosphorus linearly
decreases.

Excess calcium also has a negative effect on growth of pigs, and this has been shown in a
number of studies already. This photo was taken in one of the calcium requirement studies
in our laboratory. We can see the big difference in the body size of pigs when they were
fed high vs. low calcium diets with the same phosphorus level.

To prevent oversupply of calcium, feed ingredients should be evaluated based on digestible
calcium. 

Like amino acids, we had a question if we need to determine the phosphorus and calcium
digestibility in ileum or total tract. Several studies indicated that the ileal
digestibility and total tract digestibility are not different, which means that we may
determine the total tract digestibility of phosphorus and calcium.

However, there were challenges with the apparent total tract digestibility (ATTD) values.
This study was published in 2001, and the authors determined the ATTD of phosphorus in
percent in diets containing different inclusion rates of soybean meal. The only source of
phosphorus was from soybean meal, and the ATTD of phosphorus increased as concentration of
phosphorus from soybean meal increased.

Same story was observed in 2013. The authors determined the ATTD of calcium in diets
containing about 12 to 50% canola meal. As the inclusion rate of canola meal increased,
the concentration of calcium increased as well. Data indicated that the ATTD of calcium
linearly increased as calcium in diets increased. These observations can be explained by
basal endogenous losses.

When looking at the composition of nutrients in fecal samples of pigs, not only
diet-originated nutrients but also basal endogenous loss-originated nutrients are
contained. Standardized total tract digestibility, STTD, is calculated by correcting ATTD
with the basal endogenous loss. And these values for the STTD is believed additive in a
complete diet.

Values for the STTD of phosphorus and calcium in feed ingredients are available in NRC
2012 and a paper by Stein and his colleagues in 2016, respectively. However, most of the
values were obtained from growing pigs, and these values were also used in sows’ practical
application.

Therefore, a question remains if the STTD values for calcium and phosphorus are also
applied to diets for sows.

The basal endogenous losses of calcium and phosphorus were 430 and 160 mg/kg of dry matter
intake, respectively, which were close to the values that were reported previously.
However, gestating sows had more than 3 times greater basal endogenous losses of calcium
and phosphorus compared with growing pigs. Unlike growing pigs, there were no data
reported previously.

Before we look at the data for the STTD of calcium, let me set up the slide. In the
bottom, different physiological states—that were gestating sows and growing pigs—were
presented and were divided into normal or high phytate levels within each physiological
state. The normal diets included corn, soybean meal, and inorganic sources of calcium and
phosphorus, and the high-phytate diets included the same ingredients as the normal diets
and 40 percent rice bran that contains a high phytate-bound phosphorus. The vertical axis
represents the STTD of calcium in percent. Interaction between phytate level and
physiological state was observed. The STTD of calcium was greater if growing pigs were fed
the low-phytate diet than if they were fed the high-phytate diet, and this agrees with
previous data. However, phytate level did not affect the STTD of calcium by gestating
sows. Regardless of phytate level, gestating sows had reduced STTD of calcium compared
with growing pigs.

A basically similar story was observed for the STTD of phosphorus. There was an
interaction between phytate level and physiological state. The STTD of phosphorus was
greater if pigs were fed the low-phytate diet rather than the high-phytate diet, but the
difference was greater if growing pigs rather than gestating sows were fed the diets.
Regardless of phytate level, gestating sows had reduced STTD of phosphorus compared with
growing pigs, and growing pigs are likely more affected by phytate level.

The previous experiment used sows in mid-gestation. It is possible that the STTD of
calcium differs among sows in different gestation periods, possibly because of different
calcium requirements or feed intake. 

Therefore, we conducted another experiment where the basal endogenous loss of calcium and
digestibility were compared among different gestation periods. I would like to point out
that feed intake increased from early- to mid- to late-gestation because the body weight
of sows increased. Now we are looking at the basal endogenous loss of calcium from
gestating sows fed the corn-based calcium-free diet. The basal endogenous loss of calcium
was greatest by sows in early gestation, followed by sows in mid- and late-gestation
periods, respectively. 

The ATTD of calcium was corrected with each value of the basal endogenous loss of calcium,
and the values for the STTD of calcium in calcium carbonate were 42, 28, and 48%
respectively for early, mid, and late gestation, with mid-gestation sows having the least
digestibility. 

Looking at the ATTD of phosphorus in diets, the values were 24, 14 and 36% respectively
for early, mid, and late gestation, with mid-gestation sows having the least digestibility
and late-gestation sows the greatest. Again, sows have very low digestibility of
phosphorus when we compare with previous data from growing pigs. I did not show in this
presentation, but we found a similar trend for retention data. 

Then what are the reasons for the differences in calcium and phosphorus balance? Sows in
late gestation need much calcium and phosphorus because of greater need for fetus growth.
Also, sows in early gestation need calcium and phosphorus to compensate the loss with
lactation. It seems that sows in mid-gestation regulate calcium absorption and retention
because they may not need as much calcium as sows in other periods.

It is well demonstrated that minerals that are positively charged tend to chelate to
phytate when phytate becomes negatively charged. Phytate from plant feed ingredients
including corn, soybean meal, and so on, can bind to calcium ions in the gastrointestinal
tract of pigs because the phytate is a strong chelator, and, therefore, calcium
digestibility decreases. However, use of microbial phytase may increase not only
phosphorus, but also calcium digestibility. 

This was published in 2010, and the authors determined the ATTD of phosphorus in different
feed ingredients without or with phytase. Data indicated that the ATTD of phosphorus in
corn without phytase was 27% and increased to 62% when phytase was used in the diet. Same
story was observed in soybean meal, that the ATTD of phosphorus was increased from 49 to
75% when phytase was used. However, use of phytase did not affect the ATTD of phosphorus
in DDGS. The reason for the no phytase effect was that in the production of the DDGS,
phytase is usually used in the fermentation process, which results in a decrease in
phytate-bound phosphorus in DDGS. This indicates that the phytase effect on the ATTD of
phosphorus is ingredient-specific because it depends on the amount of substrates, phytate.

This study was published in 2015, and this graph shows the STTD of calcium in inorganic
calcium supplements in percent. As expected, the STTD of calcium in calcium carbonate
increased from 60 to 73% by supplementing microbial phytase to the diets. This clearly
indicates that calcium from calcium carbonate binds to phytate from corn and use of
phytase released calcium from the calcium-phytate complex. However, the STTD of calcium in
MCP and DCP was not affected by using microbial phytase. We repeated this for many times,
but same was observed. This can be explained by calcium in MCP or DCP strongly binding to
phosphate rather then to phytate.

We conducted another experiment to investigate the effects of different suppliers of
calcium carbonate and phytase. There were 4 different sources of calcium carbonate from 4
different suppliers, and 8 corn-based diets were formulated without and with phytase.
Interaction between phytase and source of calcium carbonate was not significant. Values
for the STTD of calcium ranged from 71 to 75% when no phytase was used. Supplementation of
phytase increased the STTD of calcium in calcium carbonate. Again, the increase in the
digestibility of calcium is because phytate from the corn chelates calcium from calcium
carbonate, and phytase can release the calcium from the phytate-calcium complex.
Regardless of use of phytase, source D had the least and source A had the greatest values
for the STTD of calcium.

We also determined the basal endogenous loss of calcium without and with phytase in
corn-based calcium-free diets. The basal endogenous loss of calcium was 463 mg/kg of dry
matter intake, and this value was close to the values obtained in the previous data. The
basal endogenous loss of calcium was reduced to 304 mg/kg of dry matter intake when
microbial phytase was supplemented to the calcium-free diet, meaning that phytate from
corn can also bind to calcium ions from the basal endogenous loss of calcium and that use
of phytase releases the calcium from the phytate-calcium complex.

Then what about in gestating sows? Is microbial phytase also working in them?

In another study, we determined the basal endogenous loss of calcium in sows in early,
mid, and late gestation fed the corn-based calcium-free diets without and with phytase.
The basal endogenous loss of calcium was approximately 1100 mg/kg of dry matter intake.
The basal endogenous loss of calcium was reduced to about 860 mg/kg of dry matter intake
when microbial phytase was supplemented to the calcium-free diet. The phytase was also
working in gestating sows to release calcium ions that are endogenous-originated from the
phytate.

In the same study, we determined the STTD of calcium in calcium carbonate and the ATTD of
phosphorus in corn and monosodium phosphate in sows. The STTD of calcium in calcium
carbonate was approximately 40% when no phytase was used. However, supplementation of
phytase did not increase the STTD of calcium in calcium carbonate. The ATTD of phosphorus
was not affected by used of phytase as well. These observations were not expected, because
usually phytase works very well in growing pigs.

In the same experiment, we found that the ATTD of phosphorus in diets containing the same
amounts of corn and monosodium phosphate fed to gestating sows were much different between
the calcium-free diet and calcium carbonate-containing diet. Only difference was the
calcium level in diets: no calcium in calcium-free, but a certain amount of calcium was
provided from calcium carbonate in the second diet.

Therefore, next experiment was conducted if increasing calcium in diets affects calcium
and phosphorus balance and some blood biomarkers. A total of 4 diets were formulated.
First diet contained corn, soybean meal, and sugar beet pulp. Calcium carbonate and
monosodium phosphate was also added, but the levels of calcium and phosphorus were only at
25% of requirements. From the second diet, more calcium carbonate was added to have 50,
75, and 100% of the requirement with a constant level of phosphorus. The 4 diets contained
0.18 to 0.72% calcium and 0.56% phosphorus.
 
Jumping to the results, let me set up the slide. The horizontal axis represents percentage
requirement of calcium that are 25% all the way up to 100%. The vertical axis represents
the ATTD of calcium and phosphorus in percent. As dietary calcium increased, the ATTD of
calcium quadratically increased. This increase can be explained by different contributions
of the basal endogenous loss of calcium when calcium concentration varies from low to
high. What about phosphorus? As dietary calcium increased, the ATTD of phosphorus linearly
decreased, and this was also observed in growing pig data. This decrease can be explained
by the calcium and phosphorus precipitation in the intestinal tract of pigs. 

Now, we are looking at the retention of calcium and phosphorus in percent in the vertical
axis. Retained calcium and phosphorus were calculated by subtracting calcium and
phosphorus in feces and urine from intake of calcium and phosphorus, respectively. As
calcium increased in diets, retention of calcium quadratically increased. The quadratic
increase in the ATTD of calcium can be one of the reasons for the quadratic increase in
retention. We found that increasing dietary calcium decreased the ATTD of phosphorus.
Then, what about retention? As calcium increased in diets, retention of phosphorus
linearly increased and this was an opposite situation to the digestibility data. It has
been known that the synthesis of bone tissues, which is the primary role of calcium and
phosphorus, requires calcium and phosphorus in the body at the same time. Therefore, as
more calcium was available in the body as calcium increased in the diets, utilization of
absorbed phosphorus increased in the body.

There are 2 bone cells that are working on bone turnover. First one is osteoblast that is
in charge of formation, and second one is osteoclast that is in charge of breaking down.
The idea was if byproducts of those cells are analyzed in blood or urine, we may predict
the bone formation and resorption.

Because the body keeps forming and breaking down the bone cells, we have calculated the
ratio between osteocalcin and CTX-I concentrations. Osteocalcin is a bone formation
biomarker and CTX-I is a bone resorption biomarker. As calcium in diets increased, the
ratio linearly increased, which indicates that there were more bone formation over bone
resorption. This result is in agreement with calcium and phosphorus balance data showing
that more calcium and phosphorus were retained to synthesize bone tissues.

Let’s move into overall conclusions. First, calcium and phosphorus digestibility and
retention are much less in gestating sows compared with growing pigs. Second, calcium and
phosphorus digestibility and retention are greater in late gestation than in earlier
periods of gestation. Third, phytase increased digestibility of both phosphorus and
calcium in growing pigs.

Fourth, phytase effects on calcium and phosphorus are ingredient-specific because of its
substrate, phytate. Fifth, phytase effects are less predictable in gestating sows than in
growing pigs. Lastly, some blood biomarkers may be useful to predict bone turnover status
of sows.

I would like to acknowledge ABVista for financial support for my PhD work and everyone
from Dr. Stein’s lab. If you want to learn about the research we are conducting in the
Stein Monogastric Nutrition Laboratory, please visit our website or search “Stein” and
“pig” on google. Thank you for listening.