EFFECT OF POTASSIUM ON ASSOCIATION OF MINERALS WITH V A R I O U S
FRACTIONS OF DIGESTA A N D FECES OF SHEEP FED H A Y 1'2
Sha H. R a h n e m a and J. P. F o n t e n o t
Virginia P o l y t e c h n i c Institute and State University 3
Blacksburg 24061
ABSTRACT
An experiment was conducted with 12 crossbred (Finn X Dorset • Suffolk) wethers (42 kg),
fitted with ruminal, abomasal and ileal cannulae to study the effect of K administration on the
association of Mg, Ca, K and Na with various fractions in digesta and feces. The wether were fed
800 g of orchardgrass hay and dosed with 2 g Cr203, as solid marker and 2 g cobalt ethylenediaminetetraacetate (EDTA), as liquid marker, daily. Six wethers were used as controls and the
other six were dosed with 100 g KHCO3/head daily, via the ruminal eannulae at the time of
feeding. Feed, feces and digesta samples were treated with tris buffer and separated into liquid and
solid fractions. The liquid fraction was treated with ethanol and separated into liquid and ethanol
precipitated parts. The solid fraction was extracted with chloroform:methanol. The residue from
this extraction was further treated with LiOH-EDTA and centrifuged into liquid and solid entities.
Administration of KHCO 3 resulted in an increase in ruminal pH with no effect on abomasal, ileal
and fecal pH. Urinary excretion of Mg was decreased (P<.05) and that of K was increased by
KHCO 3 administration. Most of the Mg in the diet and feces was associated with the solid fraction
(75%) and, in the abomasum, with the liquid fraction (74%). Magnesium in the rumen and ileum
was divided approximately equally between the solid and liquid fractions. All the Mg and 50 to
70% of the Ca present in the liquid fraction in the rumen were associated with protein. None of the
Mg or Ca in the abomasum was precipitated with ethanol. Administration of KHCO 3 had no effect
on Mg solubility, but it resulted in an increase (P<.05) in ruminal solubility of Ca. It is concluded
that solubilities of Mg and Ca in the digesta may not be indicators of their availability for absorption. Also it is possible that the effect of increasing K above 3.2% on Mg absorption may be limited.
(Key Words: Sheep, Magnesium, Potassium, Fractionation, Solubility.)
I ntroduction
Considerable i n f o r m a t i o n is available regarding h y p o m a g n e s e m i c t e t a n y and some of
the factors affecting Mg m e t a b o l i s m ( F o n t e n o t
et al., 1983). The q u a n t i t y and site o f Mg
absorption has been investigated also (Phillipson and Storry, 1965; Grace and MacRae,
1972; T o m a s and Potter, 1976a,b; Greene et
al., 1983b,c). However, i n f o r m a t i o n is limited
concerning f o r m and association of Mg and
o t h e r minerals with various fractions present in
r u m i n a n t diets (Todd, 1961; M o l l o y and
Richards, 1971a,b) and the changes that m a y
1Supported by John Lee Pratt Anim. Nutr.
Program.
2Sincere appreciation is extended to Mrs. V.
Bowman for her assistance in laboratory analysis.
3 Dept. of Anim. Sci.
Received January 3, 1986.
Accepted May 30, 1986.
occur w h e n passing through the digestive tract
o f these animals (Storry, 1961; Smith and
McAllan, 1966; Grace et al., 1977).
High dietary K has been shown to suppress
Mg absorption in the r u m i n a n t ( N e w t o n et al.,
1972).
The r u m i n a n t s t o m a c h has been
implicated as the site of the deleterious effect
o f dietary K on Mg absorption (Tomas and
Potter, 1976b; Greene et al., 1983b; Wylie et
al., 1985), b u t the effect of dietary K on the
f o r m and association of Mg with fractions in
the digestive tract has n o t been elucidated.
This e x p e r i m e n t was c o n d u c t e d to d e t e r m i n e
the association o f Mg, Ca, K and Na with
various fractions in the diet, digesta and feces
o f sheep, and the effect of K on these associations in digesta and feces.
Experimental Procedure
A m e t a b o l i s m e x p e r i m e n t was c o n d u c t e d
w i t h 12 crossbred (Finn X Dorset X Suffolk)
wethers (42 kg) fitted with ruminal, abomasal
1491
J. Anim. Sci. 1986. 63:1491-1501
1492
RAHNEMA AND FONTENOT
and ileal cannulae, fed (as-is basis) 800 g
orchardgrass hay in two equal portions at 0800
and 1900 daily. Mineral composition of the
orchardgrass hay is presented in table 1. Wethers
were dosed via the ruminal cannulae with 1 g of
Cr203 as a solid marker and I g of cobalt
ethylenediaminetetraacetate (Co-EDTA) as a
liquid marker at each feeding time. The markers
were in powder form. Wethers were blocked by
breed and weight, and were randomly allotted
to two groups. Six wethers were dosed through
the ruminal cannulae with 100 g of KHCO3
(reagent grade) daily (50 g at each feeding) at
the same time as the markers, and the other six
were dosed with the markers only. The wethers
were kept in metabolism stalls similar to those
described by Briggs and Gallup (1949).
The experiment consisted of a 10-d
preliminary period followed by 7 d of total
feces and urine collection and a 3-d ruminal,
abomasal, ileal and fecal sampling period. Fecal
samples were dried in a forced-air oven daily at
a maximum of 60 C and a 10% portion was
composited for each wether. Urine samples
were diluted with deionized water to a constant
weight and a 2% aliquot was taken for each
wether. Dried feed and feces were ground
through a 1-mm screen.
During the 3-d sampling period, at least 80
ml of ruminal and abomasal and 50 g of ileal
and fecal samples were collected and frozen at
6-h intervals advancing by 2 h each day to
obtain 12 samples at 2-h intervals. At the end
of the experiment, samples were thawed, pH
was determined
and the samples were
composited for each wether. The fecal and
ileal samples were composited based on equalweight (25 g) basis and abomasal samples on
equal-volume (25 ml) basis. The composited
samples were divided into two portions. Onehalf was used for dry matter (DM) and total
Mg, Ca, K and Na determination, and 80 g of
the other half were blended (3 rain) either
before (ruminal, abomasal and ileal) or after
(feed and fecal samples) the addition of 100 ml
of Tris 4 buffer. The pH of the Tris buffer was
adjusted with HC1 to a pH similar to that of the
sample it was added to. Eighty milliliters of the
blended samples were put into polypropylene
4.05 M tris HCI buffer.
s Dupont Company, Biomedical Products Division,
Wilmington, Delaware 19898.
centrifuge bottles s and partitioned according to
a modification of the method reported by
Grace et al. (1977), as shown in figure 1.
Blended samples were centrifuged at 25,000 X
g for 30 rain and separated into solid and liquid
fractions. To the liquid fraction, 100 ml of
absolute ethanol were added and the mixture
was shaken for 1 h at 2 C and centrifuged
at 25,000 x g for 30 rain, separating this into
liquid (deproteinized) and residue (ethanol
precipitate) fractions. To the solid fraction of
the first centrifugation, 50 ml (v/v) of 2:1 ratio
of chloroform:methanol were added and the
mixture was shaken for 1 h at 23 C. This
mixture was then filtered, using no. 42 Whatman paper into liquid (extract) and solid
(filtrate) fractions. Fifty milliliters of .5 N
LiOH and .1 N EDTA acid were added to the
solid fraction of the filtration. The mixture was
shaken for 4 h at 23 C and separated into liquid
extract (alkali soluble) and solid fractions.
Upon the completion of the experiment,
jugular blood samples were collected 3 h
post-feeding.
Feed, feces, digesta and the partitioned
samples were wet-ashed (Sandel, 1950) for Mg,
Ca and Co determination by atomic absorption
spectrophotometry and K and Na by flame
emission spectrophotometry using a Perkin
Elmer 403 atomic absorption spectrophotometer. Phosphorous was determined by the
method of Fiske and Subbarow (1925) and
Cr203 by a modification of the method outlined by Kimura and Miller (1957). Nutrient
flow rates were adjusted to 100% Cr203 and
CoEDTA recoveries; flow rates were calculated
based on the procedure described by Faichney
(1975). The control and KHCO3 treatments
were compared using the General Linear Models
Procedure of Statistical Analysis System (SAS,
1979).
Results and Discussion
Potassium bicarbonate administration increased (P<.05) ruminal pH (table 1), but had
no effect on the abomasal, ileal or fecal pH.
Serum mineral concentrations were not affected (P>.05) by KHCOa treatment, and were
within the normal ranges (table 2).
Greater (P<.05) quantities (g/d) of K were
absorbed (21.51 vs 58.20) and excreted in the
urine (18.07 vs 50.77) by wethers treated with
KHCO3 (table 3). Potassium retention was not
affected by treatment. Fecal excretion of Na
MINERALS IN DIGESTA AND FECES
1493
SAMPLE
I Added I00 ml
tris buffer and
blended for 3 min.
BLENDED
SAMPLE
Centrifuged at
25,000 x g
for 50 rain.
SOLID
LIQUID (Soluble)
Added 50 ml (v/v)
\
Added I00 ml of absolute
2:1 ratio chloroform
hanoi. Shook for I h
/ at 25 C, and filtered using
no. 42 Whotman paper.
\ e t h a n o l , shook for I h Ot 2 C,
\and
centrifuged at 25,000 x g
\ for 30 min.
LIQUID
"~
(Extract)
SOLID
/
u
LIQUID
.
~
Added 50 ml of .5 N
~"
RESIDUE
(Deproteinized)
LiOH and .I N EDTA acid,
shook for 4 h at 23 C
nd centrifuged at
g for 30 min.
LIQUID EXTRACT
(Alkali Soluble)
SOLID
Figure 1. Partitioning schematic used for fractionation of minerals in diet, digesta and feces (modification of
the method of Grace et al., 1977).
and P and urinary excretion of Mg decreased
(P<.05) with KHCO3 treatment.
A trend for a decrease in Mg absorption
(table 4) was observed from dosing with
KHCO3, similar to the findings reported by
Kemp et al. (1961). They noted a 2.5% reduction in Mg absorption when 400 g KC1 were
added to a fresh-cut grass diet fed to dairy
cows. Greene et al. (1983a,b,c), reported
reductions in Mg absorption with increased
dietary K. However, in the present experiment,
the control diet (orchardgrass hay) contained
higher amounts of K (3%) than were used in the
basal diets of these workers (.6%). In the
present experiment the reduction in urinary
excretion of Mg (P<.05) in the treated wethers
would indicate a decrease in absorption and
agrees with previous findings (Newton et al.,
1972; Greene et al., 1983a,b,c). It is also
interesting to note that in this experiment Mg
and Ca were poorly absorbed and P was highly
absorbed, similar to results reported by Pfeffer
et al. (1970), Grace et al. (1974) and Chester-
Jones (1982). No additional effects (P>.O5)
were noted due to increase in K intake on Mg,
Ca or P metabolism.
No differences (P>.05) due to KHCO3
administration were noted in the Mg fractions
of digesta or feces (table 5). Of the 2.82 g Mg/d
ingested for the control diet, .95 g (33%) was in
soluble form, of which .69 g remained in
TABLE 1. DRY MATTER AND MINERAL
COMPOSITION OF ORCHARDGRASS HAY
Item
%
Dry matter
Magnesiuma
Calciuma
Phosphorous a
Potassiuma
Sodiuma
90.63
.39
aDry matter basis.
.55
.34
3.14
.14
1494
RAHNEMA AND FONTENOT
TABLE 2. RUMINAL, ABOMASAL, ILEAL AND FECAL pH VALUES IN WETHERS FED
ORCHARDGRASS HAY WITH OR WITHOUTPOTASSIUMBICARBONATE
Item
Control
Treated
SEa
Rumen
Abomasum
Ileum
Feces
6.67
2.97
8.16
7.49
7.05b
3.01
8.07
7.51
.06
.04
.03
.06
astandard error of the mean.
bDifferent from control (P<.05).
solution after extraction with ethanol. This is
somewhat less than the values noted by Grace
et al. (1977) and Todd (1961) for ryegrass. The
chloroform:methanol extracted value o f . 12 g/d
of Mg for the control diet, represents the
fraction associated with chlorophyll and lipid
fractions. This is lower than the acetoneextracted value of ryegrass reported by Todd
(1961). The EDTA-LiOH extract of the residue
from the chloroform:methanol extraction for
the control and treatment diets represented
about 20% (.57 and .55 g/d respectively) of the
total Mg.
There was a loss of total Mg between the
ingested quantities (g/d) and that present in the
rumen. Approximately 47% of the total Mg
present in the rumen of control wethers was in
soluble form, compared with 34% in the diet.
Grace et al. (1977) reported no change in water
soluble Mg of ryegrass between diet and ruminal contents. The soluble ruminal Mg (47%)
noted here is similar to the 49% reported by
Storry (1961) for concentrate. However, this
value is lower than the 56% reported by Grace
et al. (1977) for ryegrass and the 77% noted for
fresh spring grass by Storry (1961).
Almost all the Mg present in the soluble
form in the rumen was precipitated by ethanol
treatment, indicating that this Mg may be
associated with ruminal microorganisms and
other protein fractions. Fitt et al. (1972)
pointed out that isolated cell walls of ruminal
bacteria are capable of linking Mg, and that Mg
uptake increases with Mg concentration,
increased pH, and degradation of the cell wall.
Grace et al. (1977) reported that 65% of
bacterial Mg was found in the water-soluble
fraction.
In the present experiment, little ruminal Mg
(.03 g/d) was extracted with chloroform:
methanol; however, more Mg was solubilized
with EDTA-LiOH extraction than that of the
diet, which may represent the fermentation
effect in the rumen,
Even though less Mg reached the small
intestine than was ingested, which is in accord
with Greene et al. (1983b) and Rahnema and
F o n t e n o t (1983), it appears that some Mg may
have been secreted into the digesta between the
rumen and abomasum (1.90 vs 2.35 g/d,
respectively, for control wethers). Of the 2.35 g
Mg entering the small intestine, 1.51 g or
TABLE 3. BLOOD SERUM MINERAL CONCENTRATIONSOF WETHERS FED
ORCHARDGRASS HAY WITH OR WITHOUTPOTASSIUM BICARBONATE
Item
Control
Treated
SEa
Magnesium
Calcium
Inorganic phosphorus
Potassium
Sodium
1.90
8.69
5.64
45.1
308
mg/1 O0 ml
1.94
8.50
6.92
48.1
305
.06
.17
.34
3.3
4
astandard error of the mean.
MINERALS IN DIGESTA AND FECES
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67% was in soluble form for control wethers.
Both Grace et al. (1977) and Storry (1961)
reported values of approximately 90% for
soluble Mg in the abomasum. These values are
more than 20% greater than that noted in the
present experiment, even though the abomasal
pH in this experiment was similar to that
reported by Storry (1961). F r o m the relative
Mg solubility in the hay, rumen and abomasum,
it is possible that the Mg present in the orchardgrass hay used in this experiment was much less
soluble than that present in ryegrass used by
Grace et al. (1977) and Storry (1961). None of
the soluble Mg entering the small intestine was
precipitated with ethanol. This indicates that
the low pH and pepsin in the abomasum may
have hydrolyzed the bacteria and protein
molecules into peptides and amino acids, which
were not precipitated with ethanol.
Chloroform:methanol and EDTA-LiOH extraction removed .15 and .06 g/d Mg, respectively, from the solid residue entering the
small intestine of control wethers. Grace et al.
(1977) reported that 1.3% of total Mg was in
the alkaline-soluble fraction, which is similar to
the value of 2.6% in this experiment.
Although more of the Mg entering the small
intestine was in soluble form than at any other
sampling site, there appeared to be a net
secretion of Mg into the small intestine. This
lack of Mg absorption from the small intestine
may be partially explained by the possible
association of the soluble Mg with the small
peptides and amino acids present in the abomasum from the hydrolyzed microbial cell wall
(Fitt et al., 1972; Grace et al., 1977) and
consequently not available for absorption.
The quantity of Mg entering the large
intestine averaged 2.45 and 2.67 g/d for the
control and KHCO3-treated wethers, respectively. Almost one-half of the total Mg
entering the large intestine was in soluble form,
which is similar to that noted in the rumen,
but, compared with the rumen, less of the
soluble Mg was associated with ethanol precipitable fractions (26 vs 47%), indicating more of
the Mg is in ionic or in non-ethanol precipitable
form after passing through the small intestine
than in the rumen.
Basically, no change in the quantity of total
Mg was noted between ileum and feces (2.45 vs
2.59 g/d, respectively, for control wethers). The
soluble and the deproteinized Mg values in feces
were similar to the amounts present in the diet.
Only .5 to .6% of the Mg in the feces was
1496
RAHNEMA AND FONTENOT
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RAHNEMA AND FONTENOT
extracted with the c h l o r o f o r m : m e t h a n o l solution as c o m p a r e d with 4% of the Mg in the diet,
which m a y indicate little Mg association with
fecal lipids. The E D T A - L i O H soluble fraction
was a p p r o x i m a t e l y twice that of the orchardgrass hay. This fraction was over five times as
high in the feces than that entering the large
intestine.
Calcium basically f o l l o w e d the same general
pattern as Mg (table 6). With the e x c e p t i o n of a
decrease (P<.05) in Ca f l o w through the
pre-intestinal region, increased K intake had no
effect on the f l o w and partial absorption o f this
mineral at any o t h e r sampling site. In contrast
with the findings of Grace et al. (1977), s o m e
Ca secretion in the small intestinal area was
noticed.
On a percent basis, less Ca than Mg was
associated with soluble fractions at each site. A
similar observation was m a d e by Grace et al.
(1977). As with Mg, and consistent with results
of Grace et al. (1977) and Storry (1961), m o r e
of the Ca entering the small intestine was in
soluble f o r m than at any o t h e r site. Decreases
(P<.05) in ethanol, c h l o r o f o r m : m e t h a n o l and
E D T A - L i O H fractions were n o t e d due to
KHCO3 t r e a t m e n t in samples entering the large
intestine.
Sodium and K followed opposite trends
(tables 7 and 8). Sodium f l o w through the
pre-intestinal region and small intestine was
increased (P<.05) and its absorption f r o m all
sampling sites was decreased (P<.05) by
KHCO3 treatment. Increased K intake resulted
in an increase in absorption and flow of K
through the pre-intestinal and small intestinal
regions of the digestive tract, respectively.
A p p r o x i m a t e l y 25% o f . N a and 54% of K
were f o u n d in the soluble fraction o f the
control diet. More (P<.05) K and less (P<.05)
Na were f o u n d in soluble fractions of digesta
collected f r o m all sites due to KHCO3 treatment. More of the fecal K and Na were in
soluble f o r m than that of the feed. In contrast
to Mg and Ca, m o r e Na was present in soluble
f o r m in the r u m e n than the abornasum. This
m a y reflect the Na c o n t r i b u t i o n f r o m saliva,
and indicates that pH appeared to have had no
effect on changing the affinity of Na for the
organic fractions present in the digesta entering
the small intestine.
The results of this experiment, supported by
those r e p o r t e d by R a h n e m a and F o n t e n o t
(1983), are interpreted to indicate that solubility
of Mg and Ca in the digesta m a y n o t be the
determining factor for availability to wethers.
There was no n e t Mg absorption f r o m the
intestinal region even though m o s t of the Mg
(more than any o t h e r sampling site) present in
the a b o m a s u m was in soluble form. Also,
KHCO3 administration did n o t result in an
a c c u m u l a t i o n o f Mg in either the soluble or
solid fractions o f ruminal digesta. This w o u l d
indicate that K had no effect of Mg solubility,
hence the inhibiting effect of K on Mg absorption is not related to Mg solubility.
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MINERALS IN DIGESTA AND FECES
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