MAGNESIUM BIOAVAILABILITY UPDATE
By George Miller, PhD
Premier Magnesia
What have we learned about magnesium nutrition in
animals during the past few years? Several research reports have been published which
increase our understanding of Mg utilization. While an earlier generation of researchers
concentrated more of their efforts on solving the grass tetany problem, recent research
has emphasized the areas of absorption, bioavailability of Mg sources, the effects of
other nutrients on Mg utilization, the importance of magnesium oxide in buffer supplements
and other aspects such as the effects of excess Mg intake. This update highlights the
results of many of these research projects.
Where and How is Magnesium Absorbed?
While non-ruminants absorb Mg primarily from the small intestine, ruminants are able to
absorb much of their Mg requirement from the rumen. In fact, the reticulum and rumen can
account for up to 80% of the Mg absorption along the entire digestive tract (Remond, et
al, 1996). Scientists have known this for awhile but some interesting new studies shed
more light on the subject of the absorption site.
For example, ewes were infused with 0 to 4 grams of Mg daily into the distal ileum so
that absorption from the large intestine could be estimated (Dalley & Sykes, 1989).
Results showed that as more Mg was infused, more Mg was absorbed. Significant amounts were
absorbed from the large intestine, apparently by passive transport.
Rectal infusion of Mg solutions can be a very effective way to help cattle that are
down with grass tetany to replenish their blood Mg levels. Calves 6 weeks of age were
given MgCl2 solutions by oral or rectal administration while fed diets containing either
0.04% (very deficient) or 0.24% Mg (Bacon et al, 1990). Plasma Mg of deficient calves was
maximized within 10 minutes following rectal infusion compared to 160 minutes after oral
dosing. However, plasma levels were sustained longer following oral dosing. While plasma
levels of both oral and rectal treatment groups were increased by dosing, those of
deficient calves were increased by a much higher percentage (16% or 47% in Mg adequate
calves vs 48% or 124% in deficient calves).
Grass tetany remains a significant cause of cow deaths every year. Blood samples were
taken from cows in a commercial dairy herd experiencing grass tetany to determine the
effects of supplementing the ration with 22.5 grams Mg per day (Contreras, et al, 1992).
That is equal to 1.5 ounces of MgO daily. Initial blood serum Mg averaged 1.29 mg/dl. This
increased to 1.92 mg and 2.16 mg on days 11 and 44 after supplementation, respectively. By
7 days after supplementation ended the blood serum Mg had dropped to 1.7 to 1.9 mg. This
is good evidence that blood Mg can be an indicator of Mg status in deficient animals.
Mg absorption occurred prior to the small intestine in cows and steers fed grass hay or
silage diets (Khorasani & Armstrong, 1992). They found that net Mg absorption occurred
also in the small intestine and both net absorption and secretion in the large intestine.
They concluded that the major site of absorption of both Mg and Ca was prior to the small
intestine while the major site of P absorption was the small intestine. Another experiment
with Holstein cows fed a TMR confirmed that the rumen was the major site of Mg absorption
(Khorasani, et al, 1995).
The pH conditions in the cecum and proximal colon can affect the solubility of Mg,
according to an experiment with sheep (Dalley, et al, 1992). By infusing volatile fatty
acids into the terminal ileum, they were able to decrease digesta pH and increase Mg
solubility. While urinary excretion of Mg (absorbed Mg) increased during the first 4 hours
of infusion, after 24 hours of infusion it was at or below pre-infusion levels. Overall,
there was little improvement in Mg absorption in the large intestine.
Mechanisms of Mg absorption were reviewed in another article (Hardwick, et al, 1991).
At usual Mg intakes, Mg absorption occurs in non-ruminants primarily by intercellular
diffusion and solvent drag mechanisms. There is evidence for active transport of Mg from
the rumen in sheep.
What Other Factors Affect Mg Utilization?
The concentration of other nutrients in the ration such as calcium (Ca), phosphorus
(P), potassium (K) and fat and the presence of ionophore feed additives can affect Mg
absorption.
Excess Potassium
Probably, the nutrient having the greatest adverse effect on Mg absorption is an excess
of K in the ration, as shown by at least four recent sheep experiments.
Increasing the K concentration in the rumen of sheep caused a decrease in plasma Mg in
12 hours, decreased urine Mg excretion and increased fecal Mg (Grace, et al, 1988).
Prolonged infusion of KCl into the rumen significantly reduced plasma Mg levels from 2.3
to 2.0 mg/dl (Yano, et al, 1990). Increasing K intake from 1.6 to 4.6% in rumen fistulated
sheep resulted in a decline in net absorption of Mg from the entire digestive tract with a
consistent reduction in plasma Mg. However, by increasing the Mg intake, they were able to
increase Mg absorption at all K levels, thus overcoming the adverse effects of K excess
(Dalley, et al, 1997). Another study found that apparent Mg absorption in ewes decreased
from 0.43 to 0.34 g/day when K infusion into the rumen was increased from 15 to 45 g/day,
most of the decrease occurring as K was increased from 15 to 25 g/day (Wachirapakorn, et
al, 1996).
The response was similar in goats. Increasing the dietary K concentration from 0.78 to
3.4% reduced Mg absorption from 29.8 to 22.1%. However, they were able to counteract this
effect by adding corn starch, but not glucose, to the ration, possibly by altering rumen
pH (Schonewille, et al, 1997).
Mid-lactation dairy cows were fed 1.6, 3.1 or 4.6% K in a TMR. The highest K level
resulted in reduced plasma Mg and reduced milk yield (Fisher, et al, 1994). Another study
with early lactation cows found that the forage source can affect mineral absorption
(Khorasani, et al, 1997). For example, the apparent digestibility of K was lower for
cereal silage than for alfalfa silage. Mg excretion in the feces was greater with higher K
intake. In dry cows feeding supplemental MgO (0.4%Mg) with a ration containing 3.0% K for
only a 2 week period failed to affect plasma Ca or Mg levels (Fredeen, et al, 1995).
However, high K did result in a higher incidence of retained placenta. Perhaps, feeding
supplemental MgO for a longer period of time could have made a difference in blood Mg
levels.
Fat
Higher levels of added fat in dairy cow rations can react with minerals such as Ca and
Mg resulting in the formation of Ca and Mg soaps, possibly leading to reduced mineral
availability.
Lactating cows in one study were fed animal-vegetable fat or Megalac at 2.5% or 5.0% of
the TMR (Rahnema, et al, 1994). While rumen absorption of Mg was not affected by fat
intake, Mg absorption in the total intestinal tract was decreased by fat intake. Ca
absorption was decreased more by the higher fat level. Fat source did not have an effect
on Mg absorption (Rahnema, et al, 1994). Cows in a later experiment were fed from 75 to
1,567g of different types of fat (Pantoja, et al, 1997). Again, increasing fat intake
decreased the apparent absorption of Mg. Ca absorption was affected more by type of fat,
with unsaturated fats causing the greater decrease in absorption. While Mg absorption was
not affected in all experiments, many nutritionists recommend feeding an additional 0.05%
Mg with higher fat rations.
Ionophores
Feeding ionophores such as monensin and lasalocid increased Mg absorption in some
experiments. For example, steers on a high forage diet were fed 0, 100 or 200 mg of the
ionophore lysocellin or 200 mg of monensin per day. Absorption percentages of Mg, Ca, K
and P were higher in steers fed lysocellin and feeding either monensin or lysocellin at
200 mg produced similar results (Spears, et al, 1989). Another study measured the effects
of feeding the ionophores lasalocid and monensin on absorption of various minerals from
different segments of the digestive tract in sheep. Feeding lasalocid and monensin
increased urinary Mg excretion (absorbed Mg) 17% and 19%, respectively (Kirk, et al,
1994). The authors concluded that ionophores may alter the flow and extent of Mg
absorption in different segments of the digestive tract. They suggested feeding a source
of Mg with high availability in the preintestinal region for the best results.
Other Minerals
High levels of other minerals can affect Mg absorption. One experiment with sheep found
that feeding high levels of Ca and K, along with reduced levels of Mg, reduced the net
balance of Mg, apparently by reducing Mg absorption (Fredeen, 1990). An in vitro study
found that increasing the P concentration caused a decrease in Mg solubility when Ca was
present (Brink, et al, 1992). Also, they found that increasing the Ca concentration caused
a decrease in Mg absorption from the ileum of non-ruminants (rats). These results confirm
the fact of interactions among minerals and emphasize the importance of maintaining a
balance of minerals in the ration.
Age
One recent report studied the effect of inadequate dietary Mg intake on bone formation
or remodeling in dry cows. Cows were fed either low Mg (0.22%) or high Mg (0.82%) for
seven weeks before calving. Plasma Mg was lower in both young and old cow groups when fed
low Mg but decreased to a greater extent in older cows. The experiments demonstrated that
younger cows are better able to mobilize Mg from the body reserves than older cows (Van
Mosel, et al, 1990).
Magnesium Bioavailability
Various methods were used during the last few years to measure the biological
availability of Mg including in vitro or lab methods such as rate of solubility in rumen
fluid or in a weak acid solution. These are less time consuming and much less costly than
in vivo methods requiring the feeding of animals and sampling various tissues or
collecting blood, urine and feces, although animal studies are preferred when feasible.
One experiment compared three commercial feed grade magnesium oxides with different
reactivities and different particle sizes by measuring soluble Mg in acid solution and
rumen fluid. The finer, more quickly reactive MgO (MAGOX) was more readily soluble in both
acid solution and rumen fluid than less reactive and coarser products (Xin, et al, 1988).
Rumen fluid Mg contents were 157.26, 128.08 and 86.01 meq/l, respectively for fine
(Magox), medium and coarse sizes. Also, the total acid consuming capacity was highest for
MAGOX (28.58 vs 20.74 and 15.72 meq H/g).
Another in vitro experiment compared Mg solubilities of various commercial MgO sources
in ruminal conditions for 48 hours then in abomasal conditions for another 2 hours. MAGOX
from Premier Magnesia was more soluble than the nearest competitive product (22.6 vs 14.6
%) in the ruminal stage and in the abomasal stage (51.1 vs48.2 %) (Beede, et al, 1989).
Results are shown in table 1 below.
Table 1. Percent of Total Mg Solubilized
| MgO Source |
Ruminal Stage |
Abomasal Stage |
Total |
Relative Solubility |
Magox
Chinese
Turkish
Spanish
Baymag
Greek #1
Greek #2 |
22.6
11.7
14.6
14.5
14.2
7.6
6.5 |
51.1
48.2
45.1
33.8
33.2
33.4
30.7 |
73.7
59.9
59.7
48.3
47.2
41.0
37.2 |
100
81
81
66
64
56
50 |
Four MgO sources, all of Greek origin, were compared for apparent availability in a
balance study with sheep. Apparent availability of Mg in the basal diet was 20.3%.
Apparent availability of Mg was significantly different among MgO sources ranging from
29.2 to 38.1% (Zervas and Papadopoulos, 1993). In this test apparent availability results
correlated well with solubility in ammonium nitrate but not well with rumen solubility
using the nylon bag technique.
Five feed grade MgO's were compared to reagent grade MgSO4 in another experiment with
lambs. One MgO source was derived from seawater and the others were calcined magnesite
products. Based on urine excretion, the seawater source was 85.3 to 86.3% as available
reagent grade MgSO4 while magnesite sources ranged from 77.5 to 81.8% (Van Ravensway, et
al, 1991). Raw, uncalcined magnesite ore had a biological availability of zero when the Mg
content of the basal diet was considered.
One experiment compared magnesium hydroxide and MgO for bioavailability in beef cattle
fed free-choice supplements. Daily Mg intakes were similar (7.4 and 7.7 g, respectively)
and plasma Mg levels were similar, suggesting the two sources had similar
bioavailabilities (Davenport, et al, 1990).
Magnesium mica was compared to MgO and MgSO4 in lambs. Diet treatments were control
(.08%Mg), Mg-mica (.27% Mg), MgO (.27% Mg) and MgSO4 (.24% Mg). Fecal Mg excretion was
highest with Mg-mica while plasma Mg was highest with MgO and MgSO4, indicating greater
availability for the latter two sources (Jackson, et al, 1989).
Magnesium Oxide as a Buffer/Alkalizer
Building upon the extensive buffer research of the early 1980's, recent researchers
continue to show the benefits of feeding magnesium oxide along with sodium bicarbonate to
lactating dairy cows. MgO acts as an alkalizer to raise the pH or decrease the acidity in
the digestive tract that results from feeding a high concentrate or high energy ration.
Following are some prominent examples of this research.
Buffer consisting mainly of MgO (30 g/day) and sodium bicarbonate (100 g/day) was fed
for 8 months to groups of 92 cows with depressed milk fat. Milk fat increased from 3.06%
(pre-treatment) to 3.68% at 4 months and 3.71% at 8 months. The number of rumen protozoa
increased from 2.85 x 105/ml pretreatment to 9.61 x 105/ml at 8 months with an increase in
acetate production (Shimada, et al, 1989).
First lactation dairy goats were fed concentrates and alfalfa (70:30) supplemented with
2.5% bicarb alone, 2.5% bicarb + 0.5% MgO or 0.5% MgO alone. Feeding MgO increased fat and
solids content and had an additive effect on milk fat. Also, feeding the combination
increased milk fat and rumen fluid butyrate content (Lee and Hsu, 1991).
Magnesium hydroxide (MgOH2) and calcium hydroxide (CaOH2) were fed alone or in
combination as buffer /alkalizer supplements for sheep fed a high barley diet. They fed
1.0% Ca(OH)2, 0.79% Mg(OH)2, or 0.5% Ca(OH)2 plus 0.39% Mg(OH)2. Dry matter intake was
increased by each supplement but intake was significantly greater with the Mg treatments.
Rumen and blood pH also were increased by each treatment (Boukila, et al, 1995).
Note:
Calcium hydroxide can be hazardous to handle but magnesium hydroxide is safe and gave the
best overall response.
Trans fatty acid formation may lead to depressed milk fat levels according to some
recent experiments. Holstein cows were fed either low or high concentrate rations with or
without 0.5% MgO and 1.5% sodium bicarbonate in combination. The high concentrate ration
did increase dry matter intake and decrease milk fat percentage. Buffer addition increased
% milk fat, as expected. Interestingly, trans fatty acids were increased in duodenal
contents and milk of cows fed high concentrate ration without buffer but not in buffered
rations (Kalscheur, et al, 1995). They suggest that trans fatty acids formed in the rumen
are responsible for milk fat depression and that feeding buffers can at least partially
correct this.
A later study by these researchers (Kalscheur, et al, 1997) again utilized high and low
concentrate diets with and without the combination of 0.5% MgO and 1.5% sodium
bicarbonate. The high concentrate diet without buffer increased the flow of trans-C18:1
fatty acids from the rumen to the duodenum and depressed milk fat percentage. Again,
feeding the combination buffer partially corrected this milk fat depression.
Various buffers were fed to early lactation cows to determine their effects on blood
components and milk composition (Thivierge, et al, 1995). Their TMR consisted of 53% grass
silage and 42% concentrate with 4% of calcium salts of fatty acids. Buffer treatments were
1.1% sodium bicarbonate and 1.1% potassium bicarbonate; 1.9% sodium bicarb alone; 0.5%
MgO; and 2.0% sodium sesquicarbonate which were calculated to provide equal acid
neutralizing capacity. Buffers in general increased the blood acetate:propionate ratio and
the removal of triglycerides from the mammary gland. Triglyceride removal from the mammary
gland was greater with MgO than with sodium bicarbonate. Feeding MgO compared to sodium
bicarbonate decreased the weight percent of polyunsaturated fatty acids in milk but
increased the cis:trans ratio, the saturated to polyunsaturated fatty acid ratio and the
proportion of monounsaturated fatty acids.
Other Magnesium Nutrition Research
Mineral status of animals in research often is best determined through bone analysis.
One study measured the Mg, Ca and P in rib bones of cattle sampled at various ages in
order to establish reference values (Beighle, et al, 1994). Bone ash was considered more
reliable than fresh or dried bone weight as a basis for expressing mineral values. The Mg
content of bone ash decreased with age of animal and was greater in steers than females.
The average Mg concentrations in mg per g bone ash were 12.37, 8.09 and 6.62 for ages 6-18
months, 19-36 months and older than 36 months.
Mineral concentrations were measured in Holstein colostrum, in later milk and in plasma
of colostrum-fed calves after one week (Kume and Tanabe, 1993). The concentrations of Mg,
Ca, P, Fe, Zn and Mn were highest at parturition and decreased rapidly by 24 hours
postpartum. Colostral Mg, Ca and P decreased as lactation number increased and stabilized
after the third lactation. Calves' plasma Mg decreased with time. Initial Mg
concentrations in colostrum were 38.6 and 29.2 mg/dl for cows in their first and fifth or
greater lactations.
Mares' milk from quarter horses was measured for mineral content during the first 180
days of lactation (Anderson and Loch, 1993). The concentration of each mineral (Ca, P, Mg,
Na and K) in milk decreased in a curvilinear manner throughout lactation. The initial Mg
concentration was 160.7 ug/g, decreasing to 90.1 ug/g after 180 days.
Increasing the Mg content of feedlot cattle rations from 0.18% to 0.32% by addition of
MgO may enhance dietary net energy in a manner independent of organic matter, starch, and
protein digestion according to a recent report (Zinn, et al, 1996). Magnesium levels were
fed either with or without the feed additive laidlomycin propionate (LP). At the higher Mg
level, LP decreased the molar proportions of acetate and increased the propionate.
Increasing dietary Mg improved average daily gain by 6%.
Magnesium oxide was added at levels of 0.25, 0.75, 1.25 and 1.75% into a
monensin-containing self limited energy supplement to determine supplement intake of
steers grazing wheat pasture (Paisley, et al, 1997). Individual supplement intakes were
measured during a 28-day period with data analyzed by two different models. There was no
significant decrease in supplement intake by increasing the MgO content.
Magnesium supplements are sometimes fed to reduce stress or produce a
"calming" effect in animals. There are few controlled studies published which
document this effect. However, two recent reports show that feeding magnesium aspartate
supplements to stress-susceptible pigs improved meat quality. In one experiment feeding 20
g of monomagnesium aspartate-HCl (to provide 25 mg Mg) per day for five days decreased
meat temperature at 45 minutes post-stunning and increased meat redness. Feeding 40 g/day
reduced percent drip loss (Schaefer, et al, 1990).
Large White x Landrace boars weighing 170 lb were fed 40 g Mg aspartate per day for
five days before slaughter in another study. Supplemented pigs had lower lactic acid in
muscles, higher muscle pH and lower drip loss from the carcass (D'Souza, et al, 1998).
We emphasize that feeding magnesium aspartate commercially is not approved for this
purpose in the United States.
Excess Mg
The effects of feeding an excess of Mg were described in three reports. In the first,
24 finishing steers were fed diets containing calculated levels of 0.3, 1.2, 2.4 or 4.8%
total Mg (dry basis, from MgO) for 130 days (Chester-Jones, et al, 1988). Control steers
gained 20 pounds while other groups lost 11, 59 and 65 pounds, respectively. Steers fed
the two higher levels became lethargic and developed severe diarrhea, with intermittent
diarrhea in group 2. Other effects were decreased feed intake, increased Mg absorption and
serum Mg levels (up to 9.04 mg/dl).
The same research group (Chester-Jones, et al, 1989) studied the feeding of excessive
Mg levels to lambs. Four Mg levels in the complete ration were 0.2% (basal), 0.6, 1.2 and
2.4% with MgO supplying the supplemental Mg. Reduced feed intake occurred only in one
animal fed 2.4% Mg. Diarrhea occurred within 24 hours in lambs fed the two higher levels,
those fed 2.4% Mg having the most severe form. Other effects were reduced dry matter
digestibility and decreased P and Ca utilization. There was little effect of feeding 0.6%
Mg and the authors suggest that the "maximum tolerance" level of 0.5% Mg is
acceptable with a narrow margin of safety.
Finishing steers were fed one of four Mg levels ----0.3, 1.4, 2.5 or 4.7% ----in a
feedlot ration for 130 days (Chester-Jones, et al, 1990). Supplemental Mg was supplied by
MgO. Steers fed the two higher levels refused some feed so their daily intakes were 2.4
and 3.7% Mg. Severe diarrhea and a lethargic appearance occurred at the two higher Mg
levels. There was noticeable damage to rumen papillae of steers fed 1.4% Mg, although not
as severe as found at the two higher Mg levels. Utilization of P, Ca and dry matter was
decreased at the higher Mg levels. A safe level appeared to be something below 1.4% Mg.
The authors conclude that accidental over-consumption of Mg, although debilitating, is
unlikely to cause fatal toxicosis under practical circumstances.
The Bottom Line
Knowledge of magnesium in animal nutrition has progressed considerably during the past
10 years. Absorption of Mg in ruminants occurs throughout the digestive tract but
primarily in the rumen and reticulum, provided the Mg source is readily soluble.
Non-ruminants absorb Mg primarily from the small intestine. Dietary factors which limit Mg
utilization include excessive K levels, added fat and an imbalance of other minerals,
especially Ca and P. Supplementing with Ionophores appears to improve Mg utilization in
ruminants. Also, the animal's ability to mobilize Mg from body reserves decreases with
age.
Continued research shows that Mg bioavailability varies among supplemental Mg sources,
even among different sources of the same compound, such as magnesium oxide. Mg sources
which are more soluble in acid solution and in the rumen are more efficiently utilized.
Readily bioavailable Mg sources include MgO, Mg(OH)2 and MgSO4.
Buffers containing MgO along with sodium bicarbonate and sodium sesquicarbonate
continue to be more effective than a single buffer/alkalizer in ruminants. Recent research
shows that rumen buffers restore depressed butterfat levels in part by reducing the
formation of trans-fatty acids in the rumen.
Feeding excessive Mg levels to ruminants results in damage to the rumen wall and
diarrhea along with reduced feed intake and lethargy. Excessive levels appear to be
between 0.6% and 1.4% of dry matter. Other research reported the Mg content of colostrum
and milk of cattle and horses and the use of a Mg compound for reducing stress levels of
pigs. While much more remains to be done, recent research on Mg nutrition in animals has
been very beneficial to the animal industry.
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