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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.


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.


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.


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 SourceRuminal StageAbomasal StageTotalRelative Solubility
Greek #1
Greek #2

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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