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OCR for page 277
Magnesium
Magnesium (Mg) is an alkaline earth metal belonging to Group IIA of
Me periodic table. It ranks eighth in abundance in the earth's crust.
Industnally, it is important as a structural material such as for fire
bricks in production of steel.
Magnesium is one of the major minerals recognized as essential for
animals. Approximately 60 percent of total body magnesium is located
in bone, where the function of the element is not known (Pike and
Brown, 1975~. About one-third of that in the bone is combined with
phosphate, and the remainder is adsorbed loosely on the surface of the
mineral structure. Magnesium occurs intra- and extracellularly in soft
tissues. The small amount present in extracellular fluid is exchanged
easily with that adsorbed on the bone surface. The serum level of
magnesium varies, usually between 1 and 3 mg/dI. Within the cells of
soft tissues, magnesium is found in larger concentrations than any other
element except potassium.
Supplemental magnesium is used at fairly high levels for dairy cows,
beef cows, and ewes. Although recommended levels are not toxic,
magnesium may be toxic when excessively high levels are accidentally
used.
ESSENTIALITY
Magnesium is essential for cellular respiration. It is necessary for all
phosphate transfer reactions, and in certain tissues it is complexed with
277
OCR for page 278
278 MINERAL TOLERANCE OF DOMESTIC ANIMALS
adenosine triphosphate (ATP3, adenosine diphosphate (ADP3, and aden-
osine monophosphate (AMP3. It is an activator for all thiamine pyro-
phosphate (TPP) requiring reactions. Also, for certain reactions mag-
nesium is involved in the metabolism of fat and protein. Magnesium is
an essential nutrient for all animals. The amount required varies among
species and between classes of animals within species.
signs of deficiency in all animals include loss of appetite, lower rate
of body weight gain, and hyperexcitability. Among farm animals the
disturbance most frequently associated.with lack of magnesium is
hypomagnesemic tetany in ruminants, also known as grass tetany,
grass staggers, winter tetany, and wheat pasture poisoning. In the
United States this disturbance usually occurs in beef cows in the early
stages of lactation, but also occurs in ewes and dairy cows (Fontenot
et at., 1973~. The disturbance is more prevalent in older animals, which
may be due to less labile magnesium in older than in young animals
Thomas, 19651. The signs of the condition are of a neuromuscular
nature. Usually, the disease is fatal if the animals are not treated.
Hypomagnesemic tetany appears to be caused by a physiological defi-
ciency of magnesium, which may result from a simple dietary defi-
ciency or lowered efficiency of utilization of the element. However,
there is some evidence that a shift of magnesium ion inside the body Is
responsible, at least in part, for hypomagnesemia (Larvor, 1976~. Per-
haps the main reason that ruminants are more susceptible to this dis-
turbance than nonrum~nants is the generally lower magnesium levels in
roughages than concentrates and lower bioavailability of the magne-
sium' especially from certain "tetany-prone" roughages. Calves on
all-m~lk diets are quite susceptible to magnesium deficiency (Duncan et
al., 1935~. This appears to be due to a low magnesium level in milk.
Magnesium deficiency was produced in baby pigs fed diets containing
125 ppm magnesium or less (Miller et al., 1965~.
METABOLISM
Magnesium is absorbed from the small intestine in simple-stomached
animals and from the first three compartments of the ruminant stomach
(Grace et al., 1974~. There is considerable excretion into the lower
digestive tract (Cragle, 1973~. The magnesium in bone can be mobilized
to a limited extent, especially in younger animals, but apparently this
is not under hormonal control (Rayssiguier et al., 1977~. In older
animals mobilization is very limited (Thomas, 1965~. Approximately
OCR for page 279
Magnesium
279
one-third of magnesium in bone is in combination with phosphorus, but
its function is not known.
Unnary excretion is usually a reflection of quantity of magnesium
absorbed. It has been suggested that in ruminants magnesium absorbed
in excess of requirement is excreted via the urine (Rook and Starry,
1962~. Chicco et al. (1972) reported a high correlation (r = 0.95) be-
tween magnesium absorption and urinary excretion.
SOURCES
Magnesium is present in variable amounts in common feedstuffs
(National Research Council, 19791. Generally, concentrates contain
higher levels than roughages. There is a large degree of variability
among forages, presumably due to soil availability (Reid et al., l970~.
Legumes are generally higher in magnesium than grasses.
I'here are a number of supplemental sources available, with the most
commonly used being magnesium oxide. The bioavailability of magne-
sium from this product is very good, and there is variation in the
availability of salts of magnesium. Another important consideration is
We level of magnesium in the different supplemental sources. For ex-
ample, the magnesium content is more than 50 percent in feed-grade
magnesium oxide, but only about 12 percent for magnesium carbonate.
Bioavailability in cattle and sheep of magnesium from crude products
such as dolom~tic limestone and magnesite has been shown to be very
low (Gerken and Fontenot, 1967; Ammerman et al., 1972~. Further-
more, supplementing with dolom~tic limestone results in a large depres-
sion in apparent digestibility of energy, resulting mainly from depres-
sions in digestibility of the carbohydrate components (Gerken and
Fontenot, 1967~.
TOXICOSIS
Toxicosis due to ingestion of natural feedstuffs has not been reported
and does not appear likely. Thus, toxicosis would occur from using
excess levels of supplementary magnesium.
LOW EEW~
Ingestion of excess levels of magnesium has generally resulted in de-
creased growth rate in chicks (Nugara and Edwards, 1963; Chicco et
OCR for page 280
280 MINERAL TOLERANCE OF DOMESTIC ANIMALS
al., 1967), guinea pigs (Moms and O'Dell, 1963), and sheep (Kerk,
1973~. The decrease in performance appears to be caused at least partly
by decreased feed intake. Scouring is also a problem with high dietary
magnesium levels (Peirce, 1959; Care, 1960~. Generally, the high intake
of magnesium increased blood serum magnesium.
Supplemental magnesium oxide was administered orally by capsule
to supply up to 5.3 percent magnesium, dry basis, to yearling wethers
(personal communication, J. P. Fontenot, Virginia Polytechnic Insti-
tute and State University, Blacksburg). High levels of supplemental
magnesium resulted in depressed feed intake, elevated serum mag-
nesium, arid diarrhea. The time required to produce diarrhea was in-
versely related to the dietary level. Diarrhea was observed 24 hours, 48
hours, and 6 days after initiation of treatment in sheep receiving 5.3,
2.0, and 0.8 percent magnesium, respectively. Feeding 2.3 or 4.3 per-
cent magnesium to Holstein bull calves resulted in severe diarrhea and
decreased feed intake and rate of gain (Gentry et al., 1978~. Mucus was
voided in feces of calves fed the high magnesium levels. In studies
designed to establish the magnesium requirement of beef cows, levels
as high as 20 g per day (0.29 percent) were fed to cows during gestation
(O'Kelley and Fontenot, 1973) and 42 g (0.29 percent) to lactating beef
cows (O'Kelley and Fontenot, 1969) with no deleterious effects. In-
creasing the level of magnesium in the diet from 0.16 to 0.22 percent
lowered rate and efficiency of gain in growing or finishing swine when
they weighed 20 to 45 kg, but had no effect thereafter (Krider et al.,
1975).
HIGH LEVELS
The presence of high magnesium levels in water (about 1 percent) was
reported to cause a weakening eject on men and livestock in an area
including parts of Minnesota, the Dakotas, and Montana (Allison,
19301. He reported that cattle and hogs could not be fattened for market
while drinking this water. Cattle developed a "run-down-ragged ap-
pearance," and many died prematurely. A degeneration of the bones
occurred. Calves were stunted and many never matured. The cows
developed depressed appetites.
Hypertonic magnesium sulfate enemas produced adverse effects in
young lambs (Andrews et al., 19651. Administration of 10 nil of 50
percent magnesium sulfate to five newborn lambs resulted in death in
23 to 46 minutes. Signs included lack of reflexes, anesthesia, and car-
diorespiratory depression. Plasma magnesium was 3.70 to 5.72 mg/dl at
OCR for page 281
Magnesium
281
deatli. Administration of 10 m! of a 25 percent solution resulted in loss
of deep tendon reflexes, deep steep, and cyanosis in two of five lambs,
but only one lamb died. When 10 m] of 50 percent solution were ad-
~ stered to five 2- to 3-months old lambs, three had absent reflexes
and slept, and only one died. Postmortem examinations showed con-
gestion of the lungs, heart, liver, spleen, and kidney.
Intravenous infusion of high levels of magnesium ions resulted in
disruption of motor function in horses, cattle, and dogs (Bowen et al.,
19701. The levels required to produce the effect were 0.1~0.14 g
magnesium sulfate per kilogram (0.02~0.028 g magnesium per kilo-
gram) of body weight. Calcium gluconate and ethylenediaminetetra-
acetic acid (EDTA) ameliorated the effects of the magnesium ions. In-
fusion of 0.22 g magnesium sulfate per kilogram (0.044 g magnesium per
kilogram) of body weight was lethal in one horse due to respiratory
paralysis and cardiac arrest.
Feeding 1.2 percent magnesium resulted in high mortality and de-
pressed growth in guinea pigs fed diets containing 0.9 percent calcium
and 1.7 percent phosphorus (Morris and O'Dell, 1963~. The animals
were lethargic, suffered from diarrhea, and exhibited a poor general
appearance. When the calcium level was increased to 2.5 percent and
the phosphorus was held at 1.7 percent, the toxic effects were al-
leviated. The level of 1.2 percent magnesium had no effect on guinea
pigs when the diet contained 0.9 percent calcium and 0.6 percent phos-
phorus.
Accidental feeding of high-magnesium pellets to sheep instead oft
concentrate pellets resulted in acute metabolic disorders, including loss
of appetite and severe diarrhea (Kerk, 1973~. In some ewes a syndrome
similar to milk fever occurred. Growth of lambs was poor, due to lower
milk production by the ewes. Following withdrawal of the pellets,
disorders of the gastrointestinal tract disappeared. The adverse effects
were ascribed to high levels of magnesium oxide in the pellets.
Ingestion of water with 0.2-0.3 percent magnesium chloride
(1.0~0.69 percent sodium chloride, respectively) was harmful to sheep
(Peirce, 1959). There was occasional diarrhea, which was more fre-
quent with animals receiving higher concentrations of magnesium chlo-
ride. Drenching cattle with 17~342 g magnesium oxide (102-205 g
magnesium) per day resulted in severe scouring in 2~48 hours (Care,
1960~. However, administration of up to 1 14 g (68 g magnesium) per day
did not affect condition or produce scouring.
Supplementing 0.64 percent or 1.28 percent magnesium to chicks
increased mortality and depressed growth rate (Nugara and Edwards,
1963).
OCR for page 282
282 MINERAL TOLERANCE OF DOMESTIC ANIMALS
FACTORS INFLUENCING TOXICITY
Increased calcium and phosphorus levels in the diet have been shown
to increase the magnesium requirement of chicks (Nugara and
Edwards, 1963) and guinea pigs (Morris and O'Dell, 1963~. Increasing
the phosphorus level from 0.6 to 1.7 percent in a diet containing 0.9
percent calcium and 1.2 percent magnesium produced high mortality
and poor growth in guinea pigs (Morris and O'Dell, 19631. Increasing
the calcium level to 2.5 percent alleviated the deleterious effect. Addi-
tion of 0.2 or 0.4 percent magnesium tended to overcome the adverse
effects of deficiencies of both calcium and phosphorus in chicks
(Chicco et al., 1967~. However, when 0.6 percent magnesium was sum
Demented, growth and bone mineralization were adversely affected
regardless of the calcium and phosphorus levels. High levels of calcium
and phosphorus have been shown to depress magnesium absorption in
sheep (Chicco et al., 1973; Pless et al., 19731. Metastatic calcification
in hearts and kidneys of rats administered high levels of vitamin D was
aggravated by high dietary levels of magnesium (Whittier and Freemen,
1971).
The interrelationships between magnesium and calcium and phos-
phorus suggest that hormones and enzymes involved with bone metab-
olism may be related to magnesium metabolism. Supplementation of
1.68 g of magnesium as magnesium sulfate per kilogram of diet to rats
increased the serum alkaline phosphatase activity (Moinuddin and Lee,
19601. Administration of magnesium chloride subcutaneously to ne-
phrectomized rats resulted in a decrease in ionic calcium in plasma, but
no such eject was observed in parathyroidectomized rats (Gitelman et
al., 1968~. Based on these results, it was suggested that hypermagnese-
mia may inhibit parathyroid gland activity. Hypermagnesemia in dogs
resulted in decreased filtered phosphorus excretion and serum calcium
(Massry et al., 1970~. These ejects were reversed by administration of
parathyroid extracts, suggesting that hypermagnesemia suppresses
parathyroid gland activity. The magnitude of involvement of dietary
magnesium level on parathyroid function is not clear, however, since
magnesium deficiency in calves has not affected plasma parathyroid
hormone (PTH) levels in calves (Rayssiguier e! al., 19771.
High dietary potassium depresses magnesium absorption in rumi-
nants (Newton et al., 1972~.
OCR for page 283
Magnesium
TISSUE LEVELS
283
There is little information concerning the tissue levels of magnesium. A
three--to fivefold increase in magnesium in kidney was found from
feeding high levels of magnesium to sheep (personal communication,
J. P. Fontenot, Virginia Polytechnic Institute and State University,
B~cksburg). The level in the unsupplemented controls was 0.1~0.21
percent. There was also an increase of magnesium in bone from 4.14
percent in controls to 4.95 percent in those administered the highest
magnesium level. There was no increase in magnesium levels in mus-
cle, liver, and heart.
MAXIMUM TOLERABLE LEVELS
In cattle, up to 0.39 percent magnesium was fed without problems
(O'Kelley and Fontenot, 1969~. Oral administering of 0.5 percent
magnesium to yearling wethers did not produce toxicity, whereas, ad-
m~nister~ng 0.8 percent or higher resulted in signs of toxicosis (Fontenot
et al., unpublished). Cattle and sheep should be able to tolerate 0.5
percent magnesium. Feeding 0.6 percent magnesium to chicks de-
creased growth rate and bone calcification (Nugara and Edwards, 1963;
Chicco et al., 1967~. Feeding diets with 0.32 or 0.4 percent magnesium
was without effect. The maximum tolerable level for poultry and swine
appears to be 0.3 percent.
SUMMARY
Magnesium is a required element for maintaining normal health and
well-being of animals and is present in soft tissue and bone. It functions
as a component or activator of aN enzymes involved in cell respiration,
but its function in bone is not clear. Magnesium occurs in most natural
feeds. Supplemental sources are magnesium oxide or salts of mag-
nesium.
Magnesium is toxic when adrn~nistered at high levels. The signs are
lethargy, disturbance in locomotion, diarrhea, lowered feed intake and
performance, and death. Toxicosis is not likely except by accident in
mixing feeds or feeding animals. Certain levels of calcium and phos-
phorus in the diet protect the animals from toxicosis.
OCR for page 284
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OCR for page 288
288 MINERAL TOLERANCE OF DOMESTIC ANIMALS
REFERENCES
Allison, I. S. 1930. The problem of saline drinking waters. Science 71:559.
Ammerman, C. B., C. F. Chicco, P. E. Loggins, and L. R. Arrington. 1972. Availability
of different salts of magnesium to sheep. J. Anim. Sci. 34:122.
Andrews, B. F., D. R. Campbell, and P. Thomas. 1965. Effects of hypertonic
magnesiu~sulphate enemas on newborn and young lambs. Lancet 2:64.
Bowen, J. M., D. M. Blackman, and J. E. Heavener. 1970. Effect of magnesium ions on
neuromuscular transmission in the horse, steer and dog. J. Am. Vet. Med. Assoc.
157:164.
Care, A. D. 1960. The effect on cattle of high level magnesium supplementation of their
diet. Vet. Rec. 72:517.
Chicco, C. F., C. B. Ammerman, P. A. van Walleghem, P. W. Waldroup, and R. H.
Harms. 1967. Effects of varying dietary ratios of magnesium, calcium and phosphorus
in growing chicks. Poult. Sci. 46:368.
Chicco, C. F., C. B. Ammerman, W. G. Hillis, and L. R. A'Tington. 1972. Utilization of
dietary magnesium by sheep. Am. J. Physiol. 222:1469.
Chicco, C. F., C. B. Ammerman, J. P. Feaster, and B. G. Dunavant. 1973. Nutritional
interrelationships of dietary calcium, phosphorus and magnesium in sheep. J. Anim.
Sci. 36:986.
Cragle, R. G. 1973. Dynamics of mineral elements in the digestive tract of ruminants.
Fed. Proc. 32:1910.
Duncan, C. W., C. F. Huffman, and C. S. Robinson. 1935. Magnesium studies in calves.
I. Tetany produced by a ration of milk or milk with various supplements. J. Biol. Chem.
108:35.
Fontenot, J. P., M. B. Wise, and K. E. Webb, fir. 1973. Interrelationships of potassium,
nitrogen and magnesium in ruminants. Fed. Proc. 32:1925.
Gentry, R. P., W. J. Miller, D. G. Pugh, M. W. Neathery, and J. B. Bynoum. 1978.
Effects of feeding high magnesium to young dairy calves. J. Dairy Sci. 61:1750.
Gerken, H. J., Jr., and J. P. Fontenot. 1967. Availability and utilization of magnesium
from dolomitic limestone and magnesium oxide in steers. 3. Anim. Sci. 32:789.
Gitelman, H. J., S. Kukolj, and L. G. Welt. 1968. Inhibition of parathyroid gland activity
by hypermagnesemia. Am. J. Physiol. 215:483.
Grace, N. D., M. J. Ulyatt, and J. C. Macrae. 1974. Quantitative digestion of fresh
herbage by sheep. III. The movement of Mg, Ca, P. K and Na in the digestive tract.
J. Agric. Sci. 82:321.
Kerk, P. V. D. 1973. Metabolic disorders in sheep and cattle caused by magnesium oxide
in the concentrate feed. Tijdschr. Diergeneesk. 98:1 166 (via Nutr. Abstr. Rev. 44:799).
Krider, J. L., J. L. Albright, M. P. Plumlee, J. H. Conrad, C. L. Sinclair' L. Underwood,
R. G. Jones, and R. B. Harrington. 1975. Magnesium supplementation, space and
docking effects on swine performance and behavior. J. Anim. Sci. 40:1027.
Larvor, P. 1976. 28Mg kinetics in ewes fed normal or tetany prone grass. Cornell Vet.
66:413
Massry, S. G., J. W. Coburn, and C. R. Kleeman. 1970. Evidence for suppression of
parathyroid gland activity by hypermagnesemia. J. Clin. Invest. 49:1619.
Miller, E. R., D. E. Ullrey, C. L. Zutout, B. V. Baltzer, D. A. Schmidt. J. A. Hoefer'
and R. W. Luecke. 1965. Magnesium requirement of the baby pig. J. Nutr. 85:13.
Moinuddin, J. F., and H. W. Lee. 1960. Alimentary, blood and other changes due to
[ceding MnSo4, MgSO4 and Na2SO`,. Am. J. Physiol. 199:77.
OCR for page 289
Magnesium
289
Morris, E. R., and B. L. O'Dell. 1963. Relationship of excess calcium and phosphorus
to magnesium requirement and toxicity in guinea pigs. J. Nutr. 81:175.
National Research Council. 1979. Nutrient Requirements of Domestic Animals. No. 2.
Nutrient Requirements of Swine. National Academy of Sciences, Washington, D.C.
Newton, G. L., J. P. Fontenot, R. E. Tucker, and C. E. Polan. 1972. Effects of high
dietary potassium intake on the metabolism of magnesium by sheep. J. Anim. Sci.
35:440.
Nugara, D., and H. M. Edwards, Jr. 1963. Influence of dietary Ca and P levels on the Mg
requirement of the chick. J. Nutr. 80:181.
O'Kelley, R. E., and J. P. Fontenot. 1969. Effects of feeding different magnesium levels
to drylot-fed lactating beef cows. J. Anim. Sci. 29:9S9.
O'Kelley, R. E., and J. P. Fontenot. 1973. Effects of feeding different magnesium levels
to drylot-fed gestating beef cows. J. Anim. Sci. 36:994.
Peirce, A. W. 1959. Studies on salt tolerance of sheep. II. The tolerance of sheep for
mixtures of sodium chloride and magnesium chloride in the drinking water. Aust. J.
Agric. Res. 10:725.
Pike, R. L., and M. L. Brown. 1975. Nutrition: An integrated approach, 2nd ed. John
Wiley & Sons, New York.
Pless, C. D., J. P. Fontenot, and K. E. Webb, Jr. 1973. Effect of dietary calcium and
phosphorus levels on magnesium utilization in sheep. Va. Polytech. Inst. State Univ.
Res. Div. Rep. 153:104.
Rayssiguier, Y., J. M. Garel, M. J. Prat, and J. P. Barlet. 1977. Plasma parathyroid
hormone and calcitonin levels in hypocalcaemic magnesium deficient calves. Ann.
Rech. Vet. 8:267.
Reid, R. L., A. J. Post, and G. A. Jung. 1970. Mineral composition of forages. W. Va.
Univ. Agric. Exp. Stn. Bull. 589T.
Rook, J. A. F., and J. E. Storry. 1962. Magnesium in the nutrition of farm animals. Nutr.
Abstr. Rev. 32:1055.
Thomas, J. W. 1965. Mechanisms responsible for grass tetany, p. 14. In Proc. Ga. Nutr.
Conf. Feed Manuf.
Whittier, P. C., and R. M. Freeman. 1971. Potentiation of metastatic calcification in
vitamin D-treated rats by magnesium. Am. J. Physiol. 220:209.
Representative terms from entire chapter:
magnesium oxide
