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VIII. Prevention of Grass Tetany
D. L. GRUNES, P. R. STOUT, AND .I. R. BROWNELL
The effect of fertilization on content of rye forage grown on coarsetextured soil is indicated in Table IV, which is adapted from Lowrey
and Grunes ( 1968). Although K fertilization decreased the concentrations
of Mg in the plants, the addition of Mg with the K did increase the concentrations of Mg. Potassium fertilization decreased the concentrations
of Ca in the plants, and the addition of both K and Mg decreased the Ca
concentrations still further. As expected, fertilization with K increased the
K concentrations in the plants. However, the further addition of Mg did
not consistently affect the K concentrations. The addition of K increased
the ratios of K/(Ca Mg). However, the further addition of Mg did not
change this ratio, since Ca concentrations were decreased.
The aforementioned rye forage was harvested at periodic intervals and
fed to milking Jersey cows. As indicated earlier, hypomagnesemic tetany
often occurs when Mg in the blood serum is 5-10 ppm. For those treatments which received no Mg fertilizer, the Mg in blood serum dropped
after the feeding of rye forage was initiated, but rose again later in the
season (Lowrey and Grunes, 1968). However, in no case did the Mg in
the blood drop below 18 ppm of Mg in the blood serum. [Rook and Storry
( 1962) have reported a temporary decrease in Mg concentrations in the
blood of dairy cattle during spring grazing.] The serum Mg level did not
drop for the cattle fed Mg-fertilized forage (Lowery and Grunes, 1968).
The concentration of Mg in the urine dropped rapidly for those cattle
fed rye forage grown on land not fertilized with Mg (Lowrey and Grunes,
1968). In fact, the level approached the concentration of 5- I0 ppm of Mg,
which Rook and Balch (1958) indicated was typical of animals suffering
from grass tetany. The Mg in the urine did not decrease below 50 ppm of
Mg for animals fed the rye forage from the Mg-fertilized area (Lowrey and
Wolton ( I 963) reviewed the effect of Mg fertilization on Mg concentrations in forage. She indicated that, in the British Isles, high rates of
Mg (365 and 1620 kg of Mg per hectare) have consistently increased
herbage Mg to levels above 0.2% and have prevented hypomagnesemia,
sometimes for a number of years. She indicated that low rates (68 kg/ha
or less) of applied Mg have often been ineffective in increasing herbage
Mg. Low and medium rates of Mg fertilization were most effective on
coarse-textured soils of low base-exchange capacity, and on soils with
a low Ca content.
She stated that calcined magnesite, Kieserite, and Epsom salts were
more effective sources of Mg than dolomite for grassland, especially on
soils with a high Ca content. (Calcined magnesite is Mg carbonate heated
to a high temperature, resulting in MgO. It contains about 50% Mg.
GRASS TETANY OF RUMINANTS
Kieserite is MgSO, * H 2 0 , and contains about 16% Mg. Epsom salt is
MgSOl . 7 H z 0 ,and contains 10% Mg. Dolomite is CaC03 * MgCOa and
contains about 12% Mg.)
R. Allcroft and Burns ( 1 968) indicated that calcined magnesite is better suited for top-dressing pastures than Mg sulfate applied as Epsom
salts, or as Kieserite. Magnesium is leached more from the soil when Mg
sulfate is applied. Also Mg sulfate is more difficult to store and to handle.
Wolton (1963) quoted work of Griffiths ( 1 959) indicating that the effectiveness of dolomite may increase with time. She also referred to work
of Reith (1954) and Stewart and Reith ( 1 956) which indicated that dolomite was useful on acid soils when it was applied at rates containing
enough Mg to be effective.
While fertilization with Mg has been effective in increasing Mg concentrations in plants grown on acid, coarse-textured soils, McConaghy
et al. ( 1963) found that, on finer-textured soils in Ireland, neither calcined
magnesite at I120 kg/ha nor Mg-limestone at 5600 kglha (2.5 tons/acre)
significantly increased Mg in pasture plants. Burns and Allcroft (1967)
reviewed literature on this problem and also concluded that Mg fertilization is most effective in increasing Mg concentrations in plants grown on
acid, coarse-textured soils. They indicated that while Mg limestone is
not as effective as calcined magnesite, it is cheaper.
Todd ( 1967) indicated that on coarse-textured soils, fertilization with
Mg at 340 kg/ha has given control for three to four years. In these cases,
the cost is similar to other methods of control. He indicated that on finertextured soils, much larger amounts are required, and that unless the cost
perunit of Mg is low, Mg fertilization should probably be limited to coarsetextured soils, or soils of low pH and low K status.
In New Zealand, McNaught and Ludecke ( I 967) applied 28-56 kg of
Mg per hectare per year (as dolomite or serpentine superphosphate) to
short pastures. (Serpentine superphosphate contains one part serpentine
rock to three parts superphosphate, and contains 5-5.5% Mg.) The Mg
contents in plants were appreciably increased only on soils derived from
pumice. Wallace ( 1967) indicated that, in New Zealand, Mg fertilization
of pastures is advised only on pumice and leached sands.
Short-term increases in Mg intake by ruminants can be accomplished
by dusting forage with calcined magnesite (or another form of MgO), or
spraying the foliage with a solution of calcined magnesite or Mg sulfate.
As indicated by R. Allcroft and Burns (1968) weather conditions are
very important for the success of these practices. Heavy rain may wash
D. L. GRUNES, P. R. STOUT, A N D .I. R. BROWNELL
the powder off, and dry and windy weather may also remove the powder.
Todd and Morrison (1964) obtained protection against grass tetany in
a dairy herd in Ireland by dusting as little as 19 kg of Mg per hectare as
calcined magnesite on a tetany-prone pasture I or 2 days before grazing
started. The grass was 23-30 cm long and was dense enough to retain
the powder. Analysis indicated that grass from untreated pastures con’ Mg for grass from
tained an average of 0.16% Mg, as compared to 0.3 1 %
R. Allcroft and Burns ( 1 968) reviewed research by McAllister and coworkers in Ireland. It was found that spraying the pasture with a solution
of calcined magnesite in water was as effective in increasing herbage Mg
as was dusting with calcined magnesite after first spraying with a dilute
solution of molasses. However, spraying the pasture with a solution of
Mg sulfate was less effective, especially in wet weather.
Kemp and Geurink ( 1967) found that. 3 days after dairy cattle started
grazing pastures dusted with 18 kg of Mg per hectare, the blood serum
contained 23.7 ppm Mg. This compared with 14.3 ppm Mg for cattle
grazing pastures which had not been dusted. Horvath and Todd (1968)
suggested that dusting winter grain pasture may be advisable in Texas,
Oklahoma, and Mississippi. Georgia would be another such area.
Horvath and Todd ( 1968) indicated that hay and silage can be fortified
with Mg by adding MgO in the windrow, or when grass is placed in the
silo. They indicated that 2 kg of a commerical grade MgO powder (containing about 52.5% Mg) per 1000 kg of fresh grass (4 Ib/ton) provides
adequate protection. If carefully mixed in, it does not interfere with the
fermentation (Todd, 1968). The Mg-fortified hay and silage would be
helpful for animals during the winter months.
Several review articles deal with oral supplementation of Mg for cattle
(R. Allcroft and Burns, 1968: Horvath and Todd, 1968: Todd, 1967:
Underwood, 1966). Many methods use MgO powder since it is the most
concentrated form of Mg. The general aim is to provide a supplement of
about 30 g of Mg (equal to 2 oz of commercial grade MgO, 85% pure)
per day. Underwood ( 1966) indicated that calves require 3.7-7.9 g of Mg
(7-15 g of commercial grade MgO per day), depending on their age. If
Mg carbonate is used instead, about twice as much is needed to obtain
an equal amount of Mg. He stated that for lactating ewes, the supplements
during the tetany-susceptible period, just after giving birth, should not
exceed 3.7 g of Mg (7 g of commercial grade MgO) per day.
Horvath and Todd ( I 968) evaluated the sources for cattle as follows:
GRASS TETANY OF RUMINANTS
Loose concentrure mixes: Soy bean oil meal, or corn meal, mixed with
MgO has been effective for feeding animals indoors, but not outdoors.
Minerul mixes: Salt mixtures, either with or without Ca and P sources,
has been used as a source of Mg. However, consumption is too erratic.
Compressed mineral blocks have not been adequately consumed by the
Liquid molasses: Liquid molasses-MgO mixtures have proved very
effective for cattle grazing limited acreage in cool, cloudy weather. However, they are less convenient than dry supplements and will cake in
warm, dry locations. In the United States, such supplements could be
useful for cattle grazing spring grass.
Molasses-MgO blocks or cubes: Compressed blocks, containing molasses, MgO. salt, and other materials have been successfully used for cattle
on pasture. About 95% of the cows are protected, since some cows consume too little to obtain enough Mg. Cubes have also been used successfully, but they require more labor, and may not be as suitable in areas
having appreciable rainfall.
Gerken and Fontenot (1967) found that the availability of Mg fed to
steers was much less for dolomite than for MgO. Approximately equivalent amounts of Mg were used in both cases.
Metson et al. (1966) and G . W. Butler and Metson (1967) have also
indicated that oral supplementation with carbohydrates could be important for cattle feeding on high-N forages. Some sources of carbohydrates
are hay, molasses, and starchy concentrates.
Ritchie and Hemingway (1968) indicated that bullets placed in the
rumen, which slowly release Mg, have been effective in decreasing the
incidence of hypomagnesemic tetany in dairy cattle in an experiment in
Scotland. Davey ( 1968) indicated some effect of smaller Mg-releasing
rumen bullets on decreasing the incidence of grass tetany in sheep in an
experiment in England. In neither study was there a pronounced effect
of the rumen bullets on increasing Mg in the blood. This might be expected since the rumen bullets used for cattle released only one gram per
day per bullet, and two bullets were administered to each animal. Foot
et al. (1969) found that adding two Mg rumen bullets per cow did not
affect the level of Mg in blood serum of cattle on pasture. Davey and Gilbert (1969) suggested that the constant release of Mg from the bullets,
into the rumen, may result in a relatively high utilization rate of this Mg.
In Ireland, Smyth (1969) set up an experiment involving 15 cattle in
each of the following four treatments: control; 2 rumen bullets; 4 rumen
D. L . GRUNES, P. R. STOUT, A N D .I.R. BROWNELL
bullets; and 2 oz of calcined magnesite daily. The experiment was carried
out in the spring on permanent pasture. There were five cases of grass
tetany in the control treatment, and none in any of the other treatments.
The mean Mg in the blood serum significantly decreased, during a onemonth period, only in the control treatment.
Hemingway and Ritchie ( I 969) separated nine herds of 2 to 4 monthold calves into two groups of 113 calves each. The calves were nursing
from their mothers. Two “sheep-size” Mg bullets were administered to
one group of I13 calves. No cases of grass tetany occurred in these
treated calves, but four cases of grass tetany occurred in the 113 untreated calves. The bullet treatment produced a sustained increase, during
the sampling period, in mean Mg in the blood plasma for those herds
which had initial plasma Mg levels of less than 15 ppm.
It appears that additional research is needed on the utility of Mg rumen
bullets in decreasing hypomagnesemic tetany.
In Section V, A, it was indicated that the concentration of Mg in legumes is higher than in grasses. As indicated there, it migh be possible to
avoid grass tetany by breeding or selecting legumes that would start to
grow as early in the spring as the grasses. It might also be possible to
select or breed grasses that would contain higher concentrations of Mg
or lower concentrations of trans-aconitate, citrate, or higher fatty acids.
IX. Magnesium Deficiency in Humans
Workers in developing countries should be especially aware of problems of Mg deficiency in protein-calorie malnutrition of childhood. Montgomery (1 960) reported that tetany associated with some cases of Kwashiorkor in children in Jamaica was dramatically improved by Mg therapy.
In Uganda, Caddell (1 965) found a severe Mg deficiency of children suffering from protein-calorie malnutrition. Caddell ( 1967) indicated that
many of the dietary staples commonly used in the Tropics (cornstarch,
yam, and cassava) are Mg-poor foods. She vividly described cases (including startling photographs) of Mg deficiency in children of Nigeria,
which resulted in death unless Mg therapy was administered. She indicated that nuts, legumes, and whole grains were excellent sources of Mg.
She suggested that similar cases of human Mg deficiency are found in
villages and clinics of Central and South America, Asia, elsewhere in
Africa, and in the islands of the tropical seas. In a recent paper (Caddell,
1969), clinical symptoms are discussed in detail. Dr. Caddell also indi-
GRASS TETANY OF RUMINANTS
cated the positive effects of Mg supplements in the treatment of Nigerian
children suffering from protein-calorie malnutrition.
Gardner et al. (1950) reported that tetany in newborn children fed
entirely on cow’s milk in a Massachusetts hospital, responded to Mg
therapy. In a recent review, Krehl (1967) indicated that dietary Mg
deficiency is one of the most common nutritional deficiencies in clinical
medicine. He indicated that clinicians are becoming more aware of it, as
hospital laboratories become more proficient in making Mg determinations. Workers interested in Mg deficiency in humans are also referred to
the reviews by Aikawa (1963) and Wacker and Parisi ( 1 968). A collection
of articles dealing with recent Mg research is contained in Flink and
Jones ( 1969).
Grass tetany occurs when cattle or sheep graze grass or small grains
forages in cool weather. Pregnant or lactating animals are most susceptible. The primary cause of the disease is low Mg in the forage, but high
concentrations of N and K in the forage can be contributing factors. Other
factors that may be involved are low concentrations of carbohydrates
and high concentrations of trans-aconitic acid, citric acid, and certain
higher fatty acids.
The disease may be prevented by Mg fertilization of acid, coarsetextured soils. However, because of higher adsorption capacities of finetextured soils, and lower solubilities of MgC03 in soils of high pH,
higher rates of Mg fertilizer would generally be required in these cases.
This is more expensive, and in these cases it may be better to supply Mg
by oral supplementation by legumes, adding Mg to feed or salt blocks, or
by foliar applications of MgO to the forage. Another possible solution is
the use of Mg bullets in the rumen of cattle and sheep, but more research
is needed concerning the effectiveness of this method. A long-term solution may be the breeding and selection of legumes that grow as early in
the season as cool-season grasses, and the breeding of grasses containing
high concentrations of Mg.
Aikawa, J . K. 1963. “The Role of Magnesium in Biologic Processes.” Thomas, Springfield,
Allcroft, R., and Bums, K. N . 1968. N.Z . V e t . J . 16,109-128.
Allcroft, W. M., and Green, H. H. 1938.J. Comp. Parhol. Therap. 51,176-191.
Anderson, G . C., Jencks, E. M., and Horvath, D. J., eds. 1959. “Magnesium and Agriculture.” West Virginia University, Morgantown, West Virginia.