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VIII. Prevention of Grass Tetany

VIII. Prevention of Grass Tetany

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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

Grunes, 1968).

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.




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



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 %

treated pastures.

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:



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



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-



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.

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VIII. Prevention of Grass Tetany

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