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II. Incidence, Climate, and Season

II. Incidence, Climate, and Season

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333



GRASS TETANY OF RUMINANTS



when the mean daily temperature was below 14°C (57°F). In addition,

there were short-term fluctuations. They indicated that 5 days after the

mean daily temperature rose above 14"C, the number of cases of grass

tetany increased. They also indicated that about 5 days after a fall in

temperature, the number of cases of tetany decreased. Figure I indicates

the distribution of tetany cases in 1952.

A v e r a y e screen temp ("C



I



1



1



Precipilation



Precipitation



1



Number of g r a s s tetany cases



40~



0

March



April



May



June



July



Aug



Sept



OCt



Nov



FIG. I . The relationship between incidence of grass tetany and temperature and precipitation. Values for incidence of grass tetany and temperature are averages for a 5-day

period. Percipitation is the amount received daily. (From Kemp and 't Hart, 1957.)



In the Netherlands, 't Hart (1960) indicated that during cold, wet

years, tetany cases occurred during the whole summer. When incidence

during the entire year was considered, 95% of the cases occurred when

the mean temperature was between 8°C and 14°C (46OF-57"F). He

also indicated that in other parts of the world the limits were reported

as between 5°C and 15°C (4O0F-60"F). This includes "wheat pasture

poisoning" in Oklahoma, tetany on winter grazing of oats in Argentina,

and grass tetany in New Zealand. 't Hart ( 1 960) concluded .that hypomagnesemia occurred mainly when temperature was rather low, and

other conditions were favorable for grass growth. The frequency of

tetany was much higher in years with a sudden, quick growth of grass

in spring than in years with poor growth. 't Hart ( I 960) indicated that



334



D. L . GRUNES, P. R. STOUT, AND J . R. BROWNELL



Inglis (1960)had found a decrease in incidence of tetany when frost

occurred.

't Hart (I 960) stated that the disease occurred only under conditions

of ample moisture supply, and that spring tetany was less frequent on

very dry and very wet pastures. Large numbers of cases of autumn tetany

occurred only in wet seasons.

The dry matter content of pasture herbage is often less than 15% in

wet weather. Nitrogen fertilization may decrease the dry matter content

of the forage. If grass or small grains forages are low in Mg, animals may

be unable to obtain their Mg requirenTent from damp and succulent grass.

Van der Molen (1964)reported that 2.7% of all dairy cows in the Netherlands were affected with grass tetany between May of 1960 and May

of 1961,including 7845 fatal cases. Leech et al. (1960)reported that in

Great Britain mortality was as high as 30% of clinical cases. Meyer ( 1960)

reported that heifers and cows in Germany often had hypomagnesemic

tetany, especially on being turned out in the spring, and that calves kept

on a pure milk diet beyond the fourth week often became hypomagnesemic. He found no reports of tetany in sheep in Germany.

R. Allcroft and Burns (1968)reported that 0.5% of all dairy cows in

Great Britain, 1.1% of all dairy and beef animals in Scotland, and I-2%

of dairy animals in the Netherlands had a clinical level of grass tetany

each year. In a Scottish study (E. J . Butler et al., 1963),grass tetany was

found in 25% of 103 herds surveyed, the mean incidence in the affected

herds being 4.5 -+ 0.65%. Overall incidence was 1.1 k 0.25%, and was

significantly lower when a Mg supplement was given. After clinically

normal cows were turned out to pasture, hypomagnesemia was detected

in 41% of the herds, with an overall incidence of 8.7 -+ 1%. Incidence

with clinical signs in individual herds was as high as 10.9%; mortality of

those showing clinical signs was as high as 30%.

Stewart ( I 954),using a different approach, defined three types of tetany

in Scotland: (1)spring type, a well defined seasonal disease occurring only

in cows with calves in late April or early May, within a few days of putting the animals on grass: (2) winter type, occurring in cattle being fed

winter rations in stalls, especially on grass silage or dry grass hay, usually

occurring in November or December, and characterized by sudden death

without warning: and (3)outwinter type, in cattle maintained on pastures throughout the winter, occurring most commonly in February and

March in animals on sparse forage and pastures. Many losses have occurred in herds receiving hay in addition to forage.

Herd et al. (1964)in Victoria. Australia reported that grass tetany affected beef and dairy cattle and ewes after lambing. The mortality rates



GRASS TETANY OF RUMINANTS



335



were estimated at I % in the beef cattle and ewe populations, and somewhat less in dairy populations. Cairney ( 1964) found in New Zealand that

incidence of presumed grass tetany averaged between 0.2% and 3.9% in

the 177 herds studied. Wet, cold, and stormy weather conditions were

associated with many animal losses. Voisin ( 1 963) discussed grass tetany

in France, Scandinavia, and Ireland.

Grass tetany has also been reported in various parts of the United

States. The publication edited by Anderson et al. (1959) contains a number of articles concerning grass tetany. Included is a report by Horvath

(1959) indicating that tetany in West Virginia occurred primarily with

mature beef cows, although some younger stock, some dairy cows,

and a few sheep were also affected. The cattle were generally fed orchard grass (Dactylis glomeruta) as pasture or hay. Attacks generally

occurred from November to January, just after calving. Some cases

occurred in February through April in cows about to calve, or which had

recently calved. Some cases occurred in late March or April on grass

which had recently turned green. Other cases occurred in April or early

May when cows were placed on “lush” pasture.

Leffel and Mason ( 1 959) indicated that hypomagnesemic tetany often

occurred in beef cattle on winter rations in western Maryland. These

animals were often fed hay composed of either timothy (Phleum prutense)

or timothy and legumes. Often the hay contained less than 0. I % Mg.

Singer et al. ( 1 958) reported on a complicated grass tetany syndrome

in Kentucky. All animals were in poor condition, and six died from apparent grass tetany. The animals were hypomagnesemic, hypocalcemic,

hypocupremic, and had high levels of K, P, nonprotein N, and urea N in

the blood serum.

Miller ( 1965) indicated that grass tetany has been a major nutritional

disease of the beef cattle industry in Georgia in recent years. The typical

problem involves a 5- to 7-year-old cow nursing a calf on a small grains

pasture. Cases generally occur during the months of December to March.

Recently, a number of cases of grass tetany have been reported with

beef cattle grazing crested wheatgrass (Agropyron desertorurn) pastures

during the spring in Nevada, Idaho, and Utah.

Hughes and Cornelius (1960) reported on grass tetany in lactating

beef cattle in California in the late 1950’s.Hjerpe ( I 964) reported a major

outbreak of grass tetany in the winter of 1963-1964 in California, with

losses estimated at 4000-6000 head. He reported that sporadic losses

from grass tetany occurred annually in California, usually during the

winter or early spring. Although deaths in affected herds were rarely as

high as 5%, they reportedly reached 20% in some herds in the 1963- 1964



336



D. L.



GRUNES, P. R.



STOUT, A N D 1. R. BROWNELL



outbreak. R. B. Bushnell (Department of Epidemiology and Preventive

Medicine, University of California, Davis, unpublished data) reported on

738 investigated losses during this same period in California. He showed

losses increasing with the duration of cloudy, foggy, or rainy weather

which occurred following the early growth of grass during October.

Losses increased during January, following the cooler temperatures that

prevailed in December.

As reported by E. J. Butler et al. (1963), hypomagnesemia is frequently

found in apparently normal animals lacking clinical signs of grass tetany.

A California study (Brownell and Bushnell, unpublished data) of a normal

herd of 444 animals showed a mean serum Mg level of 19 ppm, with a

standard deviation of -t 0.35 ppm. The range in serum values, however,

was from 7.2 to 34.8 ppm, with 2.5% of the animals below 10 ppm, and

34.2% below 18 ppm. This herd had been subdivided into six different

management groups, the principal differences being: ( 1 ) the pasture they

had been grazing; (2) whether or not supplemental feed had been provided. In these management groups, one group of 180 animals had a mean

serum Mg level of 16 ppm and had been on dry range pasture. The group

with the highest serum Mg (mean = 24.4 ppm) was composed of 17 animals which had been receiving intensive supplemental feedings in the

form of alfalfa (Medicago sativa) hay. This herd was not sampled during

the normal grass tetany period, but a t the end of California’s long, dry

summer, when grass tetany is usually not present. Brownell and Bushnell

(unpublished data) found that independent of the time of sampling, 10%

of the animals were hypomagnesemic, but appeared normal. This is based

on sampling 18 herds ranging in size from 18 to 444 animals.

Crookshank and Sims (1955), Sims and Crookshank (1956), and R. E.

Davis and Crookshank (1956) reported on wheat pasture poisoning,

which occurs primarily in sexually mature cows in the late stages of pregnancy or with a calf. In the Texas and Oklahoma panhandles, most cases

developed sometime between 60 and 150 days after the start of grazing

on winter wheat, and in cows with calves under 60 days old.

111. Symptoms of Animals



A. CLINICAL



Underwood ( 1966) gave the following description of the clinical signs

of grass tetany: “The initial signs are those of nervous apprehension,

with ears pricked, head held high and staring eyes. At this stage the animal’s movements are stiff and stilted, it staggers when walking and there

is a twitching of the muscles, especially of the face and ears. Within a

few hours or days of the initial signs, extreme excitement and violent



GRASS TETANY OF RUMINANTS



337



convulsions develop; the animal lies flat on its side; the forelegs ‘pedal’

periodically and the jaws work, making the teeth grate. If treatment is

not given at this stage death usually occurs during or after one of the convulsions or the animal may pass into a coma and die. Mortality is high in

clinical cases without treatment.

“A chronic form of hypomagnesemia, characterized by a stiff gait and

gradual loss of condition, can occur for several weeks without affecting

appetite or milk yield. This disturbance is followed either by recovery or

by the acute form of tetany, as just described. Preconvulsive clinical

signs in sheep are less clearly defined than in cattle and can easily be

confused with those of hypocalcaemia or pregnancy toxaemia.”

Barker (see R. Allcroft and Burns, 1968) subdivided cases into two

groups: hypomagnesemia accompanied by hypocalcemia; and hypomagnesemia accompanied by normal serum Ca. R. Allcroft and Burns ( 1968)

concluded that 76% of those with hypomagnesemia also had hypocalcemia, and that 36% of these cases developed within 4 days of calving.

This syndrome, with both serum Mg and Ca low, was called preconvulsive tetany by Barker (1939). The related symptoms include tetany of

forelegs and hind legs, hyperesthesia (unusual sensitivity), of which one

symptom is fluttering of the eyelids, and convulsions that may be very

sudden in onset. Type two (hypomagnesemia accompanied by normal

serum Ca) is further subdivided into two distinct clinical categories:

slight symptoms (only slightly nervous, some twitching, but not convulsions); and acute severe symptoms (including the most dramatic examples

of hypomagnesemic tetany, with tetanic convulsive cases often occurring

suddenly and the animal dying within minutes). Swan and Jamieson ( 1956)

reported twelve such acute cases of grass tetany. Rook (1 963) described

such a case in an experimental cow which developed very acute tetany

and convulsions and died in about 10 minutes.

R. E. Davis ( 1 959) indicated that the symptoms of “wheat pasture

poisoning” observed in the Texas and Oklahoma panhandles are almost

identical with those of grass tetany in the Netherlands, Great Britain,

and New Zealand. He wrote: “The first symptoms are usually excitement,

incoordination, and loss of appetite. Viciousness, staggering, and falling

come later. Nervousness becomes more apparent with muscular twitching, which is particularly noticeable in the extremities. The animal often

grinds its teeth and salivates profusely. The third eyelid will protrude or

flicker as in tetanus.

“General tetanic contractions of the muscles follow until the animal

nears a state of prostration. A sudden noise or touching the animal causes

a reflex response often resulting in clonic-tonic convulsions. Some muscles are in tetanic contraction.



338



D. L.



GRUNES, P. R. STOUT, A N D J. R. BROWNELL



“The next symptoms are labored breathing and a pounding heart. Convulsions with periods of relaxation occur. During these periods of relaxation the animal attempts to get up and if successful will run into obstructions without attempting to avoid them. Following the period of extreme

excitement the animal goes into a comatose condition. The animal usually

becomes comatose 6 to 10 hours after the onset of the initial symptoms.

If treatment is started before the coma stage begins the chances of recovery are good. Very few animals survive if treatment is started when

the animal is in the comatose state.”

The diagnosis of grass tetany by clinical symptoms alone is difficult.

A more positive diagnosis can be made when the level of Mg in the blood

is also known.

B. CHEMICAL

Sjollema ( 1930), in a series of papers, described grass tetany as a hypomagnesemic disorder. Normal animals in his study had 13-20 ppm of

Mg in blood serum, whereas animals with tetany had serum Mg contents

of 5-10 ppm. He also observed that the serum Ca:Mg ratio varied considerably, normal animals having a ratio of 5.6; tetany animals a ratio of

14.6; and animals with “milk disease,” probably what is now known as

milk fever, a ratio of 2. Meyer (1960) classified animals as hypermagnesemic if they had greater than 32 ppm of Mg in their serum; normal

from 18-32; slightly hypomagnesemic 12- 18; severely hypomagnesemic

less than 12. Storry and Rook (1963) in an experimental feeding trial

showed that animals with a Mg-sufficient diet had serum Mg levels of

20-27 ppm, and that animals approaching tetany had Mg levels of 10- I5

ppm. In the same study, urine losses of Mg ranged from I to 2 g per day

on the sufficient diet, and decreased to 0 when the blood serum Mg fell

below 20 pprn. At the same time, Mg lost in the feces was about 2 g

per day on the sufficient diet and fell steadily to 1 g per day when the animals were placed on a deficient diet.

Crookshank and Sims ( I 9 5 9 , in a study of wheat pasture poisoning in

Texas, established normal serum levels for 185 Hereford cows as being

20.5 k 2.5 ppm for Mg; 1 10.8 f6.7 ppm for Ca. Sixty animals with wheat

pasture poisoning had an average serum Mg level of 13.5 ? 7.5 ppm, and

Ca levels of 66.8 & 11.2 ppm. Potassium levels appeared to be elevated

but quite variable, the normal mean being 197 -1- 22 ppm K, and affected

animals being 234 ? 83 ppm K. The affected animals were much more

variable than the normals, 10% being greater than 400 ppm K. Serum

albumin was slightly depressed and serum globulin greatly increased in



GRASS TETANY OF RUMINANTS



339



the affected animals as compared to the normals, giving a very significant

depression in the a1bumin:globulin ratio. They concluded that the wide

range of values observed in many of the constituents measured indicated

effects of the wheat pasture poisoning rather than the cause.

W. M. Allcroft and Green ( 1938) indicated that, under certain conditions, cattle may tolerate extremely low blood serum Mg levels without

showing the clinical symptoms of grass tetany.

Cow’s milk normally contains approximately 120 ppm Mg (Meyer,

1960). Rook and Storry (1962) quote a normal range of 70-180 ppm.

They and others (Cunningham, 1933; de Groot, 1959) present evidence

that the Mg content of milk does not decline if feed or Mg intake is reduced or if hypomagnesemia occurs. T. H. Arkley (unpublished data,

1964) found in California that regardless of whether samples were taken

under good or poor conditions, the Mg concentration of the milk of range

animals was always between 100 and 120 ppm and seemed unrelated to

serum Mg content.

As noted above, urine Mg concentrations dropped very low for Mgdeficient animals (Storry and Rook, 1963). The kidneys function to eliminate Mg whenever serum levels are above the renal threshold of about 18

ppm Mg. Above the renal threshold, the concentrations of Mg in the

blood plasma and Mg excreted in the urine are linearly related. When

the plasma Mg level falls below the renal threshold, reabsorption in the

kidney exceeds excretion and essentially no Mg is lost in the urine.



c.



INDUCTION OF



TETANY

SYMPTOMS



In the past, it has been a problem to study grass tetany, because of the

difficulty of artificially inducing the disease in animals. However, recently

there appears to be progress in this direction.

Bohman et al. ( I 969) induced symptoms, resembling field cases of grass

tetany, in cattle by administering KCI and either trans-aconitic acid or

citric acid. The materials were suspended in water and administered by

rumen tube within a 5-minute period. When one of the acids or KCl was

administered alone, the symptoms of tetany were not induced. Administration of both citric acid and KCl reduced the concentration of Mg

measured in the blood plasma 24 hours later. Further work, Scotto et al.

(1969), indicated that the level of citric acid in the blood increased as

the amount of KCI added with the citric acid was increased. It is possible

that the citric and trans-aconitic acid were complexing Mg, thus reducing

the availability to the cattle.

Suttle and Field (1969) induced grass tetany symptoms in sheep by



3 40



D. L. GRUNES, P. R. STOUT,AND J. R. BROWNELL



decreasing the dietary intake of Mg, while increasing the intake of K.

The Mg concentrations in the blood plasma were reduced by increasing

the K intake, as well as by reducing the Mg intake.

Ward ( 1 966) added a high rate of KCl to a cow by means of a stomach

pump; although the cow died, no tetany symptoms were observed. Geelen et al. (1966) induced tetany in an animal with a previous history of

tetany by intravenous administration of histamine. Montgomerie et al.

(1929) reported a case of tetany in Welsh mountain ponies .just after a

long railway journey. The ponies were found to be both hypocalcemic and

hypomagnesemic. Hjerpe and Brownell (1966) reported on four animals,

which were selected from a herd of 12 because of their low concentration

of Mg in the blood serum. When they were trucked 38 miles, all four animals were lower in serum Mg than was measured on the ranch 4 days

previously. The two animals with the lowest serum Mg levels developed

tetany symptoms during the trip, and serum Ca was also considerably

below that measured 4 days earlier.

IV. Soils



Magnesium in soils has been reviewed by Beeson (1959), Salmon

(1963), and Metson (1968). The relation between soil Mg and plant nutrition has been reviewed by Bould (1964). Thompson (1960) reviewed

the relationship between soil Mg and plant and animal nutrition.

Beeson ( 1 959) indicated that rocks average about 2% Mg, but the range

is extremely wide. Since soils are developed from parent rocks, soils exhibit a wide range in total Mg content. As soil development increases

under strong weathering and leaching conditions, the Mg content may

decrease. In general, acidic igneous rocks are low in Mg and more basic

rocks are high. Sedimentary rocks, especially sandstones and shales,

are generally low in Mg.

Beeson (1959) indicated that Mg deficiency in plants occurs frequently

on coarse-textured soils of the Atlantic and Gulf Coastal Plains of the

United States. However, Mg deficiency of crop plants also occurs on

finer-textured soils of the somewhat less humid Middle West (Beeson,

1959). He indicated that it is likely that continued cropping and heavy

applications of K and other non-Mg fertilizers may be responsible. Beeson (1959) also indicated that the Mg content of waters in the United

States correlates well with the occurrence of deficiencies of this element

in the soil.

Attempts to relate Mg in the soil to Mg in plants have been only moderately successful. In the United States, an extraction technique is common-



GRASS TETANY OF RUMINANTS



341



ly used to obtain exchangeable Ca and Mg (Heald, 1965). This technique

involves displacing the adsorbed ions with a concentrated salt solution

such as NH4Cl, NH4 acetate, NaCI, Na acetate, or BaClz. Some investigators have also used dilute acids such as 0.4 N HCl or 0.5 N acetic acid

(Heald, 1965). Heald recommended against determining exchangeable

Ca or Mg on soils containing free carbonates, gypsum, or excess soluble

salts.

Salmon ( 1964: Fig. 2) obtained correlations of 0.99 between the concentration of Mg in ryegrass grown in a greenhouse and a ratio involving

ion activities of Mg, Ca, and K in equilibrium soil solutions. The Mg concentrations in the plants were directly related to Mg activities in soil extracts, but inversely related to K and Ca activities in these extracts.

Magnesium concentrations in plants were also inversely related to soil

pH. A further discussion of this concept is presented by Arnold ( 1967).

In New Zealand, Metson ( I 968) indicated that consideration is being

given to assessing Mg status of the soil by including both exchangeable

Mg and Mg present after boiling the soil with normal HCl. The acidsoluble Mg is used as a measure of Mg reserves.

In England, Salmon and Arnold ( I 963) exhaustively cropped soils for

up to 1 I months in a greenhouse. Although the “exhaustion” Mg (Mg

taken up by crops plus residual exchangeable Mg) was greater than the

initial exchangeable Mg, the two measurements were highly correlated

(r = 0.99). In some of these intensively cropped soils, the exchangeable

Mg was increased by wetting and drying. If this occurs in the field, Mg

lost in cropping could be replenished by only small releases of nonexchangeable Mg.

In North Carolina, Rice and Kamprath ( 1 968) found that Mg uptake

by corn (Zea mays L.) from exchangeable Mg was closely related to the

initial exchangeable Mg in sandy coastal plain soils. However, a large

part of the Mg absorbed by plants also came from nonexchangeable

forms. They suggested that H ions exchanged from the roots may have

been effective in releasing nonexchangeable Mg.

Embleton (1966) reviewed the literature relating analyses of Mg in

soil extracts to Mg deficiency in plants. Several investigators found that

when the exchangeable Mg was less than 6% of the cation exchange capacity, a plant growth response to added Mg was likely. Embleton (1966)

indicated that Prince et al. (1947) had found in New Jersey that an ideal

amount of Mg would be up to 10% of the exchange capacity of the soil.

The Mg status of New Jersey soils is further discussed by Bear et al.

(1951).

Felbeck (1959) suggested that Mg fertilization be recommended when



342



D. L. GRUNEJ, P. R. STOUT, AND J . R. BROWNELL



the exchangeable Mg is less than 10% of the cation exchange capacity,

or less than 100 Ib of exchangeable Mg per acre. The latter figure is about

0.41 meq per 100 g of soil, assuming 2,000,000 lb per acre 6 inches of soil.

This would equal 0.5 mg of Mg per 100 g of soil.

In West Virginia, the recommendation for good plant growth is that

Mg sould equal lO-15% of the exchange capacity, or not less than twice

the exchangeable K percentage (Horvath and Todd, 1968). They also

recommended that the Ca: Mg ratio in the soil should be about 5 : I .

(Wider ratios would be expected to tend to induce Mg deficiency of plants.)

In Scotland, Reith ( 1 967) indicated that plant growth response to Mg

can be expected when there is less than 3 mg of readily soluble Mg per

100 g of soil. He extracted soils with either 2.5% acetic acid or neutral

normal ammonium acetate, the values being similar for both extractants.

For red clover growing on slightly acid, mineral soils, he recommended

a value of not less than 16 mg of Mg per 100 g of soil as a level to ensure

that the herbage content of Mg is as high as possible under practical

conditions.

All in all, it appears that much research needs to be done before soil

analyses can be used to accurately predict Mg availability to plants. As

will be discussed later, the situation is complicated by the fact that, for

some plants, low temperatures result in low concentrations of Mg in the

plants.

V. Forage



A. SPECIES

Tetany occurs most frequently on grasses accomplishing most of their

growth during cool weather, such as occurs during the spring. It often

occurs on perennial grasses such as ryegrass (Lolium perenne) in the

British Isles, the Netherlands, and New Zealand; crested wheatgrass

(Agropyron desertorurn and Agropyron cristatum) in Nevada and Idaho;

and orchardgrass (Dactylis glomerata) in West Virginia. In California,

tetany often occurs on annual grasses such as soft chess (Bromus mollis)

and mouse barley (Hordeurn leporinum).

In the Southern United States, tetany is often observed on wheat (Triticum aestivurn) (“wheat pasture poisoning”), rye (Secale cereale), and

oat (Avena sativa) forage used as green pastures.

Legumes and herbs contain higher concentrations of Mg and Ca than

grasses (see Table I). Todd (1966) also indicated that clovers have higher

concentrations of Mg than pasture grasses. However, the clovers do not

grow as early in the spring as the ryegrasses. Todd (1966) indicated that

timothy (Phleum pratense) has lower Mg concentrations than the various



GRASS TETANY OF RUMINANTS



343



TABLE I

Mineral Content of Swards in the Netherlands”

Dry matter composition

Sward constituent



% Mg



% Ca



% K



% Na



%N



Clovers

Herbs

Grasses



0.27

0.37

0.19



1.84

I .44

0.75



4.03

3.98

3.70



0.16

0.19

0.10



4.45

3.34

3.52



“Data presented by van der Molen (1964) were obtained from an investigation by P. F. J .

van Burg and G . H. Arnold in 1961.



ryegrasses (Loliumspp.) or cocksfoot (orchardgrass, Dactylis glomerata).

Nitrogen fertilization reduces the clover content of pastures and would

thus reduce the Mg concentration in the pasture forage. Todd ( 1 966) also

indicated that while herbaceous weeds have higher concentrations of Mg

than grasses, they reduce the productivity of the sward.

One way of avoiding grass tetany might be to breed or select legumes

that would start to grow as early in the spring as the grasses.



B. MINERALLEVEL

The incidence of hypomagnesemia is related to low Mg concentrations

in forage. However, grass tetany is sometimes not observed even when

Mg concentrations in the forage are low. Kemp (1960) plotted values for

Mg in blood serum against the corresponding values for Mg in herbage

for 822 dairy cows in the Netherlands. He indicated that no cases of

clinical tetany occurred at blood serum Mg values above 9 ppm or at

herbage Mg levels above 0.19%.This established 0.20%Mg as the “safe”

level for Mg in forage.

Kernp’s data also indicated that for each level of Mg in the forage, increasing the concentration of K and crude protein in the forage decreased

the level of Mg in the blood serum, and increased the likelihood of tetany.

Kemp indicated that low concentrations of Mg in the blood, as well as

frequent cases of tetany, were obtained when Mg in the herbage varied

between 0.175 and 0.200%, while K and total N averaged 3.88 and

3.79%, respectively.

Metson et al. ( 1966) indicated that on New Zealand pastures associated

with outbreaks of grass tetany in beef cattle, K concentrations in forage

averaged 3.29% (ranging from 2.0 to 4.0%), while N concentrations

averaged 5.28% (ranging from 4.2 to 6.3%). The Mg concentration in

these forages averaged 0.19% (ranging from 0.14 to 0.25%).They sug-



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