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VI. Animal Factors in Hypomagnesemia

VI. Animal Factors in Hypomagnesemia

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GRASS TETANY OF RUMINANTS



355



traction, and oxidative phosphorylation. Therefore, he believes that the

function of Mg probably extends to all of these enzymes.

In a review, Aikawa ( 1963, pp. 49-52) illustrated that in carbohydrate

metabolism, most of the enzymes along the glycolytic pathway to pyruvate require Mg at least in vitro. The initial oxidation of pyruvic acid requires Mg. In the tricarboxylic acid cycle, Mg is necessary for converting

a-ketoglutaric acid to succinic acid.

Aikawa also reported that, in lipid metabolism, transformation of pyruvic acid to acetyl-CoA and cholic acid to cholyl-CoA both are dependent

on Mg. He wrote that in vitro, the synthesis of DNA and RNA is inhibited by lack of Mg, and Mg activates all the enzymes that catalyze

the transfer of phosphate from ATP to a phosphate receptor or from a

phosphorylated compound to ADP. In vitro, many of these enzymes may

also be activated by Mn.

Wacker (1965) reported that the activity of the ATPases in vitro was

usually maximal when the concentrations of Mg and ATP were equimolar. Magnesium is known to form a 1 : 1 complex with ATP in the physiological pH range. This has been interpreted to indicate that either a Mg

ATP complex is the active substrate or that a Mg enzyme complex is the

active catalyst in phosphorylation.

Skou ( 1 965) proposed an enzyme system that functions in the active

transport of Na and K across cell membranes. The enzyme or enzymes

involved break down ATP by a two-step reaction. The first step requires

Na and Mg for maximum rate of transfer of the energy-rich phosphate

bond to a compound in the system. The second step is a dephosphorylation due to the effect of K. The optimum Mg-ATP ratio has been found

to be 2: I , for this enzyme system, when Mg, Na, and K are in the medium.

In a review article concerning tetany in cattle, Mershon indicated

(1964) that the four major cations of physiological importance (Na, K,

Ca, and Mg) play a role in nerve impulse transmission. He suggested the

physiologically maintained concentration gradient of K and N a propagates

the electrical potential at the membrane surface, and Ca and Mg stabilize the membrane. Further, the normal impulse transmission depends

on the antagonistic effects of Ca and Mg ions on the synthesis and release

of acetylcholine (ACH). At peripheral nerve endings ACH is formed,

stored, released, and hydrolyzed whenever a nerve impulse is transmitted. When Ca ions are relatively less depleted than Mg ions in tetany, the

rate of spontaneous ACH release increases. Tetany results, then, from

the increased irritability and spontaneous discharge of the sensory, motor,

and muscle fibers. Cells become more irritable as they more readily admit

Na ions or lose K ions. When the sum of Ca and Mg ions is reduced, the



356



D. L.



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



integrity of the semipermeable membrane is lost, allowing the above

leakage. Mershon (1964) suggested the following unique biochemical

lesion of tetany : an electrolyte imbalance that permits spontaneous peripheral generation of nerve impulses. He presented a table recalculated

from Harbaugh and Dennis (1947) suggesting that Ca:K ratios are important factors in the physical response of the animal. He also implicated

low P levels, with tetany induced by a limiting of the ATP phosphorylation system, that is, a failure to maintain the last step in the energy-transfer system.



B. TYPES

OF HYPOMAGNESEMIA

Underwood ( 1966) indicated that the hypomagnesemia that develops

in calves on milk diets, or on low-Mg rations or pastures, appears to be

a dietary deficiency of Mg. It develops gradually over a period of weeks,

and is accompanied by a fall in bone Mg to as low as 30-60% below normal. He also indicated that some of the slowly developing seasonal hypomagnesemia that develops in cows, as well as the tetany that occurs in

Norwegian cows maintained for long periods on rations of fodder cellulose, may also be attributed to a dietary deficiency of Mg, complicated

in the latter case by other factors such as undernutrition. He indicated

that the hypomagnesemia with tetany that arises suddenly in cows and

ewes turned out to spring grass cannot be considered a simple dietary

Mg deficiency because it is too rapid, it can occur on pastures or rations

of normal or even high Mg content, and it is not accompanied by any

gross fall in skeletal Mg. He emphasized, however, that several dietary

factors, including the level of Mg, exert a profound influence on the incidence of grass tetany.

C. TYPEOF ANIMALFEED

G. K. Davis (1965) stated that one of the peculiarities of the whole

picture of hypomagnesemic tetany has been that chemical analysis of the

forage has usually shown Mg to be adequate for nutritional requirements.

Why, then, is the Mg not available or utilized by the livestock? The problem is really one of Mg availability to the animal from the plant, and then

utilization within the animal after absorption from the intestinal tract.

Rook and Storry (1962) reported that ruminants on diets consisting mainly of roughage and succulent feeds absorbed only 5-30% of the Mg ingested. They presented evidence suggesting that Mg was less available

than average in young grass cut early in the season, especially if N and

K were high.



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357



Considerable attention has been given to Mg availability as related to

the chemical composition of the feedstuff. Kemp ( I 958) showed hypomagnesemia and tetany in a field trial to be positively related to high rates

of K fertilization, with a superimposed (less drastic) effect from high N

fertilization. G . W. Butler and Metson ( 1967) compared forages from

tetany and nontetany pastures. They found that tetany pastures contained

lower Mg, Ca, Na, crude fiber, and possibly soluble carbohydrates, while

having higher concentrations of N and K. No generalization could be

made as to critical levels, however.



D. NITROGEN

As indicated in Section V,D, Kemp et af. (1966) indicated that the concentration of higher fatty acids increased with the N concentration in the

forage. They also indicated that additions of higher fatty acids to winter

rations of cattle decreased the availability of Mg to the animals. Lomba

et af. ( 1968) and Paquay et af. ( 1968) conducted a comprehensive feeding

trial on 162 dairy cows and 55 dietary rations. They showed that.Mg absorption was enhanced by increasing Mg and Ca intake. However, N and,

to a lesser extent fat, gave higher fecal losses. They found no significant

correlation between K intake and the fate of dietary Mg.

It is often mentioned that Mg in the ruminant gut may be precipitated

as Mg ammonium phosphate and is therefore unavailable for absorption.

Phosphate-dependent precipitation of Mg in the ileal digesta of the ruminant calf has been demonstrated (Smith and McAllan, 1966). Further

experiments (Smith and McAllan, 1967) indicated that increasing the

NH, concentration increased the precipitation of Mg when phosphate,

but no Ca, was added. They concluded that precipitation of Mg with

phosphate would be appreciable only in the most distal portion of the

ileum where the pH is above about 6.5-7.0, and then only with fairly

high phosphate concentrations.

L’Estrange et al. (1967) found no association between serum Mg concentrations and high rates of N fertilization, the milk yield, or energy

intake of lactating ewes. The N fertilization more than tripled the estimated yield of herbage from April to July and increased the percentage

of nitrate N in the forage, but did not depress either the Mg or Ca concentration of the herbage. No clinical grass tetany developed, although

some of the ewes became hypomagnesemic, especially during the first

3 weeks of the experiment. The hypomagnesemia was independent of the

fertilizer treatment. Serum Ca concentrations (with one exception) remained normal throughout the trial and also were not affected by the N

treatment.



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D. L. GRUNES, P. R. STOUT, A N D I. R. BROWNELL



E. ORGANIC

ACIDS

As indicated in Section V,F, Burt and Thomas (1961) showed that

adding sodium citrate to the diet of Ayrshire heifers for 8 weeks reduced

serum Mg below values obtained for cattle on a diet containing NaCI.

The level of Na was the same in both diets. Hirst and Ramstad (1957)

reported citric acid levels of 0.4-0.7% in fresh perennial ryegrass. Jones

and Barnes ( 1 967) reported a significant positive correlation between concentrations of nonvolatile organic acids in grasses and “preference rankings” (a visual estimation of animal grazing preference). The inference

was that animals tended to select those grasses with higher organic acid

concentrations. However, a higher correlation was found between “preference ranking” and total water-soluble dry matter.

Burau and Stout ( 1965) and Stout et al. ( 1 967) reported on trans-aconitate in range forage species. Forty-seven percent of the grasses studied,

and 17% of the nongrass species studied, contained higher than 1 % transaconitate. They suggested that aconitate and citrate could chelate Ca and

Mg, and might thus affect the normal transference pathways of Ca and

Mg within the animal. J . F. Hodgson (U. S. Plant, Soil and Nutrition

Laboratory, Ithaca, New York, unpublished data) found that citric acid

complexed Ca more strongly than did trans-aconitic acid. As indicated

in Section lll,C, Bohman et al. (1969) induced visible symptoms of grass

tetany in cattle by oral administration of KCI and either trans-aconitic

acid or citric acid. It was also mentioned that Scotto et al. ( 1 969) found

that adding KCI and citric acid increased the level of citrate in blood

plasma of cattle. High levels of citrate or aconitate in blood may be complexing Mg, and thus rendering the Mg less available to animals.

Camp et al. (1968) dosed sheep with K trans-aconitate and trunsaconitic acid. They then assayed serum samples for pyruvic acid, lactic

acid, citric acid, Na, K, Ca, Mg, P, globulin, albumin, total protein, and

urea-N. The most significant changes observed were an increase in organic P and progressive decrease in serum Mg. The trans-aconitic ion

was toxic to sheep; however, the mode of action of this substance could

not be attributed to any specific biochemical constituent assayed in that

investigation. They indicated that it was not possible to conclude that

trans-aconitate is the most common or essential etiologic factor for all

forms of the entire grass tetany complex.

Kennedy ( 1 968) indicated that sheep fed trans-aconitate maintained

normal levels of blood citrate, ketones, and aconitate, but showed large

increases in urinary citrate. The aconitate additions did not substantially

change the concentration of Mg and Ca in blood plasma and serum. Intravenously injected Na trans-aconitate increased citrate concentrations



GRASS TETANY OF RUMINANTS



359



in blood and urine. Plasma Ca and Mg concentrations were not depressed

by intravenous trans-aconitate administration, but urinary Ca excretion

increased and urinary Mg decreased.



F. ENERGY

BALANCEOF



THE



ANI M AL



Underwood ( 1966) stated that evidence from Norway and New Zealand indicates that the energy balance of the animal is a further complicating factor in hypomagnesemic tetany. The utilization of Mg is decreased in animals on low-energy rations. This raises the dietary requirement for Mg, and makes it more difficult to maintain serum Mg values.



G. LACTATION

A N D PREGNANCY

Underwood ( 1966) stated: “Hypomagnesaemic tetany is most common

in older milking cows in the first 10 weeks after calving, although it can

occur at any stage of lactation in animals of any age, including suckling

calves 3 to 6 months old. All European breeds of cattle, whether dairy

or beef breeds, are susceptible and the disease is not restricted to females.

In ewes the highest incidence occurs in those with twin lambs, during the

first month after parturition.”

Hjerpe (1964) indicated that during a major outbreak in California in

which 4000-6000 head were lost, generally only pregnant and lactating

female cattle were affected. However, there was one confirmed case of a

bull with grass tetany, and 12-15 cases of tetany in steers. The disease

appears to be secondarily sex-linked, being related to Mg demands for

lactation and pregnancy. Thomas ( 1 965) concluded that the most likely

mechanism causing grass tetany is a decrease in Mg intake (from low Mg

concentrations in the feed) and/or low forage intake coupled with comparatively high Mg requirements. He believes that the tetany manifestation probably results from ionic imbalances within body tissues that influence the enzymatic activity of ion transport mechanisms and muscle

response.

Blaxter and McGill ( 1956) presented a table (Table VI) calculated from

the data of other British workers. The greatest incidence of hypomagnesemic tetany occurred in cows which had had their third or fourth calf.

However, one needs to consider the age structure of the herds. In the last

column of Table VI, it can be seen that animals which had had six calves

were 15.2 times as susceptible to tetany as those which had had only one

calf. No breed differences were apparent, and both beef cows and dairy

cows were affected. In West Virginia, Horvath (1959) indicated that grass

tetany generally occurred in beef and dairy cows that had calved I to 3



3 60



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

TABLE V I

Effect of Number of Lactations on the Susceptibility

of Cows to Grass Tetany"



Age

calculated

lactations



Incidence of

hypomagnesemia

(% of total calved cows)



1

2

3

4

5

6

More than 6

Total



4.1

8.9

20.9

23.6

14.0

13.4

15.1

100.0



in



Relative

susceptibility

to tetany

(1st lactation = 1)

1 .o



2.3

5.Y

10.6

10.1

15.2

14.0



From Blaxter and McGill (1956).



weeks previously. The tetany generally occurred the second time they

had calved and often the fourth or fifth.

Hjerpe ( 1 967) considered hypomagnesemia to occur when Mg absorption from intake was exceeded by Mg losses in milk, feces, and urine.

As already stated (Section III,C) when the serum Mg level falls below

the renal threshold, Mg losses in urine fall essentially to zero. He presented a diagram showing total Mg storage in an animal and indicating

relative losses through feces, lactation, and urine.

H. ENDOCRINE

FACTORS

Allcroft and Burns (1968) suggested that hormonal effects may be

involved in grass tetany. The increased incidence of grass tetany during

cold, wet, windy weather may be associated with changes in thyroid activity. Thyroid, parathyroid, and adrenal hormones may have regulatory

effects on absorption of Mg from alimentary tract, and on plasma Mg

concentration (Care et al., 1967).

Wilson (1964) suggested that a controlling factor may be wide variations in the amount of secretion of endogenous Mg into the digestive

juices. He speculated that spring herbage may have some pharmacologically active component that would cause an increase in the endogenous

excretion of Mg. He reported that aldosterone has been found to inhibit

the uptake of Mg by cells of the small intestine, and that aldosterone production was increased in response to high-K and low-Na intake. Therefore,

a high-K, low-Na ratio would tend to decrease Mg uptake. R. Allcroft and



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361



Burns (1968) and Dobson et al. (1966) indicated that secretion of aldosterone by the adrenal gland may decrease the availability of dietary Mg.

However, Dobson et al. ( 1 966) suggested that increased secretion of

aldosterone probably has only a minor role in the production of hypomagnesemia.

R. Allcroft and Burns (1968) indicated that the onset of estrus can precipitate clinical symptoms in cattle in which some degree of hypomagnesemia is already present. However, the influence of steroid hormones

on the metabolism of Mg and Ca is not yet known.

I. MISCELLANEOUS

FACTORS

I N STUDIES

WITH SHEEP



Care et al. ( 1967) studied the effects of Na, K energy, and of bacterial

activity on the absorption of Mg from the mid-ileum of sheep. Absorption

of asMg from the gastrointestinal tract was much less on a diet of mowed

spring grass than on hay. This was not merely the consequence of bound

Mg in spring grass. The Mg concentration of the supernatant liquid from

duodenal digesta (obtained by centrifuging at 30,000 g for one-half hour)

was 16% higher with grass diet than with hay. Bound Mg, then, was meant

to be the Mg that was removed by centrifuging at 30,000 g. Magnesium

determined in the supernatant would include ionic as well as that chelated

and that in the form of ion pairs, such as that associated with sulfate or

phosphate, but still essentially in the solution.

Factors that may affect the absorption of Mg from the mid-ileum were

studied in conscious sheep with a Thiry-Vella loop. Magnesium absorption increased with either Na concentration or osmotic pressure within

the loop, but decreased with an increase in K concentrations. To test the

theory that bacterial absorption of Mg reduced the amount available for

absorption by the sheep, chloramphenicol was administered orally, greatly reducing the bacterial population in the mid-ileum, but this did not prevent acute hypomagnesemia.

J. VARIATION

AMONG ANIMALS



Kemp (1958) followed serum Mg levels in 16 animals throughout the

tetany season. At any given time, the levels in the group varied over a

considerable range. This variation was greatest when the group mean was

somewhere in the midrange, 16-20 ppm of Mg. The variability decreased

as the group mean either increased or decreased from this point. He also

stated that “it was quite obvious, however, that there were individual

differences between the serum Mg contents, and some interesting information comes to light when these contents are studied over an entire



362



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



year, for instance, in the case of a number of individual animals. It is then

found that serum Mg contents of 2 animals over an entire season rise and

fall fairly simultaneously but on a different level.”

In California, Stout, Brownell, and co-workers have used this observation to select animals for field tetany research, selecting animals from the

low portion of an entire herd. However, even within a group selected in

this manner, individuals were found to differ and to have very consistently

followed this same pattern. That is, the animal that was high at any given

time was likely to be found high at any other sampling time. This is based

on four groups of four animals sampled 28 times during the tetany season.

Independent of the supplemental feed treatment, animals within groups

maintained relative rank based on serum Mg level.

Kemp et al. (1961), summarizing several experiments, stated “that all

cows with serum Mg levels ranging from .5 to 1.8 mg./ 100 ml. (5- 18 ppm)

showed very low percentages of Mg not excreted in the feces (7-14%).

Other cows in the same experiments, receiving the same herbage, showed

normal serum Mg concentrations ranging from 2. I to 2.75 mg./ 100 ml.

(2 I to 27.5 ppm) and higher percentages of Mg not excreted in the feces

( 1 1-23%).” This is in agreement with the general assumption that some

cows are more susceptible to hypomagnesemia than others, although

differences in Mg intake and bone mobilization may play a role and were

not evaluated in the experiments summarized here.

G. W.Butler and Metson (1967) stated that the most consistent blood

chemical abnormality associated with grass staggers (grass tetany) was

low blood Mg. The fact remains, however, that low Mg levels are also

quite common in unaffected cows. It may be that much of the individual

variation in blood Mg level is genetically inherited, and that selected

breeding might reduce proneness to grass tetany. In a trial comparing

identical twin lactating cows, it was not possible to incriminate a particular nervous or hormonal mechanism to explain the variation in blood Mg

levels. The sudden failure or inadequacy of the regulatory mechanism involving Mg, or the significant effect of the negative energy balance on Mg

requirements, could not be explained in physiological or biochemical

terms. However, Care et al. ( 1 967) have suggested that much of the wide

individual variation in the reaction of different animals to the factors

causing hypomagnesemia may be ascribed to endocrine influences.

Smith ( I 963), in studying Mg absorption by milk-fed calves, considered

intestinal transit time to explain the variations in efficiency of Mg absorption. The regression line of absorption against transit time through the

distal ileum showed a mean increase of 8.5% Mg intake for each extra

hour of transit time.



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363



VII. Treatment of Affected Animals



A detailed discussion of treatment of affected animals is presented by



R. Allcroft and Burns ( 1 968). The usual treatments involve administration of Mg and Ca compounds. If the convulsions and tetany are severe,

sedatives may also be administered. In the United States, commercial

solutions often used for intravenous injections contain Ca and Mg gluconate with added dextrose and phosphate. R. Allcroft and Burns ( 1968)

indicated that intravenous hiections should be given slowly, and the response watched carefully, because of the danger of death from cardiac

arrest. They mentioned that Mg lactate is considered safer than Mg sulfate, but even so must be administered with care. They stated that intravenous Ca injections are sometimes combined with subcutaneous Mg

injections.

Unless the animal is removed from the tetany-producing pasture and

fed hay and concentrates, the blood Mg level is likely to become dangerously low 24-37 hours after the intravenous or subcutaneous injection.

R. Allcroft and Burns ( 1 968) suggested that follow-up treatment should

consist of daily oral administration of about 30 g of Mg (2 oz calcined

magnesite, containing 85% MgO). After a week, the oral dose can be

gradually reduced. They indicated that absorption of Mg by animals has

been shown to be better from the oxide than from sulfate or carbonate.

VIII. Prevention of Grass Tetany



A. FERTILIZATION



As was discussed in Section IV, Mg fertilization is recommended for

good plant growth when the exchangeable Mg is less than 10% of the

cation exchange capacity of the soil, or less than 100 Ib of exchangeable Mg per acre. The latter figure is about 0.41 meq per 100 g of soil,

or 5.0 mg of Mg per 100 g of soil. When cattle are grazed on pastures,

higher values for soil Mg are advisable. For grazing animals in West

Virginia, it has been recommended (Horvath and Todd, 1968) that soil

Mg equal at least twice the percent saturation of K , and that the Ca:Mg

ratio not be higher than 5 : 1.

There has been appreciable research relating Mg fertilization to concentrations of Mg necessary for good plant growth. Magnesium deficiency

symptoms of plants frequently occur at 0.05-0.10% Mg, although some

higher values for Mg-deficient plants are also found in the literature (Embleton, 1966). However, as discussed earlier, cattle require Mg concentrations of 0.20% Mg or more in the forage.



364



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

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.



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