CHAPTER 26

Mineral imbalances in herbage causing tetany

	much too rich in potassium;
	extremely low in sodium;
	low in magnesium;
	low, perhaps, in copper and manganese.9

"It contains a great excess of potassium with, at the same time, a very small amount of sodium and a barely adequate amount of magnesium. In many cases it does not contain sufficient manganese and copper. This herbage just covers the requirements of calcium and phosphorus."

There is no correlation between the magnesium content of the herbage and that of the blood serum of the animal

It is logical to deal first of all with the magnesium content of the herbage, because the animal that suffers from grass tetany is hypomagnesaemic. Various workers have tried to see the possible correlation existing between the magnesium content of the herbage and that of the blood serum of the cow grazing it. The results have been variable and even so contradictory that some workers, not without reason, have tended towards the opinion that there is no correlation between the magnesium content of herbage and that of the blood serum of the cow grazing that herbage. Fig. 16, taken from KEMP, is a good illustration of these contradictory results. If the normal magnesium content of the blood serum of the cow is assumed to be 2-0 0-20 mg./100 c.c. it will be seen that such a content can exist in cows grazing herbage containing either 0-12 or 0-24% magnesium in the dry matter, the one figure being twice the other. Moreover, it has been found that, given herbage with a low magnesium content (0-12%), one may find cows with a very low and extremely dangerous magnesium content of the blood serum (0-5 mg./100 c.c.) as well as cows with a content above normal (2-7 mg./100 c.c.). It may be seen, in addition, that even with very low contents (lower than 0-7 mg./I00 c.c.) of magnesium in the blood serum, tetany does not necessarily occur.
This diagram lends expression to many findings, among them those of STEWART, who observed tetany on a pasture where the herbage had a low content of magnesium but not a single case of tetany on a neighbouring pasture ¹⁰ whose herbage contained exactly the same quantity of magnesium.

Dangerous minimum magnesium content in herbage

As has been stated frequently in the course of this book and as will be seen later in the present chapter, it is the balance between the elements, mineral or organic, that determines, above all, the biological quality of the diet. It is frequently necessary, however, for a certain absolute content of an element to be attained, whether it is a minimum or a maximum, before particular metabolic disturbances can take place, whatever the ratios of that element to the others present.
This is admirably demonstrated by Fig. 16. In fact, not one case of hypomagnesaemia ¹¹ or tetany appeared when the dry matter of the herbage contained more than 0-20% magnesium.
It is only if the content of the dry matter of the herbage is below 0-20% that there is a marked risk of hypomagnesaemia and tetany.¹² But, if it is essential for herbage to contain less than 0-20% magnesium if hypomagnesaemia and tetany are to appear, this condition is not sufficient, because, as has just been said, even although the magnesium content of the herbage is low, that of the blood serum of the cow grazing it can be normal. To recall an experiment by DE GROOT: this showed that a large supplement of potassium had no effect on the magnesium of the blood serum if the dry matter of the ration contained 0-27% magnesium, but the same supplement caused hypomagnesaemia if the ration did not contain more than 0-16% magnesium, that is, less than the safety limit of 0-20%. This limit of 0-20% must naturally only be considered as valid within the framework of KEMP'S observations.¹³ Moreover, it is total and not available magnesium that is concerned here. A practical conclusion of immense importance may nevertheless be drawn, namely, that the most effective means of ensuring protection against grass tetany is to keep the magnesium content of the dry matter of the herbage above a certain level, generally 0-20 by applying the necessary dressings of magnesium fertilizers if need be.¹⁴

Extremely low sodium content of herbage giving rise to tetany

NAUMANN'S figures agree well with the finding of most investigators, namely, that herbage causing grass tetany is extremely low in sodium.¹⁵ During an enquiry covering farms attacked by tetany in Schleswig-Holstein, BECKER, of the University of Kiel, also confirmed that the herbage on those farms was extremely low in sodium. His co-worker PETERS has found contents falling to 0-03 and 0-02% sodium in the dry matter, which means that the herbage, to all intents and purposes, contains almost no sodium at all. The result is that cattle grazing such herbage develop a "salt thirst" that makes them seek out grass on banks and at gates, lick the soil and trees, drink the urine of other cows, etc. As the Dutch worker FRENS states, sodium deficiency is one of the commonest symptoms to be observed in cows grazing "intensively managed" pasture, or, to be more exact, pasture that one imagines is "intensively" managed. Similarly, BECKER is of the opinion that this extreme deficiency of sodium is one of the main causes ¹⁶ of tetany.¹⁷

The balance of potassium in relation to calcium and magnesium

Having examined these few figures relating to absolute contents of certain macro- elements in grass, let us now turn to the problem of their balance. The ionic balance of grass, the influence of which on grass tetany has been most studied, is the ratio of potassium to calcium plus magnesium .¹⁸

Table 21: Influence of the mineral imbalance of grass on the incidence of grass tetany

calculated in milliequivalents (per kg. of dry matter). Table 21 shows that tetany is very rare when the ratio is lower than 1-80 and unknown when this ratio falls below 1-40. When the ratio exceeds 2-20, the incidence of tetany increases, to become common when the figure rises beyond 3-00. In other words, the "probability" of tetany is greater, the more marked the excess of potassium ions in relation to total calcium and magnesium ions. And this is a probability, not a certainty. It does not mean that, given a ratio of greater than 1-80, tetany is inevitable; and ALTEN has been able to quote pastures with a higher ratio than this on which not one single case of tetany occurred.
Conversely, WERNER has observed tetany on pastures where the ratio was 0.84.¹⁹

In tetany many mineral ratios enter into play

When the ratio is considered:
The antagonistic effects of the calcium and magnesium ions with regard to the potassium ion are added together, but no account is taken of the balance between the calcium and magnesium ions, which plays an extremely important role in the animal's metabolism.²⁰
The potassium : sodium ratio (K : Na) is not considered. This is generally very high in tetany pasture, as NAUMANN points out, who found mean values of 30 for the ratio, and is confirmed by other observers, such as BECKER and PETERS who, in Schleswig-Holstein, recorded K : Na ratios of as much as 100. These two workers, with good reason, considered that this high ratio must contribute towards the development of tetany.²¹ But the sodium ion has no place in the ratio.
It is known, moreover,²² that the and K : Na ratios do not necessarily vary similarly.
In conclusion, it may be said that the ratio gives expression, among other things, to one of the mineral imbalances that may help to trigger off tetany. It is an indication, but not a certainty.²³

1. On many of these farms considerable quantities of liquid manure are spread on the pastures, with a consequent considerable mineral imbalance in the herbage composition. *

2. It has been said that available quantities might be very low in relation to total quantities. As for the requirements, they are calculated on the basis of what, after all, are unfortunately very theoretical tables. The fact remains, however, that NAUMANN's figures offer a very interesting overall picture. *

3. The mean absolute contents of potassium and the elements that follow are taken from a report Of NAUMANN's dealing with the herbage for 1958. *

4. It has been said that, if grass is to meet the sodium requirements of ordinary dairy cows, it is wise that it should not contain less than 0-25% in the dry matter. It is even desirable for high-yielding cows that the content should be 0-50%. *

5. The lower limit of safety appears to be around 0-20% (see Fig. 16). *

6. See Soil, Grass and Cancer (p. 69). *

7. PETERS considers that a total content of 80 p.p.m. manganese in the dry matter represents the lower limit of safety. This investigator, in his study of the herbage of pastures giving rise to tetany in Schleswig-Holstein, found that 36% of the samples had a lower content than this nevertheless high limit of safety. *

8. It must be understood that these figures are merely indications. Cf. on this subject two papers by the author: "Soil and Metabolism" and "The Health of Man is Bound up with the Health of the Soil".
   In the course of their enquiries on tetany farms in Schleswig-Holstein, BECKER, with his co-worker PETERS, took the lower limit of safety as being 8 p.p.m. copper in the dry matter. Almost 70% of the grass samples examined had contents below this limit.
   The grass had sometimes the extremely low contents of 1-7 and 1-2 p.p.m.
   This result is comparable with that of JUNGERMAN, who took 6 p.p.m. copper in the dry matter as being the lower limit of safety. Nevertheless, he found that 70% of several hundreds of hay samples analysed had a content below this limit. Note that in JUNGERMAN'S investigation the samples came from any pasture and not from tetany pastures. This investigator has concluded that more attention should be paid in future to the copper content of forage crops.
   These limiting values for the copper content of grass are merely indications, because it is the total element and not the available element that is concerned. *

9. NAUMANN has applied the equation
   QS =   + , where the ratio is calculated in parts per thousand and Mn and Cu as p.p.m. in the dry matter. Tetany is estimated to occur if QS exceeds 3-5. This is an indicative mathematical formula, to which the same remarks would apply as will be made below concerning the K : Ca + Mg ratio. NAUMANN'S formula, moreover, takes no account of sodium. *

10. Table 24 shows, for example, that the herbage from the two plots A and B had an identical, low content of magnesium (0-17%). Plot A herbage, however, that had received no sulphate of ammonia and contained 18-3% crude protein, causes neither hypomagnesaemia nor tetany. The herbage on plot B, on the other hand, that had received sulphate of ammonia and contained much more crude protein, causes hypomagnesaemia, and many of the cows grazing it are affected by tetany. *

11. The magnesium content of the blood serum then never falls below 1-5 mg./100 c.c. *

12. These cases of tetany correspond with magnesium contents in the blood serum always below 0-7 mg./100 c.c. *

13. It appears to be valid, however, in many cases and under many conditions. *

14. For the influence of magnesium fertilizers on the magnesium content of grass, see Chapter 7. For their influence on grass tetany, Tables 23 and 24 and Fig. 18. *

15. It has been seen that repeated dressings of potassium fertilizers, solid, or liquid, bring about a considerable diminution in the sodium content of herbage. *

16. Another Dutch worker KEMP considered that the sodium content of grass did not play an important part in grass tetany and that he had observed serious cases of hypomagnesaemia on pastures where the herbage was rich in sodium. This might quite well be perfectly correct because many other mineral balances contribute towards grass tetany. *

17. For the influence of this deficiency of sodium on magnesium metabolism, see Chapter 24. *

18. The Belgian worker VERDEYEN has established another ratio to take into account the influence on tetany of the balance of the three ions, potassium, calcium and magnesium. He considers that the important factor is non-antagonized potash, which he represents by the difference:

    K₂0 -(CaO + MgO)

    calculated in milliequivalents. Some Dutch workers, e.g. BOSCH, have studied other, different balances such as:

    alkalinity proper - K + Na - Cl - S;
    alkaline-earth alkalinity - Ca + Mg - P;
    BT = Total of the bases - K + Na + Ca + Mg;
    ZT = Total of the acids - Cl + S + P;
    basic excess = BT - ZT;
    base : acid ratio = BT : ZT. *

19. Of the cases of tetany observed by SEEKLES, only 13-4% were associated with pastures with a ratio of less than 1-80, which, in the long run, indicates a "tendency" in accordance with Table 21, taken from KEMP. *

20. To take account of the antagonism between magnesium and calcium RAHMAN is of the opinion that it would be preferable to adopt the ratio, which he believes to be better correlated with tetany. *

21. Mention has already been made above of how this increased K : Na ratio throws the adrenal cortex out of gear and causes very much greater production of aldosterone, which promotes the development of tetany. It is apparently desirable for the K : Na weight ratio not to exceed 8, or even 5. *

22. See Table 9. *

23. As the Dutch worker DE GROOT rightly states, this ratio is particularly valid within certain limits of potassium contents in grassland, we would add, within certain limits of sodium contents and probably other elements also. *