CHAPTER 16

Hypomagnesaemia caused by under-feeding

	1. an imbalance in the composition of the herbage;1
	2. under-feeding.

	Primary or absolute under-nourishment due to inadequate quantities of food being made available to the animal. This is true of a pasture with a poor crop of grass from which the animal can "harvest" only very small quantities of herbage.
	Secondary or relative under-nourishment resulting from digestive upsets and particularly from rumination disturbances.3

These changes in diet, however, act more quickly in both cases on the calcium ⁵ than on the magnesium. Many of the cows were subject to convulsions. Injections of calcium and magnesium salts were effective in some cases but not in others.

INGLIS, for example, observed that in the case of ewes kept in the cold underfeeding brought about a rapid drop in the magnesium content of the blood serum which fell in four days from 2-1 to 1-3 mg./100 c.c. As soon as the diet was complemented by forage crops the magnesium level of the serum rose again rapidly, reaching its almost normal figure in less than three days (Figure 8).

Hypomagnesaemia caused by under-feeding does not always react in tile same way to magnesium injections

Hypomagnesaemia resulting from insufficient quantities of herbage being harvested by the animal makes a slow appearance and is sometimes called "slowly developing seasonal hypomagnesaemia". The animal suffering from tetany caused by hypomagnesaemia of this kind reacts little or not at all to parenteral injections of magnesium. Twenty-four hours after such an injection the magnesium content of the blood serum falls again to its previous low level, whereas spring hypomagnesaemia ⁷ frequently responds to a single injection, allowing the magnesium level in the blood serum to rise again, perceptibly and permanently.

Secondary under-nourishment in cows grazing very young spring grass

Secondary under-nourishment, the result of disturbed rumination, makes the cow "harvest" inadequate amounts of grass. In the case of spring grass tetany, caused principally by unbalanced herbage composition, this effect of under-nourishment can accentuate still further the tetany-producing nature of the herbage.
This was confirmed by LADRAT, who studied the quantities of herbage "harvested"' by cows on a temporary cocksfoot sward that gave rise to grass tetany at the Grignon School of Agriculture in the spring of 1959. During the period preceding the spring grass tetany attack they found that a cow harvested on the average per day only 9 lb. [4 kg.] of dry matter: obviously a very low figure ⁸ compared with the 20 - 30 lb. [9-13 kg.] normally harvested by large cows.⁹ These workers were of the opinion that this diminution in the amount of grass harvested was due to a progressive distaste on the part of the animal for a herbage containing too little fibre to allow normal rumination. The result is an ever-increasing lack of palatability, which appears to be accentuated by the excessive moisture content of the herbage ¹⁰ and its low content of sodium.¹¹
These reasons appear to be correct, but the author is of the opinion that there is an important additional cause of rumination disturbance and lack of appetite in the cow, namely, the production of excessive amounts of ammonia in the rumen brought about by very young grass.

Under-nourishment and ketosis

The importance of under-nourishment, be it primary or secondary, in grass tetany is now obvious. But under-nourishment, as has been pointed out, is generally accompanied by acetonaemia. Consequently, it has often been stated that hypomagnesaemic grass tetany in the cow is accompanied by moderate acetonaemia with, in this case, simultaneous hypocalcaemia.
This links grass tetany with ketosis,¹² although ketosis is chiefly associated with stall feeding.

Entero-toxaemia, a form of tetany in the grazing animal

To end this chapter a few words will be said on the subject of another disease, entero-toxaemia of sheep: this is essentially a grazing disease caused by the micro-organism Clostridium Welchii.¹³ Two factors link entero-toxaemia and grass tetany:

	The external symptoms are very similar.14
	The conditions that promote spring grass tetany are also favourable to entero-toxaemia.

The second point would appear to require some discussion because it brings to mind a fundamental principle that has unfortunately received too little attention.

"The microbe is nothing, the environment is all"

Clostridium Welchii is a normal inhabitant of the intestine of the sheep and only multiplies rapidly and becomes toxic if certain changes take place in this environment. Entero-toxaemia provides an excellent illustration of CLAUDE BERNARD'S statement: "The microbe is nothing, the environment is all." (Le microbe West rien, le milieu est tout.) ¹⁵
These favourable conditions for Clostridium Welchii are created by the following three factors:

	Sudden alteration in the diet, particularly the change from a poor to a rich pasture.
	Very young herbage that has received large quantities of fertilizers.
	Recently sown temporary swards.16

These are exactly the same three conditions as promote spring grass tetany: suddenly being put out to graze a very young sward that has been inconsiderately swamped with fertilizer, especially if it is a temporary pasture, favours tetany as well as the development of Clostridium, the cause of enterotoxaemia. Consequently, the best methods of protection against enterotoxaemia are exactly those that will be recommended below as offering protection against grass tetany.

1. Which seems to assume the dominant role in spring grass tetany. *

2. In Norway during the Second World war stall-fed animals were often fed rations low both in magnesium and in energy units (calories). The resulting incidence of tetany was fairly high. *

3. A third type may be added, namely physiological under-nourishment, which can be caused by an increased metabolic level, high milk yield, etc., the greater requirements of the organism not being accompanied by an increase in the amount of food consumed. This physiological under-nourishment was produced experimentally in cows at the beginning of lactation by the administration of thyro-protein which raises both the basic metabolism and the production of milk. In this instance moderate hypomagnesaemia and hypocalcaemia, accompanied by acetonaemia, were observed. *

4. HALSE believes that this is due to a direct connexion between the calcium content of the serum and calcium absorption in the digestive tract. One may equally well think, however, that the mechanisms that maintain a constant level of magnesium in the blood serum act more rapidly than those that control the calcium content. *

5. The ketone content increased from 2-32 to 7-23 mg/100 c.c. Note that at the same time the mineral phosphorus content of the blood serum increased. *

6. Due principally, but not exclusively, to the neuro-endocrine system of the organism being suddenly thrown out of gear as a result of the imbalances in the composition of the herbage. *

7. To determine the quantities of herbage harvested on the average by each of the cows in the herd the three investigators determined the quantity of herbage present per unit area before and after grazing. Obviously, this method is only approximate. *

8. For the amounts of dry matter harvested by the cow see Grass Productivity (pp. 75-86). *

9. According to the calculations made by these workers, the energy supplied by the herbage harvested supplied only 50% of the cow's requirements. Just before the tetany attack was triggered off this proportion fell to as little as 30%. *

10. LADRAT held the opinion that a high moisture content in the herbage can contribute to these effects.
    In wet weather, according to 'T HART'S observations, a very young grass can have extremely low contents of dry matter, sometimes less than 15%. The Dutch worker estimates that the quantity of dry matter ingested (as well as the quantity of magnesium ingested) diminishes as the moisture content of the herbage increases. She also thinks that a high moisture content in herbage reduces the quantities consumed.
    These experiments were concerned with spring grass tetany. Some workers, however (such as MACKELLAR), think that the majority of the cases of autumn grass tetany are likewise due to the absorption of a herbage with a relatively high protein content but with a low content of starch equivalent by contrast. *

11. The cocksfoot consumed by these cows was extremely low in sodium (0-07% in the dry matter). This led LADRAT and his co-workers to suppose that this marked deficiency of sodium could aggravate the lack of palatability. *

12. Ketosis in ruminants is generally described as: acetonaemia in the cow, occurring particularly during lactation and 1-6 weeks after calving; pregnancy toxaemia in the ewe and goat (it may also occur, but more rarely, in the cow), sometimes referred to as "twin lamb disease" because of its greater frequency in ewes carrying twins.
    Ketosis can be looked upon as the result of a negative dietary balance. The lipids (fats) are mobilized to make up the deficit in the diet, and this favours the occurrence of ketosis. Ketosis is characterized by the sweet, so-called acetone, smell of the animal's breath, milk and urine. *

13. The two most common types of entero-toxaemia are designated by D and C. The disease most frequently observed is of type D and is generally accompanied by degeneration of the kidney known as "pulpy kidney". It is essential to distinguish clearly between enterotoxaemia and pregnancy toxaemia in ewes, the latter, as was stated above, being primarily caused by under-nourishment. Note that it is very rare for entero-toxaemia to occur in lambs less than 2 weeks old. *

14. As STEVENS emphasizes, the external symptoms, that is the nervous disturbances of entero-toxaemia and hypomagnesaemia, are so similar that the two diseases are often confused. Although the death of sheep suffering from hypomagnesaemia is often sudden, it seems to be a little less rapid than in the case of entero-toxaemia. Autopsies on sheep that have died from entero-toxaemia generally reveal degeneration of the kidney (pulpy kidney), cardiac lesions, pulmonary congestion and fatty degeneration of the liver. Entero-toxaemia is apparently rarely accompanied by hypomagnesaemia. Unfortunately, death due to entero-toxaemia is so rapid that there are not sufficient results for absolute certainty on this point. *

15. PASTEUR is said to have repeated these words on his death-bed, adding: "Yes, Claude Bernard was right." Unfortunately modern medicine has its eyes fixed almost wholly on the microbe and pays little attention to the environment which, if it were not favourable, would not allow the microbe to develop. *

16. See chapters 31 and 32 regarding the relatively high incidence of tetany on the temporary pastures (leys) that one associates with the idea of "grassland intensification". A British specialist, STEVENS, writes: "The incidence of entero-toxaemia is greatest in lowland flocks particularly when intensive methods of grass production are practised and the incidence may, therefore, rise if grassland management continues to improve."
    The word "improve", the author feels, must be tinged with a large dash of British humour. *