In the various chapters of Parts III and IV some of the aspects of magnesium metabolism will be examined as well as the relationships of this metabolism with the neuro-muscular system and endocrine glands. This study will be brief and simplified, omitting many problems. As was stated in the Introduction, attention here is focused on practical methods of "protecting" the animal against grass tetany, in which event only such physiological questions as are absolutely essential for the better understanding of these methods will be studied. The physiologist, however, or the research scientist may find that the following chapters are not sufficiently detailed. Their indulgence is craved on the grounds that the aim of these simplified chapters is to help the veterinary surgeon, agricultural adviser and practical farmer who need to become acquainted with only some of the theoretical aspects of the problem.
Furthermore, to simplify matters for the reader, a summary has been printed at the beginning of each chapter of Parts Ill and IV. Non-specialists may find these sufficient for their purposes.
Certain sections in these chapters offer material that may interest agriculturists and agricultural advisers. For the main scientific text smaller type has been used, while the development of agricultural matters is printed in the same size as the normal text.
Agricultural advisers may find it sufficient to read the material printed in the normal text type in Chapters 12-25.

CHAPTER 12

Resorption of magnesium in the digestive tract and the factors influencing that resorption

This phosphorus effect may be direct and due to the formation of an insoluble and unavailable phospho-ammonium-magnesium compound, particularly in the case of herbage rich in nitrogen which favours the formation of large quantities of ammonia in the rumen.

It is equally possible for the action of phosphorus on magnesium to be attributable to the fact that part of this phosphorus is present in the form of phytate.²
ROBERTS and YUDKIN have demonstrated in the case of rats that phytates produce toxic effects, particularly convulsions reminiscent of the manifestations of magnesium deficiency.³ These toxic effects disappear more or less completely when magnesium sulphate is added to the ration. The two workers conclude that the phytates can give rise to magnesium deficiency by forming insoluble, mixed calcium and magnesium salts that cannot be resorbed in the digestive tract. They further conclude that the presence of phytates in grass can promote hypomagnesaemia ⁴ and tetany.⁵
Unfortunately the factors responsible for the variation in the phytate content of grass are still unknown: as is the extent to which the application of phosphoric fertilizers (depending on their nature) may participate in this increase in phytate content.

Another factor affecting the availability of magnesium is the potassium content of the herbage. Generally speaking, a high potassium content in the ration has an unfavourable effect on the availability of magnesium. The results,⁸ however, are far from being uniform.⁹ The differences probably originate from the different experimental conditions. Of the variable conditions that explain the varying effect of adding potassium salt to the ration, the absolute magnesium content of the latter deserves special mention. DE GROOT, for example, has observed that the addition of potassium hicarbonate to a ration for cows caused the content of magnesium in the blood serum ¹⁰ to fall in three days from 2-1 to 1-5 mg. per 100 c.c. when the animal was ingesting in its ration ¹¹ 21 gm. magnesium per day. When the cow ingested 35 gm. magnesium per day, on the other hand, the magnesium content of the blood serum was not appreciably altered.¹² Transferring these figures to a cow at grass and assuming that it "harvests" ¹³ 29 lb. [13 kg.] dry matter per day we find that:

	In the first case, where the potassium supplement caused the magnesium content of the blood serum to drop, the dry matter contains 0-16% magnesium.
	In the second case, where no effect of the potassium supplement on the magnesium in the blood serum is observed, the dry matter contains 0-27% magnesium.

The risk of hypomagnesaemia and tetany will be seen to increase considerably when the content of magnesium in the dry matter of grass falls below 0-20%. DE GROOT's experiment completes this observation by showing that below this limiting value a high potassium content in grass will have more chance of exercising a hypomagnesaemic effect on the animal.

The results that have been obtained regarding the influence of Vitamin D on the availability of magnesium are not in agreement. It appears, without having been established, that Vitamin D shows a slight tendency to increase the level of magnesium resorption,²¹ but since this is accompanied by greater urinary excretion of magnesium, the ultimate result is a lower content of the element in the blood serum. Some workers think that a diminution in the amount of magnesium in the blood serum produces effects similar to those produced by an excess of Vitamin D.²²
If the Vitamin D results are very vague it seems, by contrast, to be an established fact that the saponins ²³ are capable of very considerably increasing the level of magnesium resorption. This has been demonstrated, indirectly, by KOFLER working with mice and frogs.²⁴
Unfortunately, there is still very little known about the saponins in plants, particularly herbage plants. Cocksfoot and rye-grass, as well as Ranunculus species and certain Umbelliferae, would appear to be particularly rich in saponins.²⁵

1. A distinction is made between two coefficients of digestibility, namely:
   

       a) The apparent coefficient (percentage calculated on the basis of the difference between the amount of magnesium ingested and the amount excreted in the faeces); the actual coefficient (percentage of ingested magnesium that is resorbed in the digestive tract).
       b) The faeces contain not only the magnesium that has been ingested and not resorbed (exogenous magnesium) but also magnesium supplied by the fluids secreted in the digestive tract such as saliva, gastric juice, bile (endogenous magnesium). It is of interest that CARE has shown, for example, that the magnesium content of the bile diminishes when the magnesium content of the blood plasma drops: this, in a period of crisis, allows the organism to reduce its endogenous magnesium losses.

   As is generally (though not always) the custom, "availability" here will refer to the apparent digestibility coefficient.
   It was only recently that the use of radio-active magnesium ²⁸Mg made it possible to separate endogenous magnesium and distinguish the two digestibility coefficients a little better. *

2. Salt of inositol phosphoric acid. *

3. Unfortunately the magnesium contents of the blood serum were not determined. *

4. This transposition of results from rat to ruminant seems reasonable. In fact G0WDA, of the Indian Veterinary Research Institute, has shown that, in ruminants, if there has been hydrolysis of the phytates from calcium-magnesium, these elements are released at a point in the intestine where they are no longer capable of being absorbed and are therefore excreted in the faeces and lost. *

5. ROBERTS and YUDKIN also believe that this hypothesis would explain why the calcium content of the blood serum of hypornagnesaemic cows is often reduced. *

6. See Soil, Grass and Cancer (pp. 25-8). *

7. In Table 24, for example, herbage containing 0-17% magnesium in the dry matter does not give rise to hypomagnesaemia when it contains only 18-3% crude protein, but does give rise to hypomagnesaemia and tetany when the protein content is raised to 23-1% following the application of nitrogenous fertilizers. Nitrogenous fertilizer (sulphate of ammonia), as was seen above, can produce other changes in the composition of herbage. *

8. In actual fact, relatively little has been published concerning the influence of large quantities of potassium (naturally present or added) on magnesium resorption and balance. The aim of most of the investigations has been to establish the effect of adding potassium salts to the ration on hypomagnesaemia and tetany in the animal concerned.
   Some have found positive, others negative effects on the blood-serum magnesium.
   These contradictory results may be due to the observation methods employed. BOMER, for example, observed that under certain conditions the buccal administration of potassium acetate to cows did not appear to have any effect on the contents of calcium, potassium and phosphorus in the blood serum when the latter was analysed every 24 hours. When the analyses were made every 2 hours, on the other hand, there was a sudden increase in the potassium content of the blood serum immediately following the administration of the potassium salt, an increase which reached its maximum 2 hours after the administration (it rose from 23 to 34 mg. in 100 c.c. blood serum). At the same time the calcium content of the blood serum fell from 10-3 to 7-0 mg. per 100 c.c. Seven to eight hours later these contents had returned to normal. *

9. Another contributory factor in altering the effect of a high potassium level in the ration (or supplementary potassium) is the accustoming of the organism to a ration rich in potassium. It has been observed, for example, that following several successive administrations of large but not toxic doses of potassium salts to rats or dogs, very much larger doses of potassium were required to poison the animal. This phenomenon of "tolerance" is general and well known (mithridatism), but unfortunately it is too often overlooked both in experimentation and in practice.*

10. With a simultaneous fall in the amount of magnesium in the urine and a subsequent (but after an interval) very considerable diminution in the amount of sodium excreted in the urine. (For the sodium in urine see Table 17.) *

11. Comprising forage, fodder beet and concentrates. *

12. In this same experiment DE GROOT studied the evolution of the potassium content of the blood serum and blood corpuscles. At the end of his observations he advanced the hypothesis that the administration of a potassium supplement causes a slight, transitory increase in the potassium content of the blood serum and red corpuscles. Magnesium ions are absorbed simultaneously by the red corpuscles, with the result that their K/Mg ratio remains constant (as it does in the intra-cellular environment in general). In a second phase that follows the movement of these ions is reversed.
    It should be noted that in grass tetany the hyperkaliaemia can be slight and of a temporary nature. SJOLLEMA, however, observed an increase of 40-50% in the potassium content of the blood serum of cows put out to graze a luxuriant young sward. The increase was quite transitory, which would confirm B0MER'S finding.*

13. For a 12 cwt. (600 kg.) cow see Table 31 of Grass Productivity, showing the quantities of dry matter harvested. *

14. Added in the form of bicarbonate. *

15. If the supplementation continues for a period of time, the potassium effect becomes less marked, the organism apparently adapting itself to this excess of potassium in the ration. *

16. The daily addition to the ration comprised 52 gm. phosphorus (P) in the form of ammonium phosphate (NH₄H₂PO₄) and 244 gm. potassium (K) in the form of potassium bicarbonate. It is possible that the ammonium ion increased the effect of the phosphate. *

17. This drop in magnesium availability as a result of the simultaneous addition of potassium and phosphate is accompanied by a very considerable diminution in the excretion of magnesium in the urine which becomes almost nil (0-05 gm. per day). There is an increase, however, in the quantity of magnesium retained, but it is unable to prevent a fall in the magnesium content of the blood serum. MEYER, therefore, was of the opinion that his balance studies indicated that potassium and phosphorus take action preferentially at two different points in the metabolism of magnesium: potassium lowers resorption in the digestive tract, while phosphate increases the retention of magnesium in the organism.
    It is equally possible that this combined effect of potassium and phosphoric acid in reducing magnesium resorption is due to the formation of insoluble double magnesium and potassium phosphate (MgKPO₄). *

18. In the dry matter. *

19. Most workers have obtained a similar result. Ross, however, observed in vitro that potassium and ammonium ions had no effect on the resorption of magnesium in the intestine. He remarked that the effects of the potassium or ammonium ions on hypomagnesaemia could have been exerted at a stage preceding the transport of the magnesium through the intestinal wall. *

20. Note that magnesium availability is very much greater in the young than in the adult animal. *

21. See, for example, the remarks and discussions on this subject on pp. 73, 110, 115, 118 and 120 of the British Veterinary Association Conference on Hypomagnesaemia, 1960. *

22. It will be seen below, however, that workers at the University of Kiel consider that supplementary feeding of Vitamin D in the stall can reduce the incidence of tetany when the animals go out to grass in the following spring. *

23. Emulsifying substances of the detergent variety which are contained in many plants. *

24. Saponins allow magnesium sulphate given by means of buccal administration to exert a narcotic effect, which magnesium can normally exert only when it is administered parenterally. *

25. Of the cereal grains, oats have a very high saponin content. If the effect of saponins on magnesium resorption were confirmed, therefore, an oat supplement might prove useful in critical periods as a protection against grass tetany. *