CHAPTER 30

Pasture flora and tetany

Timothy, for example, is particularly low in magnesium, and it has been confirmed in the U.S.A. that timothy hay, which is fairly widely used in some regions, can help to produce hypomagnesaemia in the stall. White clover contains twice as much magnesium as timothy.¹ The high magnesium content of red clover, a very popular forage crop for dairy cows in Normandy, should likewise be noted.
It will be seen, moreover, as has already been stressed, that the magnesium content of all these species is higher in summer than in spring (cf. Fig. 3 and Table 6).

Legumes and "miscellaneous species" contain more magnesium than grasses

On the average, legumes can be said to contain more magnesium than the Gramineae. In addition, the "miscellaneous species" have a higher magnesium content than the legumes. This is demonstrated by Table 26 and Table 27. More generally it will be obvious that the legumes and "miscellaneous species" are richer in mineral elements (major or trace elements) than the Gramineae.²

TABLE 26: Mean mineral element content of grasses, clovers and miscellaneous species

TABLE 27: Mean and relative contents of major and trace elements in grasses, legumes and miscellaneous species

The extremely low and almost non-existent sodium ³ contents established by these German and British investigators gives rise to some anxiety. The serious physiological consequences of this deficiency have already been referred to.

Does white clover favour a high content of magnesium in the blood serum of the grazing animal?

It will be seen from Table 26 that the ratio is higher in the grasses than in the legumes or miscellaneous species. In certain circumstances this ratio, if it is high, can favour grass tetany. This being the case, therefore, the presence of white clover, or of miscellaneous species, would reduce the risk of tetany, tending as they do to lower the ratio of the herbage.
The "Netherlands Society for the Importation of Potash" has compared the magnesium contents of the blood serum of cows grazing swards with and without white clover. The results are reproduced in Table 28.

TABLE 28: Influence of the white clover in pasture on the magnesium content of the blood serum of cows

Before going out to grass the magnesium content of the blood serum was 2-58 mg./100 c.c. on the average. On the 1st May Group A, which was put on to a pasture low in clover (1-3%), had a mean magnesium content in the blood serum of 0-88 mg./100 c.c., whereas Group B, grazing herbage containing 30% white clover, kept its blood serum magnesium level at 1-96 mg./100 c.c.
The groups were then reversed:

	Group A changed to the pasture with clover and the magnesium content of the blood serum rose from 0-88 to 2-50 mg./I00 c.c.
	Group B that had left the pasture with clover for that without clover also has a higher magnesium content in the blood serum: the level rose from 1-96 to 2- 56 mg./100 c.c.

The two results are contradictory.

Various disadvantages of pasture rich in white clover

Although this experiment is not convincing, there are some conditions under which a high percentage of white clover may reduce the tendency towards hypomagnesaemia and the risks of tetany. But other factors enter into play.
AS KERGUELEN points out, the production potential of white clover is less than that of the grasses. Moreover, in our climates the growth and production period of the Gramineae is longer than that of white clover, which has a greater heat requirement.
Moreover, to maintain a considerable percentage of white clover in a pasture it is essential to apply sometimes quite large ⁴ dressings of potassium fertilizer, and the resulting mineral imbalances can entail for the animal all the dangerous consequences that have been outlined. One may wonder, therefore, whether, in some cases, what is gained from encouraging white clover will not be lost (and more) by the mineral imbalances caused by the potassium fertilizer.⁵ The author is not aware of any investigations having been undertaken on this point, nor is there any scientific basis for stipulating the optimum white clover content of a pasture from the point of view of the health of the animal, who is still supreme judge.
There is worse, however, to come. It was stated previously that the new, pedigree strains of white clover ⁶ tended to increase the level of thiocyanate in the blood serum of the animal, and this substance sensitizes the neuro-muscular system to potassium. These new white clover strains have still more serious disadvantages. The thiocyanate they produce in the organism is an anti-thyroid factor which can have a serious. toxic effect on the foetus; moreover, it passes into the milk.⁷ Finally, bloat is often frequent on pasture rich in these strains of white clover. In addition, these new white clover strains contain an oestrogen ⁸ which reduces animal fertility.⁹ These effects are obviously more marked, the more white clover the sward contains.
In conclusion, although a certain percentage of white clover may help to avoid tetany (and this is not yet proven), a high proportion of clover, especially the new strains currently in use, involves the danger of other disadvantages which are perhaps even more serious.¹⁰
From the practical point of view the problem is to know what is the optimum percentage of white clover for optimum animal production. This percentage is obviously dependent on many factors: type of stock farming, soil, climatic conditions, etc. It appears that in general and on the average 15-20% clover in summer is assumed to be desirable.

Casualties with "grazing chains"

With regard to the influence of the flora on tetany, is it possible that some herbage species are more tetanigenic than others?
Although few data are available, some information on this question has become available in recent years thanks to a technique not very widely used but sometimes advised: the so-called "grazing chain" method. This method consists in using temporary swards comprising a single grass species" which are grazed one after the other. The aim is to develop a rotation regulated by the rate of regrowth of the different species. Great plans have been drawn up in theory, but the results in the field have been less startling and the movements of the herd could not be organized to give satisfaction in practice. In addition, serious mishaps occurred, even more serious than with temporary swards comprising several grasses and legumes. An English veterinary surgeon, WHITE, has reported that where a farmer was using pure grass swards of this nature that had received large quantities of nitrogenous fertilizer, 25% of his cows were attacked by tetany each year and 10% died of the disease, despite the care taken to administer magnesium injections in time. These calamities have at least had the merit of revealing the more or less marked "tetanigenic" nature of certain individual grass species.

Tetanigenic nature of certain grasses

Cocksfoot has been seen ¹¹ to have a particularly unbalanced mineral composition. In 1959, at the French National Agricultural School, Grignon, a very serious outbreak of tetany ¹² occurred on a temporary pasture composed solely of cocksfoot. The mineral composition of this cocksfoot, following excessive rates of fertilizer application, was so unbalanced ¹³ that tetany was inevitable; the fact remains, however, that, by its very composition, cocksfoot is favourable to tetany.¹⁴
It has been reported from many countries (Australia, New Zealand, Great Britain, France) that animals (cattle, sheep or horses) grazing pastures in which there is an abundance of rye-grass have been attacked ¹⁵ by hypomagnesaemic tetany.¹⁶ These casualties would appear to occur particularly when the rye-grass has headed and the seeds are milk-ripe. As FIOCRE, a veterinary practitioner, who has given a good description of the disease, has pointed out, excessive dressings of nitrogenous and potassium fertilizer increase the tetanigenic effect of rye-grass.

1. WALSHE, however, found that S.50 timothy is one of the grasses richest in magnesium. *

2. This refers to total elements: the available elements do not necessarily vary similarly. *

3. It has been stated that it is unwise to drop below a sodium content of 0-25% in the dry matter. *

4. See Better Grassland Sward (p. 200). *

5. Remember that to maintain a high percentage of white clover it may be necessary to limit the application of nitrogenous fertilizers also. See Better Grassland Sward (pp. 213-23). *

6. S.100, ladino, N.Z. strain I etc., so-called broad-leaved strains. *

7. See Soil, Grass and Cancer (pp. 79 and 81). *

8. Female hormone, in this case coumestrol. For the effect of plant oestrogens, see Soil, Grass and Cancer (pp. 61-7). *

9. It has been claimed, for example, that ewes grazing ladino white clover had their fertility reduced by about 20%. *

10. Cases of tetany have also been reported on pastures consisting exclusively, or almost exclusively, of legumes. This tetany would appear to be particularly frequent among animals grazing legume catch-crops in the autumn. *

11. Sometimes with white clover. *

12. Table 4. Tetany caused by the application of sulphate of ammonia to a temporary cocksfoot-dominant pasture has also been studied (Table 24). Many cows were affected and several died. *

13. In the dry matter:
    3-06% potassium
    0-13% magnesium
    0-44% calcium
    0-07% sodium
    A very high content of potassium is therefore associated with a very low content of magnesium and the almost complete absence of sodium. *

14. Which does not mean that it is not advantageous to have a certain proportion of cocksfoot in pastures in view of the qualities of the species: earliness, drought resistance, etc. *

15. Hypomagnesaemia and tetany caused by a temporary pasture of rye-grass that had received simultaneous dressings of nitrogenous and potassium fertilizers were discussed in Table 23. *

16. Magnesium therapy is effective. Anti-histamines (neo-antergan or phenergan) are equally effective, but the animals suffer a relapse if, following the treatment, they are put back on to the rye-grass sward responsible. Concerning histamine, see Chapter 20. *