by André Voisin
PART VIII
THE CAUSES OF GRASS TETANY IN RETROSPECT
CHAPTER 38
Characteristics of tetanigenic herbage and their origin
Knowledge of the causes of tetany is still far from perfect
Before examining in the next Part practical measures of protection against grass tetany, the causes will firstly be reviewed. It must be admitted that the theoretical aspects of tetany are still rather obscure. For this there may be three reasons:
1. The problem involves several metabolic phenomena about which little is known,
2. The experimental work necessitates the use of many animals as well as large, well-equipped laboratories: which means that it is extremely costly.
3. It is much more difficult to study animals at grass than in the stall.
The present chapter will be devoted to summarizing the characteristics
of herbage that produces tetany, while the next chapter will deal with
the physiological effects of this herbage on the animal-effects that
trigger hypomagnesaemic tetany.Imbalance in the organic composition of tetanigenic herbage
It is young, rapidly growing herbage that causes tetany. It is too rich in nitrogenous substances relative to carbohydrates. These nitrogenous substances are capable of rapid breakdown by the rumen microflora. The herbage contains little dry matter or crude fibre. It is possible, moreover, that this young herbage in the process of growing may contain a specific, tetanigenic factor.
More mature herbage tends to lose these characteristics and dangerous
imbalances (see Figs. 14 and 15).Mineral imbalances in tetanigenic herbage
There are two principal mineral imbalances in tetanigenic herbage:
| 1. | too low or too high an absolute content of certain mineral elements; |
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| 2. | abnormal relationships between these elements. |
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From the point of view of absolute contents in the dry matter, tetanigenic
herbage is:
| (a) | extremely low in sodium, which may sometimes be almost non-existent; |
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| (b) | too rich in potassium, the content of which is often more than 3-0% in the dry matter; |
|
| (c) | not necessarily low in magnesium, but a lower magnesium content than 0-20% increases the risks of hypomagnesaemia and tetany; |
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| (d) | relatively low in copper and manganese. |
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With regard to the mineral ratios there is:
| (a) | an excess of potassium over sodium: that is, the K:Na ratio is too high, often exceeding 10 (weight ratio); |
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| (b) | although it is not an absolute rule, an excess of potassium in
relation to the total of calcium and magnesium: that is to say, a high
 ratio, which,
calculated in milliequivalents, often exceeds 1-80, the
limit considered as dangerous under certain conditions.
|
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Sward flora and tetany
White clover and the miscellaneous herbs reduce the
ratio, but an
excess of white clover and the invasion of a pasture by herbs can present
problems both from the point of view of the pasture and of the health of
the animal.
Mineral imbalances are more marked in temporary than in permanent pastures.Fertilizers and the tetanigenic nature of herbage
The excess of rapidly decomposable nitrogenous substances in grass is accentuated by the application of excessive quantities of nitrogen (Table 20) that the grass has not had time to "digest" because the interval between the application of the fertilizer and the commencement of grazing is not long enough. Ammonium ions, more than nitrate ions, tend to exert an antagonistic effect on the soil magnesium. By a cumulative effect, therefore, they can create deficiencies of magnesium which will help to produce in the herbage a mineral imbalance that will favour the triggering of tetany. Moreover, nitrogenous fertilizers, by a slow, cumulative effect, give rise to copper deficiency.
In most instances, potassium fertilizers:
| (a) | increase the potassium content; |
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| (b) | very considerably reduce the sodium content; |
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| (c) | clearly diminish the magnesium content of the herbage. |
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The most frequent result is a marked increase in the K : Na and
ratios.
Liquid manure must be considered as a fertilizer relatively much too rich
in potash. Not only does it diminish very considerably the sodium content
of the grass, as does potassium fertilizer, but it diminishes the
magnesium content of the herbage much more than the latter (Table 11).
Its ammonia helps to create copper deficiencies. The hydrogen sulphide it
contains causes both copper and iron deficiencies.
Dressings of fertilizer with sodium or magnesium increase in many cases
the sodium and magnesium contents of the herbage respectively. This helps
to improve the poor balances resulting from excessive potassium fertilizer
applications, whether in solid or liquid form. Applications of copper
fertilizer remedy the copper deficiency caused by nitrogenous fertilizers
and liquid manure.
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