CHAPTER 34

Influence of external conditions on grass tetany

Commenting on these curves, KEMP is of the opinion that tetany becomes almost unknown, after the mean daily temperature in spring remains above 57° F. (14° C.) for six consecutive days. It is only in the autumn, when the temperature falls below this level, that tetany reappears. Moreover, the lower the temperature when the heating-up process starts, the greater the increase in the number of tetany cases. This is clearly shown by Fig. 20, where the peak of spring and autumn tetany occurs after a rise in temperature during a period in which the temperature had previously been relatively low. The remarkable fact is that there is a regular interval of about 5 days between the variation in temperature and the appearance of tetany.¹ This time lag is reduced ² when the mean temperature rises.

Rainfall accentuates the effect of cold in favouring grass tetany

Most workers have found that the effect of a low temperature favouring grass tetany is accentuated by heavy rainfall.³ 'T HART considers that grass tetany is particularly frequent in a damp, cold season ⁴ where the temperature is generally lower than 59° F. (15° C.). Autumn or winter, like spring tetany is therefore favoured by cold, wet weather conditions. This is well illustrated by Fig. 21. The months of November - December in 1958 were both colder and wetter than in 1957. The result was that autumn-winter grass tetany was four times more frequent in 1958 than in 1957 (the cows in question were South Devons).

Figure 21: Incidence of spring, autumn and winter tetany as a function of temperature and rainfall

External temperature and herbage composition

The question is why this rise in temperature following a cold spell,⁵ as well as dampness, should favour grass tetany. Damp weather tends to diminish considerably the dry matter content of herbage, the effect being accentuated by the application of large dressings of nitrogenous fertilizer. It has been observed, under these conditions that tetanigenic herbage may possibly contain less than 15% dry matter, a low content which has been seen to be capable of giving rise to under-feeding, which in turn accentuates the hypomagnesaemic effect resulting from imbalances in the composition of the herbage.
In addition, the rapid rise in temperature (accompanied by moisture) triggers a sudden spurt of herbage growth and, as many workers have found, sudden grass growth after a dormant period favours tetany. It is not impossible, moreover, that this rapidly growing young grass contains a tetanigenic factor. But, leaving aside this particular factor, the question remains whether the mineral composition of this grass whose rapid growth has been triggered off by the rise in temperature may not promote tetany.
First of all, what is the effect of a rapid rise in temperature following a cold period on the mineral balance of grass? It has been said (see Table 21) that, under certain conditions, an increase in the ratio of potassium to calcium and magnesium in the herbage can promote tetany.

Figure 22: Relation between soil temperature, potassium excess (in relation to the equivalent total of calcium and magnesium) and incidence of grass tetany

Figure 22 shows (starting from the bottom upwards) for the first grazing period running from 20th April until 12th May:

	incidence of grass tetany;
	mean temperature of the soil;
	variations in the potassium content of the herbage;
	alterations in the ratio.

When the temperature rises after having been low for a period ⁶ there is an immediate increase in the potassium content of the herbage. It appears that a fraction of the potassium "fixed" in the soil is released rapidly, although there is no reasonable explanation for this. The grass then gorges itself with this excess of available potassium that appears in the soil, effecting "luxury consumption". Since the contents of calcium and magnesium have only varied insignificantly (at least under the conditions of these observations), the curve of the ratio of the grass is more or less parallel to the curve of the potassium content. This ratio reaches its maximum on 5th May, and it is about this date, too, that tetany spreads considerably.
The high incidence of tetany, therefore, following a rapid rise in temperature after a period of low temperature, might be explained by the increase in the ratio of the herbage.

Humidity mobilizes soil potassium

When humidity is added to the sudden increase in temperature, a second phenomenon appears to participate in rapidly increasing the potassium content of the herbage. It seems, in effect, that, under the influence of the higher humidity of the soil, the "fixed" potassium is converted to the available form.⁷ This is well illustrated by Table 32, relating to a sandy and to a clay soil.

TABLE 32: Influence of humidity on the potassium : magnesium ratio in the solution of two soils

When the humidity of the soil rises, the K : Mg ratio can be multiplied by three in the soil solution, that is, in the plant nutritive solution. This excess of potassium over magnesium (its antagonist) in the nutritive solution of the soil will help to create a similar imbalance in the plant,⁸ which means that the K : Mg ratio is higher in the ration of herbage "harvested" by the cow and this could help to trigger off tetany.

Weather to fear

It may be seen that the cumulative effect of increased temperature and humidity following a period when there has been no grass growth tends to produce herbage that is too rich in potassium.⁹ The dangers of this to the animal are well known.
In practice, the farmer must be particularly on the look-out for tetany when the atmosphere heats up after a period of cold weather, especially if the heating-up process is accompanied by rainfall, be it spring, autumn or winter. It is wise at such a time to apply certain protective measures, if necessary, and to inspect the herd for the possible appearance of various precursory symptoms of tetany. This is particularly advisable in spring, a critical period for tetany, so long as the atmosphere has not become warmer.

1. In the U.S.A., in West Virginia, it was found that attacks of tetany were triggered off in April, generally after 5 warm days in which the mean temperature rose suddenly to 15-16° C. *

2. It has been seen that an alteration in temperature affects the thyroid gland only after an interval of time. This phenomenon may probably be entering into play. *

3. Note, moreover, that the magnesium content of the blood serum of the cow is higher on the average in hot, dry weather than in cold, wet weather. In England ALLCROFT found the highest blood serum magnesium levels when the external temperature was high and there had been little rain previously. On the other hand, she recorded the lowest contents of magnesium in the blood serum in cold, very wet weather.
   For seasonal variations in the magnesium content of the blood serum of the grazing cow, see Fig. 2. *

4. ANDERSON reports having observed spring grass tetany following very wet spells of weather. The cattle involved were cows of a beef breed that had calved several months previously and were feeding a large calf. *

5. Regarding the very low temperatures that may encourage grass tetany, as will be seen in the next chapter, it has been said that the resulting greater thyroid activity increases the requirements of magnesium (Fig. 10). In addition, this low temperature has been seen to accentuate the effects of under-feeding (Fig. 8). *

6. The magnesium content of herbage is lower at low temperature. It is possible, therefore, that low temperature makes a dual contribution to hypomagnesemia:

   	by diminishing the magnesium content of the herbage;
   	by increasing the magnesium requirements of the animal.

   
   In KEMP'S experiment, however, the magnesium and calcium contents appear to have hardly altered at all. *

7. This is similar to the effect of an increase in temperature. The two effects may therefore be additive. *

8. It is well known, moreover, that magnesium deficiencies in plants are more frequent in a wet than in a dry spring. *

9. It would be interesting to study the influence of these same factors on the other mineral elements. *