by André Voisin
Grass tetany in sheep
Grass tetany in sheep has become much more widespread
Following the study of winter grass tetany, it might be useful to include here a short discussion of grass tetany in sheep, which, in many countries, is mainly a winter tetany. Grass tetany in sheep, it is true, does not differ essentially from grass tetany in cattle, but it does have some particular features that deserve mention. This appears all the more justified, since in most countries in recent years different workers have reported an alarming development of grass tetany among sheep. ALLCROFT claims that attention was first directed to grass tetany in sheep in 1952 by STEWART, who described the sudden appearance and considerable spread of the disease in Scotland. She reports that, since that date, grass tetany has appeared frequently in Great Britain in all hill or low-ground flocks of sheep. She recalls that the disease has also been observed in Ireland and in North America. Although there are no statistics available, Mrs. ALLCR0FT is of the opinion that the disease has become much more widespread in Great Britain in recent years.
Causes of grass tetany in sheep
As with grass tetany in general, there are two main, common causes in the case of sheep:
1. Temporary pastures developed by the ley-farming policy or so-called "grassland intensification". Grazing of a particular type of temporary pasture, green wheat, is also favourable to sheep tetany.
2. Application to pastures of excessive dressings of nitrogenous and potassium fertilizers (mineral or organic).
CONWAY, for example, pointed out that heavy rates of nitrogen and potassium fertilizer application to pastures effected a considerable decline in the magnesium content of the blood serum of ewes. Simultaneous application of a magnesium fertilizer (magnesium sulphate) enabled this content to be maintained.
One particular form of grass tetany in sheep occurs frequently when herds are moved, being brought down from poor hill grazings to lower-lying pastures that have received a great deal of fertilizer. The phenomenon is dangerously accentuated if nitrogenous and potassium fertilizers are involved.1 Here is another example of the dangers of sudden transition from one type of feeding to another, especially when the new diet is one of young herbage unbalanced in composition.
Sheep tetany is mainly a winter tetany. It is encouraged by cold winds and snow, the low temperature accentuating the effects of under-nourishment. At all times of the year tetany in sheep, as in cows, is favoured by dampness and by a sudden rise in temperature following a cold spell.
Ewes are particularly affected
Grass tetany affects all sheep, but ewes in particular, especially if they have two lambs. As in the case of cows (Table 31), it is the oldest ewes that are most affected, which is not surprising in view of the fact that an old ewe, on the average, has significantly less magnesium in its blood serum (1 -62 mg. 1100 c.c.) than a young ewe (2-26 mg./100 c.c.).
Grass tetany in sheep takes place shortly before lambing and in the six weeks that follow, the highest incidence of the disease being 1-4 weeks after lambing, when the milk production of the ewe is at its maximum.
Some breeds of sheep may possibly be more susceptible than others,2 but, as in the case of cows, the statistics are far from being uniform.
Symptoms of grass tetany in sheep
This is a hypomagnesaemic tetany, almost always accompanied 3 by hypocalcaemia.
Four-year-old ewes that had changed over from a permanent to a temporary pasture that had received large quantities of fertilizer were attacked by grass tetany. In the ewes of this flock it was found that the mean contents of magnesium (0-7 - 0-8 mg./100 c.c.)4 and calcium (3-2 - 4-2 Mg./100 C.C.)5 in the blood serum 6 were very low.
The symptoms of hypomagnesaemic grass tetany in sheep are very similar to those in cows: the ewe is timorous and nervous, her head is tense and her eyes staring as if she is listening to some alarming noise; when the symptoms worsen, the ewe rolls her eyes and grinds her teeth, the face muscles quiver and then the animal falls down in a convulsion.7 It must be emphasized that the course of the disease's development is much more rapid in the ewe thanin the cow. Thus, it frequently, if not almost always, happens that the shepherd finds his ewes in a coma or dead, without previously having noticed anything abnormal. It takes an experienced shepherd to notice in his flock the symptoms that have just been described, announcing the onset of an attack.
As has been said, it is often difficult to distinguish grass tetany from entero-toxaemia in sheep, bearing in mind that the conditions that cause grass tetany are the same as those that favour the development of Clostridium Welchii, the causal micro-organism of entero-toxaemia.
Therapeutic and protective methods
The therapy is the same as for cows: parenteral injection of a magnesium and calcium salt. It is very difficult to apply in practice, however, because, as has been said, the disease develops extremely rapidly, making it almost impossible to intervene in time.
As a result, only "protective" methods can be applied, and fortunately these are often highly effective. Buccal administration of magnesium supplements has proved an excellent measure, but is obviously very difficult to implement in a large flock, especially if the grazings are far from any shelter. The most practical, and at the same time the most efficient and safe method, therefore, consists in applying the necessary dressings of magnesium fertilizer 8 so that the magnesium content of the herbage never falls below the critical level, generally 0-20%, of magnesium in the dry matter. These applications have proved very effective.
From the point of view of grazing methods, all the general recommendations apply. In addition, when flocks are being moved from poor hill grazings to so-called "improved" pastures sudden transition from one type of grazing to another must be avoided.
It is advisable when sheep are congregated together on rich, lower-lying pastures that they should be fed old and somewhat hard grass at the same time. Here again, the best insurance is to avoid excessive rates of potassium fertilizer application, but the necessary magnesium fertilizer must certainly be applied.