by André Voisin
Supplementary feeding as a means of protection against grass tetany
Rectify the faults in tetanigenic herbage at their foundation
Leaving aside the question of under-feeding, the main cause of hypomagnesaemic tetany is known to be an imbalance among the mineral as well as the organic elements.
The most effective, safe and simple method consists, as has been described above, in:
For want of anything better, however, supplementary foodstuffs can also be distributed to make up for the deficiencies and imbalances in very young herbage that has received excessive dressings of fertilizer.
Some of the common supplements will be discussed below.
Fibrous foodstuffs at the beginning of the grazing season
The commencement of the grazing season is an extremely dangerous period for tetany in the case of animals that have spent the winter indoors. It would be wise to wait until the first of the grass has reached a height of 5-6 in. [12-15 cm.]. But winter keep is scarce and expensive, which means that, as soon as the grass has begun to grow a little, the farmer is compelled to put his stock out to graze. The problem. therefore, is to take the necessary steps to minimize as far as possible the risks of grazing this first growth of grass. The rules to be observed in this initial period at grass have already been outlined. During the transitional period and for a few days subsequently (say twelve days or so) 2 it is advisable to compensate for the lack of crude fibre and carbohydrates in the very young grass 3 by feeding the animals hay, straw and cereal grains such as oats, barley, etc.
Buccal administration of magnesium
It has been seen that the risks of tetany appear to be very much greater when the magnesium content of the herbage is relatively low. It was logical, therefore, to feed supplements of magnesium in an attempt to afford protection against hypomagnesaemia, an attempt all the more normal, as supplements of magnesium salt fed to cows that were hypomagnesaemic but not suffering from tetany had been observed (Fig. 13) to clearly and rapidly raise the level of magnesium in the blood serum. This effect, unfortunately, persists only during the period of the buccal administration, and the magnesium is immediately "lost" in the urine. The organism, moreover, has hardly any mobilizable reserves of magnesium. It is understandable, therefore, that the administration (during a relatively short time) of magnesium supplements in the ration will not allow the animal to build up the magnesium reserves that would make it better able to resist the hypomagnesaemic effects of the herbage. In other words, preliminary supplements of magnesium, once they have been stopped, do not continue to afford protection against the factors causing hypomagnesaemia, should these factors continue to be present.
The most common and cheapest supplement of magnesium is magnesium oxide (MgO),4 the recommended dose of which is 50-60 gm./day for a cow and 6-8 gm./day for a ewe. Distribution should commence two days before the animals go out to grass, and continue through the transitional grazing period (5-6 days) and for a fortnight subsequently. Even if given over periods of several months, these doses will have no harmful secondary effects.5 The magnesium oxide supplement can be mixed with wet beet pulp. If none is available it can be given with crushed oats, but the beasts will sometimes refuse it because of its bitter taste. Concentrate cubes are now available containing magnesium, and cows will eat these without any trouble.
In Holland, in 1960, 330,000 cows received concentrated foodstuffs of this nature supplying 50 gm. magnesium oxide (MgO) per head per day in the ration. The consequent diminution in tetany cases was estimated at 70-80%. If these results are correct this would appear to be a very efficient method of "protection".
Other supplements of magnesium salts are also possible: for example, magnesium sulphate, but this unfortunately presents the risk of scouring.6 Magnesium bicarbonate has also proved effective (Fig. 13).
Administration of such supplements to animals at grass is obviously easy in practice only in the case of milch cows being milked under cover. The simplest method of supplementing the diet of a cow or ewe at pasture with magnesium remains, therefore, as has been repeated again and again, to supplement the herbage itself, increasing its magnesium content by applying the necessary dressings of magnesium fertilizer. This is all the more to be recommended, as these dressings of magnesium fertilizer are known to alter not only the magnesium content of the herbage but also its organic composition.7
Lack of uniformity in results obtained from the buccal administration of magnesium
Despite the success of supplementary magnesium administration, irregular results have sometimes been obtained.8 In the first instance it seems obvious that the mechanisms of magnesium metabolism can be so profoundly upset that the organism is incapable of using efficiently the exogenous magnesium supplied to it. It sometimes happens even that the magnesium balance remains negative despite magnesium supplementation.
ALLCROFT, for example, administered a daily supplement of 56 gm. magnesium (MgO) to each cow on three farms when they were put out to grass. The results on two farms were favourable, but on the third farm the treatment had no effect. The "dietary record" of the herd on the third farm might explain why it was incapable of utilizing magnesium supplements efficiently.
Supplements of sodium as a protection against grass tetany
As long ago as 1930 the Dutch pioneer SJOLLEMA expressed the opinion that sodium deficiency played a decisive role in grass tetany and claimed to have got protection against the disease by administering sodium.
Since then, practical men and scientists alike have obtained the most contradictory results. In the long run, however, it was to appear as if sodium does indeed play a decisive part in grass tetany.9 It was noted, moreover, that cows on tetany pastures experienced a great "salt hunger",10 and this logically led to their being fed a sodium supplement.
An average dairy cow has been seen to require 80 gm. salt per day. On the other hand, it would seem wise to assume that grass supplies practically no sodium at all, as is frequently the case,11 and take the precaution of feeding a daily salt supplement of about 80 gm. per cow (and 10 gm. per ewe). As in the case of magnesium, it is only easy to distribute salt supplements to milch cows that are being milked under cover. The old method of making salt licks available is still to be recommended, therefore.
Another method consists of adding salt to the drinking-water. FRENS advocates that water containing 2-5% salt (NaCl) should always be made available to animals grazing young grass, the mineral composition of which is unbalanced due to dressings of potassium fertilizer or liquid manure.
Another old forgotten method is to "salt" hay.
The most effective measure, however, is still to increase the sodium content of the herbage by applying the required amounts of sodium to the soil via the different sodium fertilizers.
As has just been said, the effectiveness of sodium supplements is much disputed. Many investigators have voiced the opinion that, although this supplement might be justified in other circumstances,12 it is not effective against tetany.
On the other hand, a practical farmer in England, PICKERING, reports that the addition of salt to the ration 13 he was feeding to sheep greatly reduced the incidence of hypomagnesaemia in the flock. More recently another English farmer, Rex PATERSON, who is responsible for 40 herds of 65 cows seriously affected by tetany, distributed salt supplements to two of the affected herds in which the cows were not only extremely nervous and hyper-irritable but exhibited a tendency towards "licking". He thought the results he obtained were dramatic: the nervousness disappeared and there was an immediate increase in the output of milk. And a point of fundamental importance: no subsequent cases of tetany were observed. A slight increase 14 in the mean magnesium content of the blood serum 15 was also noted. In Germany BECKER found that distribution of 50 gm. salt per cow per day helped to eliminate grass tetany.
These contradictory results are not surprising for, as has been seen, sodium deficiency makes its pathological effects slowly felt. Conversely, supplementary feeding is slow to take effect on the magnesium metabolism of the cow. It is not a case, therefore, of administering sodium supplements as a therapeutic, but of seeing that sodium plays its part as a protector. To this end care must be taken that the ration always contains a sufficiency of the element, supplied either by sodium supplements or by the required applications of sodium fertilizer to the herbage. This is all the more essential, as sodium deficiency, in the long run, may have so seriously impaired the adrenal cortex that no sodium supplement will allow it to resume its normal functions.