by André Voisin
PART VI
INFLUENCE OF GRAZING METHODS ON TETANY
CHAPTER 29
Putting out to grass: the critical time for grass tetany
Mature grass reduces the risks of tetany
It has been seen that very young grass, especially if it has received very large amounts of nitrogenous fertilizer, produces large quantities of ammonia in the rumen which favour the occurrence of hypomagnesaemia and tetany. These quantities of ammonia diminish considerably when the grass is allowed to mature.
It has also been said that the organism responds to any
disturbance of its magnesium metabolism by reducing the excretion of
magnesium in the urine. An experiment by ROOK (Table 19) has shown that
when the animal is fed on very young grass magnesium excretion in the urine
becomes absolutely nil and there is a simultaneous and considerable
diminution in the magnesium content of the blood serum. But if the herbage
is allowed to mature the diminution in the magnesium content of the urine
is less marked and hypomagnesaemia is both transitory and presents little
danger. In other words, if we allow grass to mature its hypomagnesaemic and
tetanigenic characteristics are considerably diminished. This means that
during the grazing season itself we must:
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In the case of continuous grazing, avoid producing "scraped" swards
consisting solely of short grass without a single, hard
tuft.1
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In the case of rotational grazing, allow an adequate rest period to
elapse 2
between two successive rotations so that the herbage can grow again and
establish a good balance both in its mineral and organic composition.
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Gradual putting out to grass
Unfortunately, when the stock go out to grass in spring-the critical time for grass tetany-the farmer cannot wait until the herbage is long enough for his animals to graze. Winter keep is expensive and often scarce. As soon as the grass begins to grow, therefore, the stock must be put out on the pastures, all necessary precautions having been taken. Because, if an exclusive diet of young grass is dangerous throughout the season, it is all the more dangerous at the beginning, with the first growth of grass. Every precaution must be taken, therefore, to avoid mishaps, be they simple scouring or grass tetany.
In the first place, any change in diet must be gradual in order to give the
microflora of the rumen time to alter and become adapted to the new diet.
This rule must be all the more strictly observed in the case of the
tremendous change in diet represented by the transition from stall feeding
to pasture grass. As FERRANDO reminds us: "Putting out to grass must be
done gradually, the more so in the case of pastures where liquid manure and
fertilizers, particularly potassium fertilizers and, secondly, nitrogenous
fertilizers, have not been applied sparingly."
This means that on the
first day out at grass the animals must be allowed to graze for a few hours
at the most: say, two hours on the first day, three or four hours on the
second day and so on, the grazing time being extended gradually.
Throughout this period grass forms only part of the total diet, the
fraction it constitutes becoming gradually larger until it is the one and
only foodstuff.
The stall ration (or supplementary ration) fed to the cows during this
transition period must comprise a fairly large quantity of foodstuffs rich
in dry matter as well as in carbohydrates and fibre, to make up for the
lack of these elements in the young grass. The mineral composition of this
ration (of forage, in particular) must help to compensate for the mineral
imbalances of the young grass. It is advisable, in addition, that the
animals should receive a supplement of magnesium (MgO) and have access to
salt licks throughout the danger period.
It is particularly recommended that pastures or paddocks in which grazing
commences should have received the necessary dressings of magnesium
and sodium fertilizers, and have had no potassium or liquid
manure applied since the previous autumn. One can only recommend that
grazing should start on permanent pastures, where the risks of tetany are
less than in the case of temporary swards.Farming traditions and modern scientific knowledge
An old traditional custom can help to reduce the risk of tetany when stock is put out to grass.
Tetany at the beginning of spring grazing has always been known, especially
in regions where a lot of organic fertilizer, particularly liquid manure,
is used. As the farmers nicely put it: "The fire of the grass is stronger
then than the animal."
In these regions graziers at the end of the grazing season, are careful to
leave at least one sward that is not grazed bare: that is, a sward where
there is some long grass and tufts of fibrous herbage. This is the sward
chosen to start the grazing in the following spring when there is some
growth of grass among or near the tufts of dried-up herbage. The lessons
to be learned from this farming custom are particularly important where
rotational grazing is practised. In effect, one of the great advantages of
this system is that it allows regular and uniform grazing of each paddock,
with the result that there are hardly any "refuse" tufts. But this
perfection on the part of rotational grazing at the end of the season has
frequently helped to trigger grass tetany at the beginning of the following
year, due to the fact that when the animals are put on to the sward in the
spring, all they have at their disposal is dangerous, very young grass
which has just made some regrowth. In the case of intensive grazing the
old farming custom can be fitted in at the end of the year, those paddocks
which are to be grazed first in the spring not being grazed bare. In this
way the animals will find somewhat fibrous herbage side by side with the
very tender, newly grown plants, and will thus be provided in the field
with a natural supplement to young grass similar to what is effected by
feeding, in the stall, a ration of straw. This mixture is even more perfect,
for, at the same time as it harvests the young grass, the cow is harvesting
with almost every mouthful a little of the old, hard herbage. Moreover, if
this grass has been "burned" by the winter frosts it is equivalent to straw
"on the hoof", which balances, at least in part, the defects in the
composition of young grass.At the beginning of the grazing season take the cows into the stall at night
One must be very careful in the cold, wet weather at the beginning of the grazing season when tetany is much more frequent in occurrence (see Fig. 21). To be more exact, care is essential until the temperature begins to rise. The critical period for grass tetany will be seen (Fig. 22) to coincide with the time when the temperature begins to increase,3 having previously remained relatively low. An elementary and essential precaution is to take cows,4 who are more susceptible to tetany than others of the bovine species, indoors overnight during this cold period.
Application of nitrogenous fertilizers before the commencement of grazing
The application of nitrogenous fertilizers before grazing commences presents a tricky problem. These fertilizers have the advantage of:
| a. | accelerating grass growth; |
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| b. | facilitating a more uniform commencement 5 of grazing where pastures are divided up into paddocks. |
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All these advantages would obviously be illusory if they were to be paid
for with improvement of the health of the stock or, even worse, the death
of some of the animals. On all farms, therefore, where there is a danger
of grass tetany in spring it is sometimes wise to forgo applying
nitrogenous fertilizer before grazing commences. But this solution is a
last resource which one must try to avoid.
To reduce the risk of nitrogen dressings at the beginning of the season the
first recommendation is that nitrate of lime or nitrate of soda should be
applied, these being apparently less dangerous from the point of view of
grass tetany. It is particularly important to see that the herbage contains
as much magnesium as possible: that is, that the soil itself is rich in
available magnesium. Some nitrogenous fertilizers do contain
magnesium.6
Above all, however, as has been stressed, the required dressings of
magnesium fertilizer must be applied to the soil, and these diminish
considerably the risks that may be incurred by applying nitrogenous
fertilizer (see Tables 23 and 24 and Fig. 18) before the commencement of
grazing. The nitrogenous fertilizers must naturally be applied a short time
before the grass begins to grow, leaving the herbage the maximum possible
time to "digest" the nitrogen.
In passing, it should be noted that the application of potassium fertilizer
in winter, that is, before grazing commences, would never be recommended
here. This rule must be even more strictly observed if nitrogenous
fertilizers are being applied, because there is always the danger that the
two fertilizers, in the first young grass of spring, may combine in their
effect on hypomagnesaemia (Table 23).
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This is particularly the case in dry weather, and especially in autumn,
another critical period for tetany. *
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See Grass productivity (pp. 20-8). *
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This holds good for any season. *
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A similar precaution is advisable for a few days if ewes have spent the
winter indoors. *
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See Grass Productivity (pp. 18 1-2). *
- Such as nitrogen magnesium (Stickstoffmagnesia). *