Grass Tetany by André Voisin

CHAPTER 31

Temporary pastures and grass tetany


Grass tetany has advanced hand in hand with the policy of encouraging temporary pastures

The grassland policy described, depending on the circumstances, as ploughing up of old pastures, development of temporary swards, grassland intensification, ley farming, etc., has been accompanied by the extension of grass tetany, not to mention many other serious attacks on the health of the animal.
A great deal of evidence regarding this extension was dealt with in Better Grassland Sward (pp. 129-31), and since the time that that book was written much more evidence has accumulated on the dangers of this policy of so-called "grassland improvement". It would be tedious to report all this evidence in detail: suffice it to quote in the first instance the report that appeared in the French Bulletin des C.E.T.A., stating:
"Almost complete sterility, scouring, enteritis, lameness, grass tetany, etc., were encountered on several farms. The farmers are in poor spirits and it is often the most experienced, energetic and progressive amongst them that are faced with the greatest difficulties. Moreover, they see their "backward" neighbour, who pays no attention to agricultural progress, surrounded by healthy beasts in fairly good condition."

What is obviously required is a definition of "progress" as applied to agriculture, but this might take a large book. Let us be content with saying here that this "progress" certainly does not consist in increasing plant production at the expense of animal (or human) health. As an eminent grassland scientist said recently to the author in Warsaw: "What we call good pastures make bad animals; and what we call bad pastures make good animals!"
Comparable with this official French report and telling Polish statement is the conclusion reached by a British scientist, who writes, not without humour: "There is little doubt that the incidence of grass tetany is increasing. This increase coincides with the marked improvement of pastures in recent years."
Doubtless it would have been preferable to say "improvement in the wrong direction", the results having been far from desirable. This is what was stressed in the course of the notable "Congress on Hypomagnesaemia", organized in 1960 by the British Veterinary Association. Most of the speakers affirmed 1 that the policy of ley farming had been accompanied by the extension of grass tetany.
In conclusion, let us quote the sad observation made by a New Zealand farmer to Allan FRASER:
"He (a New Zealand farmer) asserted in language rather more forthright than I should care to quote here, that the modern New Zealand temporary ley, while undoubtedly productive, had, in addition, raised so many animal health problems that it was, in certain districts, becoming a positive poison to stock."

Why do young temporary pastures favour grass tetany?

The problem is to determine why the methods popularized under the name of "grassland intensification" have given rise to such difficulties, if not sometimes veritable catastrophes, on so many of the farms where they were applied. In other words, why do temporary, newly sown pastures tend to favour tetany more than old, permanent pastures? The reply can be divided into two parts:
1. the peculiar tetanigenic nature of temporary pastures;
2. the widely recommended policy of applying too much fertilizer that has accompanied the extension of temporary grass swards.

Characteristics of the flora of a temporary pasture

The first characteristic of the temporary pasture that can contribute towards tetany is the simplicity of its flora, this being particularly marked, as was stated in the preceding chapter, in the case of mixtures for "grazing chains". It was seen previously that the "miscellaneous species", wrongly referred to as weeds, were richer in magnesium, to say nothing of other elements, than the grasses and legumes. Thus William DAVIES recommends that when temporary pastures are being sown, seeds of these "miscellaneous species", particularly chicory, should be added.2

Herbage composition is not the same irrespective of whether the soil has or has not been disturbed by Man

Herbage characteristics are not necessarily the same on the soil:
a. of a permanent pasture untouched by Man and "worked" solely by its own enormous microflora and microfauna; 3
b. of a temporary pasture which has just been disturbed and aerated by Man's implements.4

When an old permanent pasture is ploughed up and reseeded, profound changes take place in the physical,5 chemical and biological characteristics of the soil. One of the results of these changes appears to be the release of a fraction of the potassium accumulated in the soil in "fixed" form, and this the newly sown grass can gorge itself with in "luxury consumption". A study carried out by VERDEYEN shows how this phenomenon, the outcome of the ploughing up of an old pasture, increases the potassium content of the herbage in the reseeded sward. The herbage of a permanent pasture in the spring of 1948 contained 1-2% calcium and 2-7% potassium in the dry matter (Fig. 19).

Figure 19: Increase in the potassium content of the herbage after a permanent pasture has been ploughed and reseeded

This pasture was ploughed, limed 6 and resown with a mixture of rye-grass, meadow fescue and white clover. In the spring of 1949 the herbage of the temporary sward contained (in the dry matter) 4-2 % potassium, which is 55 % more than the herbage 7 of the old pasture. The ploughing up of the old pasture and the chemical phenomena that developed subsequently in the soil thus gave rise to a mobilization of potassium by the herbage. On this subject VERDEYEN writes:
"These diagrams (of potassium content) of the herbage of temporary swards provide us with quite a natural explanation of the spread of grass tetany on leys. The exteriorization of the mineral imbalance resulting from a seeding is characteristic; and it is normal that stock maintained exclusively on leys should suffer more easily from tetany than stock fed exclusively on old pasture."

In an experiment undertaken by ZURN the lime fertilizer, contrary to VERDEYEN'S practice, was applied only to the old and not to the reseeded pasture. The result obtained by the German worker is similar, nevertheless, to that arrived at by the Belgian. Table 29 shows that the content of potassium in the dry matter of the herbage of the new sward, compared with that of the old pasture, was higher by:
51% in the grasses;
60% in the legumes;
95% in the miscellaneous species.

TABLE 29: The herbage of the recently sown sward contains more potassium than that of the old permanent pasture

This is a very considerable difference. It is known that this higher potassium content in herbage is one of the factors helping to accentuate certain mineral imbalances that favour grass tetany. It must be borne in mind, moreover, that oxidation renders soil manganese unavailable.8 Any extensive and sudden aeration, such as follows the ploughing of an old permanent pasture, will tend to make a fraction of the manganese present in the soil unavailable. Grass that favours the occurrence of tetany has been seen to be generally low in manganese, but there is no explanation available as yet as to whether this manganese deficiency does actually favour tetany,9 or how.
For chemical reasons that are still not very clear the grass of a temporary sward has a higher molybdenum content, the more recently the sward was sown.10 Molybdenum is an antagonist of copper, and it was stated previously that copper deficiency may probably help to promote grass tetany.
It would be of great importance if more results could be made available enabling the composition of herbage of the same physiological age from old and reseeded pasture on the same type of soil to be compared. But the results to hand are already sufficiently indicative of the tendencies towards mineral imbalances that develop in recently reseeded pastures.

Magnesium content of the blood serum of animals grazing a permanent or temporary pasture

These various comparative analyses of the herbage of permanent and temporary pastures help us to understand the extension of grass tetany that has accompanied the policy of ley-farming. But more than herbage analyses we would like to have analyses of the magnesium in the blood serum of animals grazing permanent or temporary pastures, situated on the same type of soil, with all other conditions equal, especially the amounts of fertilizer applied. Despite the importance of the problem, however, hardly any comparative study of this nature is available. Only MICHAEL'S investigation can be cited, comparing the magnesium content of the blood serum of ewes when grazing:
(1) temporary pastures only;
(2) temporary and permanent pastures simultaneously.

The blood-serum magnesium was higher in the second case. Moreover, there was one clinical case of tetany and two sudden deaths among the ewes grazing temporary pasture only, compared with no casualties when permanent grass was available to them. The differences would probably have been even more marked if the ewes, in the second case, had only grazed the temporary pastures.
The other very few studies available include so many variables and peculiar circumstances that no conclusion can be drawn from them. Such is the case, for example, with one experiment in which the conclusion was reached that the magnesium content of the blood serum was higher in cattle grazing leys than old pastures; but, when the conditions of the experiment are examined, it becomes obvious that more than reservations 11 have to be made with regard to these conclusions, and this the investigators, very honestly, do not hesitate to do.
The same is true of another experiment with sheep: after being fed indoors, they were put out in spring on to permanent pastures, which caused a drop in the magnesium and calcium contents of the blood serum. After ten days or so these contents had returned to normal values. Some weeks later the animals were put on to leys and the magnesium fell once again. One could assume, therefore, that the effect of the ley on hypomagnesaemia is more marked than that of permanent pasture. In the light of the experimental conditions, however, such a conclusion does not appear to be justified.
It would be advantageous in future if systematic and truly comparative experiments could be undertaken.

There is every chance that the farmer who ploughs and reseeds will apply large quantities of fertilizer

The development of grass tetany that has accompanied the policy of ley farming is attributable not only to a particular type of flora and the accentuation of certain mineral imbalances resulting from changes in the characteristics of the soil. To these must be added excessive mineral fertilizer application, the reason for which is to be found in the psychological and economic background.
The general tendency among farmers is to spend as little as possible on permanent grass and apply little fertilizer. The costs of ploughing and reseeding a temporary pasture are considerable. This has led some fertilizer firms to believe (and with good reason) that if a farmer is willing to face the expense of ploughing and reseeding he will not hesitate to sacrifice still more of his money in the purchase of fertilizer, the aim being to pay off the costs of establishing the swards through the highest possible herbage output. On the basis of this logic many fertilizer firms have supported the policy of systematic ploughing of old pastures and the establishment of temporary swards. The advisory services have often done likewise.
As these seedings are far from always being a success, one of the main lines of "sales talk" has been: "Spread a lot of fertilizer to 'sustain' your seedings; in this way you will be more successful."
Matters have been taken still further in an attempt to boost the consumption of fertilizer still more, and the suggestion has been made that reserves of fertilizer should be incorporated at the time of sowing. One reads regularly in French agricultural papers: "single applications of fertilizer must be made for several years".
For the "incorporation" of these so-called reserves it is advised that, at the time of sowing, 179 lb./acre [200 kg./ha.] potash ( K20) should be applied. In addition, it is further recommended that heavy dressings of potash and nitrogen be applied each year. A French periodical in December 1961 pointed to annual dressings of 214 lb./acre [240 kg./ha.] potash (K2O), in two applications, and 179-223 lb./acre [200-250 kg./ha.] nitrogen (N). This herbage, gorged with potassium, excessively rich in different nitrogenous substances, low in magnesium and almost lacking in sodium, could not help but produce tetany. There have been, and frequently still are, veritable catastrophes on farms where this advice has been followed.
It must be noted, moreover, that if these excessive dressings of fertilizer on temporary pastures have necessarily accentuated the natural tetanigenic character of the latter, they are equally capable of giving rise to tetany, not to mention other serious stock difficulties, especially sterility, on permanent pastures.

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Notes



[Click on asterisk (*) at the end of a note to return
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  1.  
       


See, for example, pp. 39, 47, 58, 62, 111, 117 of the Proceedings of this 
Symposium. *


    

    
 

  2.  

 This seeding is particularly recommended as an insurance against bloat. 
It will now be obvious to the reader that ploughing out of pasture is 
recommended to destroy the much cursed "weeds", and then the advice is 
given that these same weeds be sown in the interests of the animals' 
well-being. * 



    

    
 

  3.  

 

 See Better Grassland Sward (pp. 117 - 24). *


    

    
 

  4.  

 Bear in mind that it is not impossible for a particular tetanigenic 
factor to be present in the sown grass. *


    

    
 

  5.  

 See Better Grassland Sward (pp. 109-16). *


    

    
 

  6.  

 But no other fertilizer was applied. *


    

    
 

  7.  

 Note that, despite the liming, the calcium content of the temporary 
pasture increased hardly at all. It is not certain, but one may deduce 
from this that, if the temporary pasture had not been limed, the calcium 
content would have fallen, increasing still further the K : Ca ratio in 
the temporary pasture grass. *



    

    
 

  8.  

 The bivalent form of manganese is relatively soluble. It is oxidized in 
the quadrivalent form which is relatively insoluble. *


    

    
 

  9.  

 On the other hand, manganese deficiency is known to cause sterility in 
animals (see Soil, Grass and Cancer (pp. 68-71)). Animal fertility is 
generally poor when only temporary pastures are being grazed. *


    

    
 

  10.  

 The exact figures are given in Table 16 (pp. 45) of Soil, Grass and 
Cancer. *


    

    
 

  11.  

Among the many anomalies, it must be pointed out that the animals were not 
put out in two separate groups at the same time to graze leys and permanent 
pastures; they grazed the swards in succession and at different times. 
Physiological age, abundance, etc., of herbage are known to play an 
important part. *
       




 



         

























                       TABLE 29

       The herbage of the recently sown sward contains more potassium than 
that of the old permanent pasture 




	 
	% in the dry matter of hay
	


	 
	Potassium (K)
	Calcium (Ca)
	


	 
	Old pasture
	New pasture
	Old pasture
	New pasture
	


	Grasses
	1-63
	2-53
	0-58
	0-37
	


	Legumes
	1-53
	2-43
	1-57
	1-55
	


Miscellaneous species
	1-71
	3-34
	1-35
	1-25
	




 N.B. 1. This was the first cut.

        
   2. The old sward had received a dressing of lime and the soil pH was 
6-0 - 7-0.

        
  3. The new sward had received no such treatment and its pH was 5-0 
- 5-5.

        
   4. The increase in the potassium content of the herbage of the new 
sward over that of the old was:
        
  5. For the potassium and calcium contents of the different categories 
of plants, see Table 26.

 From ZURN  *