Grass Tetany by André Voisin

CHAPTER 28

Influence of nitrogen and magnesium fertilizers on the magnesium in the blood serum of the cow at grass


Sulphate of ammonia, under certain conditions, can favour tetany

Some of the possible effects of nitrogenous fertilizers on the composition of herbage were discussed, while the way in which the combined action of nitrogen and potassium fertilizers may promote hypomagnesaemia and tetany and how this effect is cancelled by the application of magnesium fertilizers form the subject of the preceding chapter (Table 23). The present chapter will deal with the effect of sulphate of ammonia on tetany and how this effect can be eradicated by the application of magnesium fertilizers.
BARTLETT grazed cattle on a temporary pasture,1 divided into three paddocks:
(a) control (no fertilizer);
(b) 6 cwt./acre [70 kg./ha.] sulphate of ammonia applied annually;2
(c) the same quantity of sulphate of ammonia as in (b) and a single dressing, two years before the experiment, of 25 cwt./acre [3180 kg./ha.] calcined magnesite (containing 87% magnesium (MgO)).

It will be seen from Table 24 that the content of magnesium in the dry matter of the herbage was relatively low on the control paddock A: 0-17%, which is less than the safety limit of 0-20% (see Fig. 16). Application of sulphate of ammonia had no effect on this magnesium content, which, in paddock B, remained at the same level of 0-17%. By contrast, the crude protein content of the herbage rose considerably, increasing from 18-3 to 23-1 %. When the animals were put out to graze the control plot the mean content of magnesium in the blood serum did not fall below 1-90 mg./100 c.c. and there was not a single case of tetany. On paddock B, however, that had received the sulphate of ammonia, the mean magnesium content of the blood serum fell to 0-88 mg./100 c.c., with even lower values in individual cases where the cows were attacked by tetany.

TABLE 24: Influence of the application of sulphate of ammonia and magnesium fertilizer on the content of magnesium in the blood serum of cows put out to grass

Why can sulphate of ammonia favour hypomagnesaemia?

Under the conditions of this experiment, one single dressing of sulphate of ammonia profoundly altered the mineral composition of the animal's blood, giving rise, as BARTLETT says, to "spectacular" hypomagnesaemia 3 and several cases of grass tetany. It may be wondered why sulphate of ammonia exercises this marked depressing effect on the blood serum of the grazing animal, thus possibly helping to trigger off tetany.
As has just been said, the sulphate of ammonia in this particular instance did not alter the magnesium content of the herbage.4 It did, however, very considerably increase the content of crude protein, which is normal. Now, it has been seen (Fig. 15) that this high content of crude protein in herbage increases considerably the amount of ammonia produced in the rumen, and thus favours hypomagnesaemia 5 and tetany. This explanation is only relatively correct, however, because other nitrogenous fertilizers (particularly the nitrates) have a less marked effect on hypomagnesaemia. It was seen in the preceding chapter, for example (Table 23), that the application of ammonium nitrate had no effect on the level of blood magnesium in the cow, at least under the conditions of this experiment. It is not impossible, therefore, that sulphate of ammonia was able to exercise a different, particular effect. It is known, in fact, to increase considerably the sulphur content 6 of herbage, and a very high content of sulphur in herbage, as was seen previously, can increase the production of hydrogen sulphide in the rumen (Fig. 7). Now, very young grass in itself tends to raise the production of hydrogen sulphide, with the result that, if its sulphur content is increased by the application of sulphate of ammonia, it will lead eventually to the rumen being flooded with hydrogen sulphide, which seems to favour hypomagnesaemia.

Magnesium fertilizer suppresses the effect of sulphate of ammonia on hypomagnesaemia

BARTLETT'S investigation of the relationship between sulphate of ammonia and hypomagnesaemia provides us with another highly important result: magnesium fertilizer suppresses the tetanigenic effect of sulphate of ammonia. This is demonstrated by Table 24. Plot C had previously received the same dressings of sulphate of ammonia as plot B, but, in addition, it had been the recipient two years previously of a large dressing of magnesite.7 The result was that the herbage contained twice as much magnesium (0-34%, against 0-17%) as on plot B.8 When the cows were put out to graze plot C the mean content of magnesium in the blood serum was 1-83 mg./100 c.c., which is more or less the same figure as was recorded for the control plot A. There were no cases of tetany, whereas there were several on plot B that had received the same quantities of sulphate of ammonia but no magnesium fertilizer.

Comparative effects of two types of magnesium fertilizer on the content of magnesium in the blood serum

An experiment carried out by ALLCROFT confirms these results of BARTLETT's and provides us with some very interesting additional information. The eminent British scientist used four paddocks:
(A) without fertilizer (control);
(B) that had received 6 cwt./acre [760 kg./ha] sulphate of ammonia each year for four years;
(C) same dressings of sulphate of ammonia as paddock (B) plus the application, four years previously, of 10 cwt./acre [1270 kg./ha.] calcined magnesite;
(D) same dressings of sulphate of ammonia as paddock (B) plus the application four years previously of 221 tons/acre [5700 kg./ha] magnesian limestone.9

Figure 17: Comparative effects of calcined magnesite and magnesian limestone on the magnesium content of herbage

Fig. 17 shows that the sulphate of ammonia barely reduced 10 the content of magnesium in the dry matter of the herbage (which remained in the region of 0-12 - 0-15%). The two magnesium fertilizers, on the other hand, almost doubled the magnesium content, although the latter is a little higher (0-24 - 0-27%) with the calcined magnesite than with the magnesian limestone (0-17 - 0-20%).
To turn now to the grazing animal: Fig. 18 shows that although the sulphate of ammonia has not altered the magnesium content of the herbage to any notable extent, it has nevertheless caused a marked fall in the level of magnesium in the blood serum of the grazing animal (paddock B). This is a repetition of BARTLETT'S result that has just been discussed (Table 24). What is of particular interest is the effect of the two types of magnesium fertilizer on the magnesium in the blood serum of the grazing animal:

Figure 18: Influence of magnesium fertilizers on the magnesium in the blood serum of cows

Magnesian limestone (paddock D) eliminates the depressing effect of the sulphate of ammonia on the serum magnesium.
Calcined magnesite (paddock C) does even more: not only does it eliminate the effect of sulphate of ammonia but it succeeds in increasing the magnesium content of the blood serum of the cow after it has been put out to grass, despite the "shock" administered by the young grass.11

It should be stressed that the two magnesium fertilizers had been applied in fairly large quantities,12 and four years before 13 these investigations were conducted into the magnesium content of the blood serum of the animal. This means that the effect of calcined magnesite and magnesian limestone are persistent, which considerably reduces the expense involved, especially in the case of calcined magnesite, which is a relatively expensive fertilizer.

Why does magnesium fertilizer suppress the tetany-producing effect of grass?

These various experiments demonstrate very clearly that magnesium fertilizer is an extremely effective aid towards eliminating, totally or partially, the tetanigenic character of grass, whatever the causes of this may be. There is obviously justification for the question: why does magnesium fertilizer have this effect on tetanigenic herbage?
There appear to be two possible answers to this question.
1. The higher magnesium content represents a "supplement" of magnesium in the herbage 14 harvested by the animal which makes up for the deficit in the magnesium balance, whatever the cause of the latter may be: insufficient magnesium ingested because of the low magnesium content of the herbage, insufficient quantities of herbage harvested, lower resorption level, endocrine disturbance, etc.
2. Magnesium fertilizer alters not only the mineral composition but also the organic composition of herbage. It is possible that the application of magnesium to the soil eliminates or neutralizes a tetanigenic factor such as the "tetany-producing factor" discussed previously.

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Notes



[Click on asterisk (*) at the end of a note to return
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  1.  
 The experiment took place in 1954. This temporary pasture had been sown 
down in 1951 under oats, with the following mixture (in lb./acre [kg./hal]):
 
    •          S. 143 cocksfoot 7 [7-77]
     
    •           S.37 cocksfoot 7 [7-77]
     
    •       S.100 white clover 3 [3-33]
     
    •                        17 [18-87] *

    


    

    
 

  2. 


 From the time the pasture was sown down, i.e. the dressing was applied in 
the three years preceding the experiment. *
 


    

    
 

  3. 


 It should obviously be pointed out that the flora consisted mainly of 
cocksfoot, which appears to be particularly tetanigenic (see Table 4).
 *

    

    
 

  4. 

       
 Note, as BARTLETT did, that sulphate of ammonia caused the white clover 
to regress, and this species is generally richer in magnesium than the 
Gramineae (see Table 25). This alteration in the flora, however, did not 
alter the content of magnesium in the herbage, which is 0.17% both in 
paddock A (without sulphate of ammonia) and 
       paddock B (with sulphate of ammonia). *

 
    

    
 

  5. 

      
 For the influence of sulphate of ammonia on hypomagnesaemia, see Fig. 
18 and Table 20. *
 

    

    
 

  6. 

      
 The sulphur content in the dry matter of herbage was seen, for example, 
to be 0-188% when ammonium nitrate was applied, but it rose to 0-312% 
(i.e. 65% increase) if sulphate of ammonia was used in place of ammonium 
nitrate. *


    

    
 

  7. 

       
25 cwt./acre [3180 kg./ha.] magnesite containing 87% magnesium (MgO). 
*

    

    
 

  8. 

       
 Note that the crude protein content of the herbage on plot C (26.8%) was 
a little  higher than that of the herbage on plot B (23 - 1%). *
       

    

    
 

  9. 

       
 The two types of magnesium fertilizer (calcined magnesite and magnesian 
 limestone) supplied the same quantities of total magnesium per unit area, 
but this obviously does not mean the same quantities of available magnesium.
 *

    

    
 

  10. 

       
 The soil in question was relatively acid. *       

 
    

    
 

  11. 

      
 Which is very obvious on control paddock A, which had received no 
nitrogenous fertilizer. *
       

    

    
 

  12. 

       
 Some authors have observed that even relatively small quantities of 
magnesium (49 lb./acre [55 kg./ha.] MgO) applied in two consecutive years 
were sufficient to raise the magnesium level of the blood, and afford 
protection against grass tetany, although the disease continued to rage on 
neighbouring pastures that had received no fertilizer containing magnesium.
 * 

    

    
 

  13. 

      
 In the case Of BARTLETT'S experiment discussed at the beginning of this 
chapter, the magnesium fertilizer had been applied three years prior to the 
experiment (Table 24). *
 

    

    
 

  14. 

      
 As effective as buccal administration of a magnesium salt. *
       


      



 



         























 

  TABLE 24

Influence of the application of sulphate of ammonia and 
magnesium fertilizer
  on the content of magnesium in the blood serum of cows put out to grass
 




	 
	Mg. magnesium in
	


	 
	% in the dry matter
	100 c.c. blood serum
	


	 
	of the herbage
	  At the end  
	  After being  
	


	 
	Crude
	 
	of stall
	put out
	


	Paddock
	Fertilizer applied
	  protein  
	  Magnesium  
	feeding
	to grass
	


	A
	None (control)
	18-3
	0-17
	2-40
	1-90
	


	B
	Sulphate of ammonia
	23-1
	0-17
	2-37
	0-88
	


	C
	Sulphate of ammonia and
	 	
	


	 
	    magnesite
	26-8
	0-34
	2-32
	1-83
	






 N.B. 1. The protein content is that of the herbage when the animals 
were put out to grass in 1954. The magnesium content is the mean for 1953.


        
     2. Paddocks B and C received 6 cwt./acre [760 kg./ha.] sulphate of 
ammonia each year in 1952, 1953 and 1954. Paddock C, in 1952, received one 
single dressing of 25 cwt./acre [3080 kg./ha.] magnesite, containing 87% 
magnesium (MgO).


        
   3. The magnesium contents of the blood serum are a mean for all the cows.


        
   4. Several cows fell victim to tetany on paddock B. There were no victims 
on paddocks A and C.


        
  5. This was a temporary pasture sown under oats in 1951 with a mixture 
of cocksfoot and S. 100 white clover.


        
 6. For the effect of sulphate of ammonia on hypomagnesaemia, cf. Table 
20 and Fig. 18.

       
From BARTLETT *