CHAPTER 27

Effects of potassium fertilizers on the magnesium in the blood serum of the grazing cow

Under certain conditions potassium fertilizer favours hypomagnesaemia and tetany

In the spring of 1957 two groups of cows (I and II) were put out to graze plots A and B that had received small and large quantities respectively of potassium fertilizer. When Group II was put on the Plot B that had received a heavy dressing of potassium the magnesium content of the blood serum of the cows fell steeply (0-58 on 3rd May against 2-40 mg./100 c.c. before the animals were put out to grass). On 1st May THREE Of the cows in Group II were attacked by grass tetany. In Group I, on the other hand, which was put out to graze plot A that had received only small dressings of potassium fertilizer, the diminution in the magnesium content of the blood serum of the cows was relatively small (2-18 on 3rd May against 2-52 mg./100 c.c. before grazing) and none of the cows was attacked by tetany.
Another section of these observations illustrates well the extent to which the application of potassium fertilizer to the soil exerted a profound influence on the magnesium of the blood, under the conditions of this experiment. On the evening of 3rd May the cows were reversed on the plots:

1.	Group I on plot A with the low dressing of potassium fertilizer was shifted to plot B with the high dressing of potassium fertilizer. On 9th May, that is, 5 days later, the mean level of magnesium in the blood serum, which until then had remained more or less normal at 2-18 mg./100 c.c., fell to the low level of 0-93 mg./100 c.c., and one cow was attacked by tetany.
2.	By contrast Group II, on plot B with a heavy application of potassium fertilizer, shifted to plot A that had received a low dressing of potassium fertilizer. The mean magnesium level rose again rapidly, and 5 days later (9th May) had almost doubled, becoming 1-10 mg./100 c.c. against 0-58 mg./100 c.c. when the change from one plot to another took place. In addition, there was not one single tetany victim.

The effect of potassium fertilizer on the magnesium of the blood serum may not be immediate

Note that in 1956, the year preceding the present experiment, although the herbage, with or without potassium fertilizer, was similar in composition to that in 1957, there was no marked hypomagnesaemia among the cows, even on the plots that had received potassium fertilizer, and not a single case of tetany was reported. It appears, therefore, that the effect of potassium fertilizers on the magnesium of the blood serum may not be immediate and that "conditioning"⁷ of the soil takes place, although this is not obvious in the composition of the herbage. Be that as it may, the observations made by KEMP in 1956 show that experiments of relatively short duration may be incapable of revealing the effect of potassium fertilizers on the magnesium of the blood serum of the grazing animal.

The effect of potassium fertilizer on hypomagnesaemia and tetany may be evident only after nitrogen has been applied

An experiment undertaken by SMYTH, CONWAY and WALSHE, in Ireland, shows that potassium fertilizer, in certain circumstances, may have an effect on hypomagnesaemia and tetany only if, at the same time, dressings of nitrogenous fertilizer are applied. The experimental pasture in question was a temporary sward of Italian rye-grass,⁸ which cows were suddenly put out to graze after being indoors all winter. The rye-grass on the control plot (No. 1) was fairly low in magnesium: 0 - 14% in the dry matter, which is below the safety limit of 0-20%. The potassium content was already high, 2 - 6 % in the dry matter, while the content of crude protein was average (15.4% in the dry matter). At the end of the stall-feeding period the mean magnesium content of the blood serum of the cows was 2-25 mg./100 c.c. When they were put out to graze the control paddock (No. 1) that had received neither potassium nor nitrogenous fertilizer, the magnesium content of the blood serum fell to 1-50 mg./100 c.c. (see Table 23).

TABLE 23: Combined influence of nitrogen, magnesium and potassium fertilizers on the magnesium content of the blood serum of cows

The single dressing of potassium fertilizer (plot No. 2) increased the potassium content of the rye-grass, which then exceeded 3%.⁹ But this increase in potassium alone was not sufficient to accentuate the hypomagnesaemia. Nitrogenous fertilizer, in the form of calcium nitrate and ammonium nitrate, applied alone, considerably increased the crude protein content of the rye-grass, which reached the high level of 25-6% ¹⁰ (plot No. 3). The magnesium content of the blood serum, however, remained the same as for the cows on the control plot. It must be emphasized that none of the cows on plots Nos. 1, 2 and 3 was attacked by tetany.
But when equal quantities of potassium and nitrogenous fertilizers were applied simultaneously (plot No. 4) both the potassium and crude protein contents of the rye-grass increased. The magnesium content of the blood serum of the cows fell to the very low mean level of 0-70 mg./100 c.c., and two of the three cows were attacked by tetany. Under the conditions of this experiment the imbalance in the mineral composition, not to mention a possible alteration in the organic composition of the grass, caused by the application of potassium fertilizer alone, was not sufficient to cause a marked drop in the magnesium content of the blood serum of the grazing animal. There must be a simultaneous increase in the protein content of the herbage due to the fact that nitrogen dressings "strengthen"¹¹ the effect of potassium fertilizer, before marked hypomagnesaemia and tetany appear. This explains how the simultaneous application of excessive quantities of nitrogenous and potassium fertilizer to the pastures was able to promote the development of grass tetany.

Magnesium fertilizer cancels the tetanigenic effect of combined applications of nitrogenous and potassium fertilizers

In the course of this same experiment the Irish investigators established that magnesium fertilizer ¹² cancelled out the hypomagnesaemic effect of combined nitrogen and potassium fertilizer dressings. This triple fertilizer combination was applied to plot No. 5 (Table 23). The potassium and crude protein contents of the herbage were more or less the same as for plot No. 4, to which equal dressings of nitrogen and potassium had been applied. The magnesium content, however, was above the safety limit of 0-20%, and the magnesium content of the blood serum ¹³ did not fall when the animals were put out to grass and there were no cases of tetany. This experiment provides a good illustration of the fact that magnesium fertilizer is a first-class weapon against grass tetany. It confirms, moreover, what was stressed in the previous chapter (see Fig. 16), namely, the importance of not allowing the magnesium in the dry matter of herbage to fall below 0-20%.

Three lessons to be learned concerning the effect of potassium fertilizers on tetany

Only these experiments will be quoted here, despite the fact that there are many others available confirming that potassium fertilizers favour hypomagnesaemia and tetany. The various results reported are sufficient to provide us with the following information:

1.	The effect of potassium fertilizer on hypomagnesaemia and tetany cannot be immediately obvious. A cumulative "conditioning" of the soil takes place, as well as "conditioning" of the animals.
2.	There are instances in which the effect of potassium fertilizer is only manifested after nitrogenous fertilizer (and this does not exclude the effect of other fertilizers) has been applied.
3.	The application of magnesium fertilizer in sufficient quantity and in the required form cancels out the effect of potassium fertilizer, alone or reinforced by nitrogenous fertilizer, on hypomagnesaemia and tetany.

It will be understood that other mineral and organic imbalances may enter into play.

It is not by closing our eyes to the facts that we will solve the problem of grass tetany

Desperate attempts have been made to try to prove that potassium fertilizer is not one of the causes of grass tetany.¹⁴ The general conclusion reached by WALSH ¹⁵ appears reasonable:

"There is no doubt that fertilizer use plays a very important part in the development of hypomagnesaemia in livestock ... closing our eyes to the fact that fertilizer use does play an important part is not going to give us the required answers."

Not only is this perfectly correct, but, in the author's opinion, "facing facts" is the most certain method of promoting the application of potassium fertilizer to pastures as and when they are required. There is no question of giving up using "artificial" potassium fertilizer or the "natural" fertilizer, liquid manure, that is so rich in potash. Their possible disadvantages must be studied and overcome in such a way that they can be applied to the benefit of, and without danger to, the animal. An extraordinary propaganda campaign has succeeded in convincing the farmer that he should apply greatly exaggerated quantities of potassium fertilizer to his pastures. The results with stock have been disastrous, and in some areas grass tetany is described by farmers as "potash paralysis". In these areas farmers refuse, and may long continue to do so, to apply potash to their grassland, even if it is required. The overskillful propaganda, therefore, succeeded in boosting immediate sales of potassium fertilizer, but in the long run it has harmed the trade, and may cause great harm to potassium fertilizers in the future unless the facts as they stand are rapidly taken into account.

Grass tetany caused by liquid manure is particularly severe

Before leaving the subject of potassium fertilizers and grass tetany a word must be said about tetany caused by liquid manure, or, to be more exact, organic liquid fertilizer.
Grass tetany caused by liquid manure (or Gulle) is an almost unknown subject. It is mentioned only rarely in the various publications, but has, nevertheless, always been known. One need only study Table 11 to understand that imbalances in the composition of herbage are much more profound following repeated applications of liquid manure than after heavy dressings of potassium fertilizer. As a result, grass tetany caused by liquid manure is particularly rapid and severe, and this, paradoxical as it may appear, has contributed towards this form of tetany remaining relatively unknown. What happens is that the farmer, without having noticed any abnormal symptoms, suddenly finds one of his beasts dead in the field. He has no very good explanation for this casualty and vaguely attributes it to the "fire" of the grass. He is often unwilling to call in a veterinary surgeon for a sick beast, and is certainly not going to pay for a visit merely to confirm a death. Consequently, no one knows that liquid manure tetany is raging on such and such a farm.

Incidence of grass tetany caused by liquid manure

The first accurate and sustained observations on grass tetany, made in 1931 by the Dutch research worker SJOLLEMA, referred almost exclusively to pastures that had received large quantities of organic fertilizer, farmyard or liquid manure.
A very recent German study reports that on a pasture that had received very large quantities of liquid manure 14 out of a herd of 25 cows were attacked by tetany, 7 of them subsequently dying.
One must go to the areas, mountainous or not, where liquid manure is spread in quantity, as, for example, Herve in Belgium, to understand to what degree excessive rates of liquid manure application to pasture can promote the development of grass tetany. The situation is obviously aggravated if potash is applied simultaneously.¹⁶ In Holland, for example, SJOLLEMA established that on farms subject to tetany the animal, owing to the combined dressings of potassium fertilizer and liquid manure applied, was absorbing daily quantities of potassium ¹⁷ equivalent to ten to twelve times its requirement. To quote another result, KEMP found low magnesium contents in the blood serum of cows grazing a sward that had received large quantities of liquid manure; at the same time the amount of magnesium excreted daily in the urine diminished to almost nothing.
The importance of grass tetany caused by liquid manure is confirmed by the statistical data compiled by LARVOR, BROCHART and THERET relative to the development of grass tetany in France. They found that 35% of the known cases of tetany were associated with pastures that had received too much liquid (or farmyard) manure.

"Protective" measures against grass tetany caused by liquid manure

The simple solution would be to give up using liquid manure. Mary CHERRY, of the Farmer and Stockbreeder, referred very recently to the gravity of grass tetany in Holland caused by the application of liquid manure, stating that agricultural advisers in that country were asking farmers to stop applying liquid manure altogether to such pastures. This is often difficult, if not impossible, because when the tanks are full, the manure has to be spread. In addition, farmers often find it impossible to make dung with this material.
Of course, liquid manure could be lost and got rid of in drains and ponds, but this is not without its dangers, both to Man ¹⁸ and animals.¹⁹
As has been stated, therefore, it is not a question of giving up the use of liquid manure as well as of potassium or nitrogenous fertilizers. They must be suitably applied. Some of the methods of remedying the defects of liquid manure have already been described, particularly the application of necessary addition of compensatory mineral elements (Table 12).
From the practical point of view there is an old rule, and that is to mow pasture that has received liquid manure. This reduces the risk to the stock, but does not prevent the hay harvested from having a very unbalanced mineral composition. On purely grassland farms, moreover, it is impossible always to mow before grazing.
In certain mountain regions farmers apply the rule of only grazing for a few hours pastures that have received large quantities of liquid manure (or Gulle). ²⁰ For the rest of the day the animals are put on to swards receiving no organic fertilizer. This method also, however, is not always easy to apply: which confirms the author in his opinion that the most efficient method is to correct the liquid manure (or the soil) by adding to it the necessary mineral elements.

1. During the preceding year (1956) and the year of the experiment (1957). *

2. For the possible influence of this ratio on tetany, given certain conditions, see Table 21. *

3. The minimum safety level is known to be 0-20%. *

4. It is unwise to fall below 0-25%. *

5. A maximum K : Na ratio of 8, if possible even 5, must not be exceeded. *

6. If this herbage had an unbalanced mineral composition, its crude protein content, although high (19.2%), did not reach very high levels. *

7. Other circumstances, such as climatic conditions, may intervene. There may also be a "conditioning" of the cows, i.e. progressive deterioration of the liver, endocrine system, renal filter, etc., as has frequently been pointed out. But this could be only partly the case here. In fact, KEMP, in a letter to the author dated 1st December 1960, stated that only some of the cows that had been grazing in 1956 were used in 1957. Of the six cows attacked by tetany in the course of the 1957 experiments, only two had been used in 1956.
   This obviously does not exclude the possibility that the new cows used in 1957 might have been grazing pastures in the previous year that "pre-disposed" them to tetany. *

8. For tetany caused by rye-grass, see Chapter 30. *

9. At the same time there was a slight increase in the content of protein. The magnesium content remained the same. *

10. There was little change in the potassium content in relation to the control. The magnesium content was higher. *

11. Probably by increasing the production of ammonia in the rumen, the effects of which have been discussed. *

12. 312 lb./acre [350 kg./ha.] were applied of magnesium (Mg), half in the form of magnesium sulphate and half in the form of calcined magnesite (magnesium silicate). *

13. It will be seen in Table 24 that the application of magnesium fertilizer also cancelled out the effect of sulphate of ammonia on hypomagnesaemia and tetany. *

14. See, for example, ALTEN'S experiments carried out at the German Potash Syndicate's Experimental Centre at Buntehof, and the severe criticism levied against them by the Dutch research worker, DE GROOT. In general, investigators wanting to prove that potassium fertilizers play no part in grass tetany have placed themselves in such circumstances that the other factors present would not allow the tetanigenic effect of potassium to be manifested. It has just been seen that this effect can only become obvious under conditions which, unfortunately, are frequently encountered in actual farming practice. *

15. In the course of the discussion following CUTHBERTSON'S paper given at the Potash Symposium, 1960. Replying to a speaker trying to prove that potash is not the cause of tetany, Mrs. ALLCR0FT, of the Veterinary Research Centre, Weybridge (Great Britain) said: "I cannot agree with Professor Alten that heavy dressings of nitrogen and potassium fertilizers on pastures do not increase the incidence of hypomagnesaemia in dairy cows." WALSH then made the statement cited. *

16. See Soil, Grass and Cancer (51), where the author records a personal enquiry into a case of this kind. *

17. 500-600 gm. per day (or 26 lb. [12 kg.] dry matter containing 4-5% potassium). *

18. Contamination of streams. See Soil, Grass and Cancer (p. 252). *

19. Particularly with regard to Johne's disease or paratubulerculosis enteritis. *

20. Not forgetting that cows find it repugnant to a certain extent to graze a sward that has recently received organic liquid fertilizer. *