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From: Proceedings of John Lee Pratt International Symposium on the Role of Magnesium in Animal Nutrition

Published by John Lee Pratt Animal Nutrition Program, Virginia Polytechnic Institute and State University Blacksburg, Virginia, 1983, Edited by J.P. Fontenot, G.E. Bunce, K.E. Webb, Jr., Vivien Allen

The Effect of Fertilizer Treatment Of Grassland on the Biological Availability of Magnesium to Ruminants


Centre for Agrobiological Research Wageningen, Haarweg - The Netherlands


Hypomagnesemia and hypomagnesemic tetany (kopziekte, grass tetany, grass staggers, wheat poisoning, weidetetanie, tetanie d'herbe, etc.) are brought on by a shortage of Mg due to reduction in the dietary supply of available Mg. About 30 years after grass tetany was first associated with a decreased Mg content of blood serum (41), a great number of feeding trials with milking cows on different rations, led to conclusive evidence that Mg intake and availability of the feed Mg were the two most important factors in causing hypomagnesemia (22, 38). In contrast with Ca and P, adult animals cannot mobilize body Mg sufficiently rapidly to prevent a fall in the blood levels in cases of dietary shortage. Hypomagnesemia and tetany in cows can be induced within some days by a shortage of available Mg. Prevention has to be focussed on a sufficient supply of Mg to meet the requirement for maintenance and production.

It follows from the above, that all factors which affect dry matter intake, Mg content of dry matter and Mg availability play a part in causing hypomagnesemia. Therefore, fluctuations in the botanical composition of the herbage, climatic and soil conditions and fertilizer treatment may be important. This illustrates the complexity of the problem. Some recent reviews on this subject have been published (8, 10, 12, 30, 32, 39, 42, 47).

With respect to the influence of fertilizer treatment on the incidence of hypomagnesemia, there are many contradictory results in the literature. The problem is due at least partly to the fact that in many experiments serum Mg values have been used as a criterion, although the Mg excretion in the urine and even urinary Mg concentration gives more reliable information. For this reason attention will be given in this paper to the relationship between available feed Mg and urinary Mg excretion and to the influence of fertilization on the biological availability of Mg.


Available Feed Magnesium and Urinary Excretion

About 40 years ago, it was already suggested a relationship existed between a low Mg concentration in the urine and the incidence of hypomagnesemic tetany in cows (6, 40). More recently, a close relationship was shown between Mg excretion in the urine and serum Mg levels (9). Moreover, experimental work led to the conclusion that the urine is the main excretory route for Mg absorbed in excess of requirement (38). A great number of balance trials with cows showed a significant correlation between Mg intake minus Mg in the feces minus Mg in the milk and Mg excretion in the urine: Mg in urine (g) = 0.62 ± 0.04 (Mg in feed - Mg in feces - Mg in milk) + 0.90 (20, 22). The conclusion can be drawn from experimental work in different countries that subnormal or low serum Mg values occur only when the excretion of Mg in the urine is less than 1 g/day (9, 22).

Figure 1 shows schematically summarized data of balance trials with over 60 milking cows in the Netherlands, fed on rations consisting of freshly mown herbage from permanent grassland with different fertilizer treatment or of hay and concentrates (20, 22). According to the intake of apparently available Mg (Mg in feed - Mg in feces) being higher, more Mg is excreted in the urine, dependent on the level of milk production. A dry non-pregnant cow requires 2.5 g available Mg per day, to keep intake and excretion in balance (R O = maintenance requirement). The excretion of Mg in the urine in this case is 2.5 g/day, so retention is zero. The accuracy of this figure for maintenance requirement is low in view of its standard deviation of 1 g. German workers reported a similar value, viz. 2.1 g (33). For each 10 kg of milk, 1.2 g of available Mg have to be supplied to meet the requirement, R 10, R 20, etc. (R available Mg = 2.5 + 0.12 M), (M = kg milk per day). Lower intakes result in decreased excretion in the urine and negative retention. When urinary excretion is lower than 1.0 g/day hypomagnesemia and tetany may occur. However, Mg absorbed in excess of requirement is excreted in the urine proportionally and retention remains zero. Determinations of the excretion in the urine after oral or intravenous administration of Mg support these findings.

The preceding illustrates very clearly that the kidneys play a key role in maintaining Mg homeostasis. The relationship between Mg intake, on the one hand, and urinary excretion and serum Mg values, on the other hand, leads to the conclusion that urinary Mg excretion is a better measure to obtain information on Mg status and Mg supply of the animals than the blood serum Mg concentration. For practical purposes, even the Mg concentrations in random samples of urine give more information than serum Mg levels (21). This is shown in figure 2 .

The good relation in figure is mainly due to urinary Mg concentration being rather regular with 1 day (unpublished data, Kemp). Little is gained by expressing the Mg content of random samples of urine on the basis of creatinine content, in the attempt to allow for differences in urine volume, because the individual differences in urinary concentration of creatinine are so great. In the Netherlands the following standards for estimating the Mg supply of cows are used:

Mg in Urine (mg/liter)

Mg supply

More than 100

Adequate to liberal



Less than 20

Severe deficiency, danger of tetany

Investigations of the urinary Mg content also opens interesting prospects in experiments in which the effects are studied of various measures influencing the Mg supply of the animals. Recently a suitable rapid method for Mg determination in urine which is applicable in practice, was developed (28).


Fertilizer Treatment and Available Feed Magnesium

About 50 years ago the first data were available on the effects of fertilizing grassland in relation to the incidence of grass tetany (40). The possible importance was pointed out of the imbalance of input and output of K and P on tetany-prone farms, the input being much higher than the output. A comparison of the chemical composition of herbage from "tetany" and "non-tetany" farms drew attention to the higher K, P and crude protein concentrations and the significantly lower contents of Ca and Mg in tetany prone herbage (16). More recently, grazing experiments and feeding trials carried out in different countries, mainly in Germany, the Netherlands, New Zealand, Norway, United Kingdom and the U.S.A., resulted in a better understanding of the relationships between fertilizer treatment on grassland and the incidence of hypomagnesemia. It is now generally accepted, that heavy dressings of N or K may have an adverse effect on the serum Mg concentration. If heavy phosphate applications do play a part it seems to be one of minor importance.

However, an unfavorable influence of heavy fertilizer treatment on the serum Mg levels was not always found. This is understandable, because the relationship has been studied in experiments in which serum Mg values of number of tetany cases were used as the dependent variable, whereas it is known that the serum Mg values do not reliably reflect the Mg supply or the Mg status of the animals. For instance, when heavy applications of K on grassland reduce the amount of available herbage Mg to an intake which is just enough to meet the requirement of the animal, the serum Mg levels do not change, but urinary excretion falls. However, when K dressing decreases the Mg supply from a level being just enough to meet requirements to amounts which are much too low to maintain a sufficient supply to the animals, both serum Mg level and urinary Mg concentration decrease. Consequently, the extent of the effect of fertilizer treatment on serum Mg also depends on the available feed Mg in herbage from untreated plots. Accordingly the urinary Mg excretion and concentration gives more reliable information about the effect of herbage treatments on the Mg status of the animals. The following discussion on the effect of K, N and Mg fertilizing of grassland on serum Mg levels and on the incidence or non-incidence of tetany must be interpreted in the light of this proposition.


Nitrogen and Potassium

To interpret the effect of fertilizer treatments on the biological availability of Mg, attention has to be given to the influence on dry matter intake, Mg content of the dry matter and on dietary availability of herbage Mg consumed by the animals. Generally, variations in Mg availability are of the same importance for the Mg supply of the animals as the differences in Mg intake.

Much information is available about the effect of N or K fertilizers applied to grassland on the incidence of hypomagnesemic tetany. Tables 1 and 2 (NOT YET AVAILABLE) show only an example of results gained from grazing experiments (23, 25). The experimental findings represent more or less a general opinion in this field. The explanation of the effects of fertilizer treatment shown in these tables, will be discussed also in the light of more recent findings.

The grazing experiment was carried out in 1957 from April 23 to October 24. Sixteen lactating cows were used, four cows grazing without supplementary feed on each plot of the four nitrogen and potassium treatments on permanent grassland situated on sandy soil. The symbols n and k in Tables 1 and 2 indicate low N and low K applications, which per year amounted to 20 kg of N as calcium ammonium nitrate and 20 kg of K2O per ha as potassium chloride, respectively. The symbols N and K imply treatments with 210 kg of N and 200 kg of K2O. Potassium was applied in early spring and nitrogen after each grazing mainly in the spring period. Blood and herbage were sampled twice each week. The sward consisted almost exclusively of grasses, mainly Lolium perenne. In table 2 the mean serum Mg concentrations throughout the grazing period are summarized, while table 1 shows more detailed information on the spring period.

After 16 cows with normal serum Mg levels had started to graze the experimental swards on April 23, four serious cases of hypomagnesemic tetany occurred on May 1 on the plots with treatments nK and NK. On May 3 the serum Mg concentrations of all the cows grazing on these plots were low, the highest value being 8 mg/liter. None of the animals grazing the plots fertilized with nk and Nk plots was found to have a serum Mg value lower than 15 mg/liter. On May 4, the remaining four cows grazing the plots fertilized with nK and NK were transferred to the nk and Nk treated plots in order to prevent possible cases of tetany. At the same time the experimental cows of groups I and II were put to the plots nK and NK. Table 1 now shows a reversed pattern on the subsequent sampling date, May 9. On this date, two more animals displayed marked symptoms of tetany, one grazing the NK treated plot, the other grazing the nK treated plot. After May 9, all the animals were given extra Mg by means of Mg-containing concentrates and within some days all the serum Mg concentrations increased to the normal range.

Table 2 shows the summarized data of the entire grazing period on the experimental plots. In all the periods, both heavy K dressings and high N applications were associated with lower serum Mg concentrations. Wide differences occurred between the nk and NK treatments. During some short periods in May, July and September, all the cows were grazing together in a pasture away from the experimental plots. At the end of the periods the mean serum Mg values of the four groups did not differ significantly.

Although many questions remain unanswered about the relationship between nitrogen and potassium fertilizer applications to grassland and the incidence of hypomagnesemia and hypomagnesemic tetany, much information has been gained, which has led to a better understanding. Intensification of grassland management by means of increasing applications of nitrogenous fertilizers will increase herbage production and will change the botanical and chemical composition of the forage produced. Clover and herbs will be suppressed and the sward will consist mainly of grasses. A predominately grass sward provides a lower Mg concentration in the forage consumed by the animals, even although in a grassy sward heavy N dressings mostly increase the Mg content in the grass slightly. Although the Mg concentrations between grass species may differ considerably, legumes and herbs contain more Mg than grasses (1). A mainly grassy sward will therefore be more "tetany prone" than a mixed sward. Grazing experiments on pastures containing different amounts of clover have shown higher serum Mg levels in cows grazing on the clover rich sward (2). There is probably no information on the availability of Mg from clover and herbs for ruminants. However, it might be possible that Mg from clover is less available than that from grasses, because the N concentration in clover is higher than that in grasses, which goes with higher contents of higher fatty acids (H.F.A.) (31). It is likely that higher concentrations of H.F.A. in the forage decrease the availability of feed Mg by the formation of insoluble Mg soaps in the gastrointestinal tract (49).

Increasing amounts of nitrogenous fertilizers applied to grassland result in higher concentrations of N and the same or slightly increased Mg in the forage, if harvested in the same stage of growth. As the herbages grow older, crude protein (N x 6.25) and Mg decrease, resulting in an increasing availability of the forage Mg for ruminants. This relationship between crude protein intake and the availability of feed Mg has been shown in feeding experiments in different countries (22, 36). Table 3 (NOT YET AVAILABLE) presents one of these experiments with two lactating cows, given rations consisting of freshly mown herbage, cut from the same field in different stages of growth (22). The crude protein contents of the herbage fed in the three experiments decreased from about 260 to 140 g/kg and the Mg contents were also lower as the herbage matured. Normally the K content also falls, but this decreasing trend was prevented by means of higher K applications on some strips of the experimental pasture. The daily Mg intake decreased as the herbage matured. The amount of Mg excreted in feces decreased distinctly and the differences in availability of feed Mg changing from 10% for the young grass to 20% for the older grass, were very striking. These increasing values for availability were associated with increasing urinary excretion of Mg. The secretion of magnesium in the milk remained almost the same throughout. The Mg retention changed from 1 g/day to about zero. These data emphasize the importance of high availability of feed Mg.

To explain these results it has been suggested that an inadequate absorption of Mg would probably be associated with high ruminal ammonia production (17). However, the fact that immature forage diets are accompanied by low Mg availability might not be a direct result of the forage N level. Several metabolism studies in which N was added to the rations in the form of non-protein N did not affect Mg absorption (11, 35). This is in agreement with the results of our experiments with milking cows which were given extra N in the form of ammonium acetate. The apparent Mg absorption did not change. It is perhaps more likely that the lower utilization of Mg in ruminants consuming high N forage is due to a change in the chemical composition of the herbage other than the increase of N content. In this respect the influence of N application to grassland on the K and H.F.A. concentration in the herbage requires attention.

With regard to the effect on the H.F.A. content, figure 3 shows the relationship between crude protein and H.F.A. in fresh grass and in hay (19). An increase in the crude protein content in the herbage is associated with an increasing concentration of H.F.A. Several other workers confirmed this relationship also in forages other than grasses (3, 31, 34). Literature data from almost 40 years ago in fact suggested, that fatty acids might affect Mg and Ca utilization (4, 5, 7). More recent experimental work confirmed this hypothesis. Increasing amounts of H.F.A. added to the rations of ruminants resulted in a lower apparent Mg availability or in a reduction of the serum Mg levels (19, 48). Although more information is needed, these results suggest that the adverse affect of N fertilizer treatment of grassland on the Mg utilization may be at least partly explained by an increase in the higher fatty acids content of the herbage. The adverse effect of the formation of insoluble Mg soaps on Mg absorption is supported by recent research indicating that the rumen is an important site of net Mg absorption (14, 15, 24, 37, 45, 46).

Another explanation of the adverse effect of heavy nitrogen fertilizer application on the Mg utilization is to be found in the K contents in the herbage. Figure 4 shows the effect of nitrogen application on the K content in herbage (25). The increase in K contents in the herbage after N dressing was found to be higher with an increasing K level in sward and soil. It seems that a low K level may lead to a decrease in the K percentage of the herbage when N is increased by fertilizer application. When the K content in the herbage exceeds 20 g/kg, K increases as a result of an increase in N, viz. There has been a relatively greater increase in K uptake than in dry matter production. Below 20 g/kg K, an increase in N application is accompanied by a fall in herbage K. At such a low K level the total K uptake is relatively less than the dry matter production, so that the K content in the herbage falls as a result of increasing the N application. Under practical conditions, however, when the K supply to the herbage is sufficient for maximum grass production, there will be mostly an increase in the K content in the herbage by higher N fertilizing, because in this situation K in the herbage will be higher than 20 g/kg.

Much information is available on the unfavorable effect of a high K intake on the utilization of Mg. In a number of experiments, feeding high levels of K resulted in a depression in blood serum Mg (13, 29, 43, 44). We have already discussed some reasons why dosing animals with extra K may not result in a fall in serum Mg levels. More important is that increasing amounts of K in the rations showed an increased fecal Mg excretion, consequently a lower apparent Mg availability and a decreased urinary excretion (11, 18, 22, 44).

Potassium applications to grassland increase the K content of herbage. Although the adverse effect of K fertilizer has been explained partly by a reduction in the availability of feed Mg, the effect of K fertilizer on the Mg concentration in the herbage also plays an important part, because it may lead also to a fall in the dietary Mg intake. Figure 5 shows the effect of K fertilizing on the Mg concentration in herbage (25). Generally, a heavy K dressing reduces the Mg content in herbage in many cases by about 15 to 20%, which results in a considerably lower daily Mg intake. Recapitulating, the effect of K fertilizer on the Mg supply to grazing cattle shown in tables 1 and 2, in so far as is presently known can be explained by a reduction in the Mg intake and by a fall in the Mg availability.

Besides the effect of N and K fertilizer on the N, K, Mg and H.F.A. concentrations in grass, changes in other constituents in the plant might be of importance, as e.g. Na, Ca, P, carbohydrates and organic acids. With regard to Na, heavy K applications may decrease the Na content in herbage very considerably. Nitrogenous fertilizers may increase Na in herbage, the response being less with higher soil and plant K status (21). Although there has been much discussion in the literature on the possible effect of Na in the diet on Mg utilization in cattle, grazing experiments (26) and feeding trials with milking cows have shown that if Na plays a part it will probably be of minor importance. Balance trials with high yielding milking cows on rations containing insufficient, sufficient and an excess of Na to meet the animals' requirement, did not show any difference in apparent Mg availability (unpublished data, Kemp). Increasing amounts of nitrogenous fertilizer generally decrease carbohydrates in plants and increase the concentrations of Ca, P and organic acids. Potassium applications decrease herbage Ca. More information about the possible role of these constituents in affecting Mg availability will be given elsewhere in these proceedings.

Only limited information is available on the relationship between fertilizer treatment and dry matter intake of ruminants. Under conditions of grassland management at low nitrogen applications and at low production levels, higher amounts of nitrogenous fertilizer may increase grass production. This increased production consists of higher digestible material, which goes together with higher dry matter intakes. On the other hand, however, there are indications that the dry matter intake of very young grass heavily dressed with N fertilizer, will be somewhat reduced, especially under wet conditions.

The preceding shows that the relationship between nitrogen and potassium fertilization of grassland and the chemical composition of the herbage on the one hand, and the blood serum Mg concentration and incidence of tetany on the other hand, is rather complicated. The nomogram in figure 6 summarizes that relation and gives an impression of the effect of variations in Mg, crude protein and K contents of herbage on the blood serum Mg values. Detailed information on the experimental data and the statistical treatment has been published elsewhere (25). The relationships presented here, are better founded and more reliable than the ratio K/Ca + Mg) in relation to the incidence of grass tetany, which was published previously (27). There is no conclusive evidence that the Ca concentration in the forage should affect the Mg supply of the animals. Both Mg and K concentrations in forage are directly involved in the Mg supply, affecting both Mg intake and absorption, respectively. Dietary N is probably not directly involved, but is correlated with H.F.A. and perhaps with other factors which may be of importance. The nomogram was developed to make an assessment of the magnesium supply of grazing milking cows. In grazing beef cattle, the estimated serum Mg values will be higher because the Mg requirement of these animals is lower.


Magnesium Fertilization, Grassland Production and Hypomagnesemia

Besides variations in the availability of feed Mg, the daily intake is essential in the origin of hypomagnesemia. Improvement of the availability is as yet not realizable. On the one hand, there is still a shortage of information on what factors influence availability, while on the other hand a considerable improvement in availability seems to be associated with a potentially large decrease in grass yield which may not be economically advisable. Of course, it is worthwhile to restrict K fertilizer on grassland to applications just adequate to obtain maximum production. However, increasing the availability of herbage Mg by decreasing N fertilizer application results in a severe drop in grassland production. In the intensive highly producing grassland systems the Mg supply in many instances is insufficient to meet the requirement of grazing high producing milking cows. Even in winter, when in situations where the rations consist mainly of home grown forage, attention has to be given to the Mg supply.

The preceding shows that the Mg supply has to be improved first of all by increasing the daily intake of dietary Mg. This can be achieved by increasing the Mg content in the forage or by providing supplementary Mg. It should be realized, however, that under intensive farming conditions the daily Mg intake should be increased by at least 30 g per cow, taking into account that the same availability applies to the grass consumed and the supplementary Mg. It should be realized, however, that under intensive farming conditions the daily Mg intake should be increased by at least 30 g per cow, taking into account that the same availability applies to the grass consumed and the supplementary Mg. If the availability of Mg in young grass is 10% only 3.0 g is available of these 30 g of supplementary Mg, supplied as calcined magnesite (unpublished data, Kemp). In the Netherlands this preventive measure has been successfully used on farms for a long time and low serum Mg levels seldom occur.

Increasing the Mg content in the herbage by Mg fertilizer offers only limited prospects, because the Mg content in the herbage cannot be increased to such an extent that the Mg supply to the animals is always safeguarded. On light sandy soils very heavy dressings of 300 to 400 kg of kieserite per ha per year increase the Mg contents in the herbage from 1.5 to 2.0 g Mg/kg dry matter. The daily Mg intake is increased in this way by only about 7.5 g per cow. On clay and peaty soils, the response to Mg fertilizer is much lower. In the Netherlands, Mg application to sandy soils to prevent hypomagnesemia is common practice. However, the administration of 30 g of Mg is both more common and successful.



About 30 years after grass tetany was first associated with a decreased Mg content in the blood serum, experimental work in different countries led to the conclusion that Mg intake and availability of the feed Mg are the two most important factors in causing hypomagnesemia.

Contradictory results in the literature on the influence of fertilizer treatment on the incidence of hypomagnesemia are at least partly due to the fact that frequently only serum Mg levels have been used as a criterion, although the Mg excretion in the urine gives more reliable information.

Intensification of grassland management by means of increasing applications of nitrogenous fertilizers changes the botanical and chemical composition of the herbage, leading to a decrease in dietary Mg intake and in the availability of herbage Mg. Large applications of potassium lead to a reduction in the Mg content in the herbage and in decreasing availability. Increasing the Mg content in the herbage by Mg fertilizer application offers only limited prospects, because the Mg content in the herbage cannot be increased to such an extent that the Mg supply to the animals is always safeguarded.

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This page was first uploaded to The Magnesium Web Site on January 30, 2001