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
A. KEMP
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
|
|
20-100
|
Inadequate
|
|
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.
Summary
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.
References not yet available in this online version.
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