ABSTRACTS
245
In the case of half-grown animals good pasture supplies an abundance of all the mineral elements
necessary to maintain optimum growth and health. In the later stages of development, both growth
and health may be adversely affected when the animals are grazing in specially deficient areas, as, for
example, in the phosphorus-deficient areas in South Africa. There is, however, no direct evidence
to show that pastures grazed by cattle are so poor in calcium that this deficiency affects the rate of
growth and health. The fact that there are no observations on record, however, cannot be taken as
evidence that such cases do not occur. In indoor feeding of half-grown animals on the ordinary
rations used in practice, there is no evidence that deficiency of calcium or phosphorus occurs to such
an extent as to cause obvious signs of malnutrition or retardation of growth.
In milk cows, good pasture, which resembles milk closely in its mineral composition, supplies both
calcium and phosphorus in the proper proportions. Further, the factors which make for assimilation
of calcium are present. On poor pastures, however, deficiency of phosphorus may be so extreme as
to limit milk production. The direct feeding of inorganic phosphates may make good this deficiency
and lead to increased milk yield. There are no observations to show whether or not deficiency of
calcium in poor pastures may be so extreme as to affect milk yield.
In indoor feeding of milk cows the cereals and other concentrates used, though relatively rich in
phosphorus, are deficient in calcium. Under experimental conditions with diets specially poor in
calcium, this deficiency adversely affects milk yield and reproductive capacity.
In the lactating animal there is difficulty in ensuring that the amount of calcium assimilated will
equal the amount of calcium lost in the milk and excreta. In most of the observations which have
been made during the period of heavy yield there is a continuous negative balance. On rations with
a low calcium content this negative balance may be reduced by the addition of calcium salts. The
presence in the ration of a sufficient amount of calcium must be regarded as the most important
factor in maintaining calcium equilibrium. Factors which aid assimilation are of secondary
importance with a sufficient supply of calcium, but become of primary importance if the supply
is likely to be inadequate, as must very often be the case at the height of lactation. Optimum
conditions for calcium assimilation would appear to be obtainable by feeding fresh green leguminous
pasture, or failing that, leguminous hay cured with a minimum of exposure and by exposing the
animals to sunshine and providing for a certain amount of exercise. Even then the addition of a
calcium supplement with or without cod-liver oil would probably increase the amount assimilated.
On the whole, the evidence indicates that in most heavy milking cows, there may be a
progressive depletion of calcium from the tissues, so that at the end of lactation there is
actually less calcium in the animal's body than at the beginning. In the case of dry cows,
there is no difficulty on good rations in obtaining a positive calcium balance, and it has been
recommended, with reason, that before the onset of lactation a diet with generous supplies
of calcium and phosphorous rich foods should be given so that reserves may be built up to the
highest limit before the onset of lactation.
Shortage of calcium with the resulting depletion has been shown to have an adverse effect
on reproduction.
With regard to other minerals, practical observations have shown that both on grazing and
on indoor feeding, there is liable to be a deficiency of chlorine which, however, can be easily
made good by the provision of ordinary rock salt or salt bricks, and this is usually done
under good farming conditions. In some districts deficiency of iodine has affected reproduc-
tion, and there is some recent evidence to show that it may also affect milk yield. To what
extent, however, this deficiency actually occurs in practice is as yet unknown.
Concerning the many other minerals present in milk we have almost no information as to either
the amounts present in the foodstuffs or the requirements, and there is no information on which
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