The Influence of the Administration of Certain Salts on the Yield and Composition of Milk

12. The Mineral Requirements of Dairy Cattle

The most important fact brought out by a review of the present position of our knowledge of mineral metabolism and the mineral requirements in the dairy cow is, that, in spite of the amount of work which has been done on the subject, our information is exceedingly scanty. We know that deficiency of calcium or phosphorus may affect rate of growth, milk yield, breeding capacity, and in all probability susceptibility to certain diseases. We know also that even on what are counted good rations, depletion of the tissues, especially of calcium, in heavy milking cows during lactation is common. In some districts deficiency of iodine causes pathological conditions, and it is common knowledge based on practical observations that even on pasture there is liable to be a deficiency of chlorine. With regard to the possibility of deficiencies of other mineral elements and the effect of such deficiencies on the health and milk production of cows, we have not yet even begun to accumulate knowledge. It is thus impossible to make a final pronouncement with regard to any of the main problems. We need many more data on the metabolism of minerals in the lactating animal and on feeding experiments of long duration with rations of different mineral content. Such information as we have, however, suggests that the subject is one of great potential practical importance, and that if the necessary information were available it might be applied to the more economical production of milk, both by increasing milk yield and by reducing mortality from disease.

With regard to the information we do possess, much of it is not immediately applicable in practice because the data have been accumulated with diets and under conditions which have no parallel in practice. The following conclusions sum up in a general way the practical bearing of our present knowledge.

197. The composition of the blood and milk of lactating cows during inanition, with a note on an unidentified constituent present in certain samples of abnormal milk

1. The yield and composition of the milk secreted during a 12-day fast is recorded. The solids-not-fat, the nitrogen partition, the chloride, lactose, fat and phosphatase contents are discussed.

2. The presence of an unknown substance in the “abnormal” milk obtained from two of the cows is recorded, and it is shown that similar amounts of an unknown constituent must also have existed in the starvation milk analysed in the course of their work by Overman & Wright(i). The nature of this substance has not yet been elucidated.

3. Certain substances in the blood were estimated before, during and after the fast, and the possibility of relating changes in them with corresponding changes in the milk is discussed.

25. The Mineral Constituents and Citric Acid Content of Milk

Though the mineral constituents normally represent less than 1 per cent, of cow's milk their importance in many dairying problems is well realised. For example, the calcium-magnesium-phosphate-citrate balance is a most important factor in connection with coagulation by heat and in the alcohol test, even minute alterations in the balance producing a very marked effect.