The composition of the milk of young and of old cows

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Abstract
Summary The milk produced by old cows frequently contains a lower concentration of solids-not-fat (S.N.F.) than that of young cows, mainly because of a low concentration of lactose. This can result from mastitis or possibly from other factors associated with ageing. The composition of the milk of 2 groups of old cows has been compared with that of substantially disease-free first-lactation cows. A comparison has also been made between the milk from the 4 quarters of each cow. The first part of the work was done with 8 old cows and 4 first-lactation cows, and the milk was examined bacteriologically, cytologically and chemically at frequent intervals throughout the lactation. There was strong evidence that most of the difference in chemical composition of the milk of the old cows compared with that of the young cows was the result of mastitis, mainly subclinical. The milk from the old cows had a higher incidence of infection, a lower concentration of lactose and casein, higher concentrations of serum albumins, globulins and proteose-peptones and contained more total and polymorph cells. There was also some indication of another effect, not directly associated with disease in the current lactation, that led to a decrease in the lactose concentration of the milk of the old cows after about 80–90 days in lactation. In the second part of the work, the milk from 23 old and 8 first-lactation cows in 8 commercial herds was examined twice, once after 58 and again after 163 days in lactation. Milk of abnormal composition as a result of disease was produced by 30% of the quarters of the old cows at the first examination and by 62% of the quarters at the second examination. Some indication is given of the probable loss of milk and milk constituents that can be attributed to disease.