1. During 1950–53 the milk of 814 Ayrshire cows was sampled six times during the lactation at intervals of approximately 5 weeks, starting towards the end of the first month of lactation. The weight of milk at successive evening and morning milkings was recorded and samples from each mixed in proportion to the yield. The milk was analysed for total solids, fat, S.N.F. by difference, crude protein (N×6·38), casein and lactose.
2. The weighted lactation average was calculated for each constituent of the milk of each cow and used in genetic studies (see Part II), and in assessing the effect of age on milk composition. The analyses of the individual samples were used to determine the separate effects of stage of lactation and of season on milk composition.
3. Advancing lactation caused the following changes in milk composition:
(a) Milk yield was highest 45 days after calving and then fell regularly to the end of lactation.
(b) Total solids, S.N.F., fat, crude protein and casein contents fell rapidly for 45 days, with fat and total solids continuing to fall for a further 30 days. The concentrations of all these constituents then increased continuously for the remainder of the lactation, rising more rapidly after about 200 days.
(c) The changes in lactose content were opposite from those of fat and protein and smaller in magnitude. The changes bore a marked resemblance to those for yield. The value rose to a maximum after 45 days, fell slowly until about 165 days after calving and then more quickly. The lactose content of milk from cows in their first lactation fell much more slowly with advancing lactation than that in the milk of older cows.
4. Seasonal effects were of smaller magnitude than those arising from advancing lactation and caused the following changes in milk composition:
(a) Yield rose steadily from January to the May-June period, and then fell to a minimum during October-November.
(b) Fat content was at a maximum in October, falling steadily to a minimum in June.
(c) Crude protein and casein contents rose to a peak in May-June and again in September. Lowest values occurred from January to March.
(d) Lactose content was at a steady high level from January to June, falling to a lower level by August at which it remained for the rest of the year. Again there was some similarity in the pattern of change in lactose content and in yield. The range of variation in the values for lactose was less than those for protein and fat, and the variations were mainly opposite in sign.
(e) Total solids and S.N.F. were at a minimum in March and April, but whereas S.N.F. reached their highest level in May-June, total solids were highest in October.
5. Increasing age of the cow resulted generally in increasing yields of poorer quality milk. The difference in composition between milk from the first and the grouped ninth and later lactations was fat 0·19%, S.N.F. 0·34%, crude protein 0·08%, casein 0·21% and lactose 0·25%.
6. On the evidence of cell counts, a number of the samples came from cows with some inflammation of the udder. The average casein numbers, however, did not indicate any serious incidence of mastitis, and it was concluded that although lactose contents may have been lowered slightly, the effect of disease on the average composition was small.
7. Although the figures are not closely comparable, it appears that there has been little change in the composition of the milk of Ayrshire cows in Scotland since 1921–22.
8. The number of individual cow samples deficient in fat was 2% and in S.N.F. 16% of the total. In 560 samples of farm bulk milk none were deficient in fat and only 1·6% deficient in S.N.F. May, June and July were the months with most fat-deficient samples, and the period from January to April provided the largest number of samples deficient in S.N.F., closely followed by the period July to October.
9. Possible causes of the seasonal changes in milk composition are discussed. These changes are thought to be caused mainly by feeding although this does not give a complete explanation of all variations.