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56
Now as regards daily variation in respect to percentages of fat and
non-fatty solids, these are small and unimportant. But if we analyse
morning and evening milk we find an astonishing figure. Dr. Crowther
showed that morning milk was quite frequently below the legal limit in
fat, and in the evening milk the fat was considerably above. The fat
variation as between morning and evening milk is very large relatively.
I shall deal with this important point more fully later.
Let us examine the natural causes of variations.
Take 1. Individuality. Admitting that the milk-yielding capacity
of cows varies greatly, there is no doubt that some difference in the
quality of the milk does take place, though it is a sweeping statement
to make that heavy milk production must necessarily be accompanied
by low quality. The effect of individuality on the composition of the
mixed milk of the herd varies with the size of the herd and is to some
extent controllable by careful selection of the cows for quality of milk.
2. Advance in Lactation. The quality of the milk yielded by the
cow gradually improves as lactation advances, at any rate after the first
month or two. If a herd of cows have calved at about the same time
there will be differences in the composition of the mixed milk at different
periods.
3. Age. Age is frequently blamed for poor quality of milk, but
the decline in quality due to advancing age of the herd is very gradual,
and over a range of ten years probably does not amount on an average
to more than 0.2 or 6.3 per cent. of fat. These are the natural causes
of variation. Turn now to the artificial causes.
The most influential of these is the interval between milkings. If
the intervals between successive milkings are approximately equal, the
morning and evening milks are practically identical in quality. The
longer interval is almost invariably the night one, and so the morning
milk is relatively poor in fat, whilst the evening milk is correspondingly
rich in this ingredient. The remedy is apparently simple, but its application
is said to be beset with difficulties. The consumer wants his
milk from the cow between 7 and 8 a.m., and in the afternoon between
4 and 5 p.m. This means an early afternoon milking, and so if the
intervals between milking are to be equal, there must be milking in the
small hours of the morning. The farmer has difficulty in securing
efficient milkers who will carry out milking at such an early hour.
A second artificial cause for variation is imperfect milking. As the
milking proceeds, the quality improves. Hurried or incomplete milking
leads to a milk of a lower quality.