Of digestible nitrogen substance the food supplied 2.64 ℔ per day, whilst the amount estimated to be required for sustenance merely is 0.57 ℔; leaving, therefore, 2.07 ℔ available for milk production. The 23.3 ℔ of milk yielded per 1000 ℔ live-weight per day would, however, contain only 0.85 ℔; and there would thus remain an apparent excess of 1.22 ℔ of digestible nitrogenous substance in the food supplied. But against the amount of 2.64 ℔ actually consumed, Wolff’s estimate of the amount required for sustenance and for milk-production is 2.5 ℔, or but little less than the amount actually consumed at Rothamsted. On the assumption that the expenditure of nitrogenous substance in the production of milk is only in the formation of the nitrogenous substances of the milk, there would appear to have been a considerable excess given in the food. But Wolff’s estimate assumes no excess of supply, and that the whole is utilized; the fact being that he supposes the butter-fat of the milk to have been derived largely, if not wholly, from the albuminoids of the food.

It has been shown that although it is possible that some of the fat of a fattening animal may be produced from the albuminoids of the food, certainly the greater part of it, if not the whole, is derived from the carbohydrates. But the physiological conditions of the production of milk are so different from those for the production of fattening increase, that it is not admissible to judge of the sources of the fat of the one from what may be established in regard to the other. It has been assumed, however, by those who maintain that the fat of the fattening animal is formed from albuminoids, that the fat of milk must be formed in the same way. Disallowing the legitimacy of such a deduction, there do, nevertheless, seem to be reasons for supposing that the fat of milk may, at any rate in large proportion, be derived from albuminoids.

Thus, as compared with fattening increase, which may in a sense be said to be little more than an accumulation of reserve material from excess of food, milk is a special product, of a special gland, for a special normal exigency of the animal. Further, whilst common experience shows that the herbivorous animal becomes the more fat the more, within certain limits, its food is rich in carbohydrates, it points to the conclusion that both the yield of milk and its richness in butter are more connected with a liberal supply of the nitrogenous constituents in the food. Obviously, so far as this is the case, it may be only that thereby more active change in the system, and therefore greater activity of the special function, is maintained. The evidence at command is, at any rate, not inconsistent with the supposition that a good deal of the fat of milk may have its source in the breaking up of albuminoids, but direct evidence on the point is still wanting; and supposing such breaking up to take place in the gland, the question arises—What becomes of the by-products? Assuming, however, that such change does take place, the amount of nitrogenous substance supplied to the Rothamsted cows would be less in excess of the direct requirement for milk-production than the figures in the table would indicate, if, indeed, in excess at all.

The figures in the column of Table VI. relating to the estimated amount of digestible non-nitrogenous substance reckoned as starch show that the quantity actually consumed was 11.71 ℔, whilst the amount estimated by Wolff to be required was 12.5 ℔, besides 0.4 ℔ of fat. The figures further show that, deducting 7.4 ℔ for sustenance from the quantity actually consumed, there would remain 4.31 ℔ available for milk-production, whilst only about 3.02 ℔ would be required supposing that both the fat of the milk and the sugar had been derived from the carbohydrates of the food; and, according to this calculation, there would still be an excess in the daily food of 1.29 ℔. It is to be borne in mind, however, that estimates of the requirement for mere sustenance are mainly founded on the results of experiments in which the animals are allowed only such a limited amount of food as will maintain them without either loss or gain when at rest. But physiological considerations point to the conclusion that the expenditure, independently of loss or gain, will be the greater the more liberal the ration, and hence it is probable that the real excess, if any, over that required for sustenance and milk-production would be less than that indicated in the table, which is calculated on the assumption of a fixed requirement for sustenance for a given live-weight of the animal. Supposing that there really was any material excess of either the nitrogenous or the non-nitrogenous constituents supplied over the requirement for sustenance and milk-production, the question arises—Whether, or to what extent, it conduced to increase in live-weight of the animals, or whether it was in part, or wholly, voided, and so wasted.

Table VII.—Percentage Composition of Milk each Month of the Year; also Average Yield of Milk, and of Constituents, per Head per Day each Month, according to Rothamsted Dairy Records.

Average Composition of Milk each
Month, 1884.
(Dr Vieth—14,235 analyses.)		Rothamsted Diary.
Average
Yield
of Milk
per Head
per Day,
6 Years.		Estimated Quantity
of Constituents in
Milk per Head per
Day each Month.
Specific
Gravity.		Butter-
Fat.		Solids
not
Fat.		Total
Solids.		Butter-
Fat.		Solids
not
Fat.		Total
Solids.
% % % ℔ ℔ ℔ ℔
January 1.0325 3.55 9.34 12.89 20.31* 0.72 1.90 2.62
February 1.0325 3.53 9.24 12.77 22.81 0.80 2.11 2.91
March 1.0323 3.50 9.22 12.72 24.19 0.85 2.23 3.08
April 1.0323 3.43 9.22 12.65 26.50 0.91 2.44 3.35
May 1.0324 3.34 9.30 12.64 31.31 1.05 2.91 3.96
June 1.0323 3.31 9.19 12.50 30.81 1.02 2.83 3.85
July 1.0319 3.47 9.13 12.60 28.00 0.97 2.56 3.53
August 1.0318 3.87 9.08 12.95 25.00 0.97 2.27 3.24
September 1.0321 4.11 9.17 13.28 22.94 0.94 2.11 3.05
October 1.0324 4.26 9.27 13.53 21.00 0.89 1.95 2.84
November 1.0324 4.36 9.29 13.65 19.19 0.84 1.78 2.62
December 1.0326 4.10 9.29 13.39 19.31 0.79 1.79 2.58
Mean 1.0323 3.74 9.22 12.96 24.28 0.90 2.24 3.14
* Average over five years only, as the records did not commence until February 1884.

As regards the influence of the period of the year, with its characteristic changes of food, on the quantity and composition of the milk, the first column of the second division of Table VII. shows the average yield of milk per head per day of the Rothamsted herd, averaging about 42 cows, almost exclusively Shorthorns, in each month of the year, over six years, 1884 to 1889 inclusive; and the succeeding columns show that amounts of butter-fat, of solids not fat, and of total solids in the average yield per head per day in each month of the year, calculated, not according to direct analytical determinations made at Rothamsted, but according to the results of more than 14,000 analyses made, under the superintendence of Dr Vieth, in the laboratory of the Aylesbury Dairy Company in 1884;[4] the samples analysed representing the milk from a great many different farms in each month.

It should be stated that the Rothamsted cows had cake throughout the year; at first 4 ℔ per head per day, but afterwards graduated according to the yield of milk, on the basis of 4 ℔ for a yield of 28 ℔ of milk, the result being that then the amount given averaged more per head per day during the grazing period, but less earlier and later in the year. Bran, hay and straw-chaff, and roots (generally mangel), were also given when the animals were not turned out to grass. The general plan was, therefore, to give cake alone in addition when the cows were turned out to grass, but some other dry food, and roots, when entirely in the shed during the winter and early spring months.

Referring to the column showing the average yield of milk per head per day each month over the six years, it will be seen that during the six months January, February, September, October, November and December the average yield was sometimes below 20 ℔ and on the average only about 21 ℔ of milk per head per day; whilst over the other six months it averaged 27.63 ℔, and over May and June more than 31 ℔ per head per day. That is to say, the quantity of milk yielded was considerably greater during the grazing period than when the animals had more dry food, and roots instead of grass.

Next, referring to the particulars of composition, according to Dr Vieth’s results, which may well be considered as typical for the different periods of the year, it is seen that the specific gravity of the milk was only average, or lower than average, during the grazing period, but rather higher in the earlier and later months of the year. The percentage of total solids was rather lower than the average at the beginning of the year, lowest during the chief grazing months, but considerably higher in the later months of the year, when the animals were kept in the shed and received more dry food. The percentage of butter-fat follows very closely that of the total solids, being the lowest during the best grazing months, but considerably higher than the average during the last four or five months of the year, when more dry food was given. The percentage of solids not fat was considerably the lowest during the later months of the grazing period, but average, or higher than average, during the earlier and later months of the year. It may be observed that, according to the average percentages given in the table, a gallon of milk will contain more of both total solids and of butter-fat in the later months of the year; that is, when there is less grass and more dry food given.