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1 gallon, or even less, towards the end of her period of lactation. At the same time, an entire herd of, for example, Shorthorns or Ayrshires, of fairly average quality, well fed, and including animals at various periods of lactation, should not yield an average of less than 8 quarts, or 2 gallons, and would seldom exceed 10 quarts, or 21/2 gallons, per head per day the year round.

For the sake of illustration, an average yield of milk of 10 quarts, equal 21/2 gallons, or between 25 and 26 ℔ per head per day, may be assumed, and the amount of constituents in the weekly yield at this rate may be compared with that in the weekly increase of the fattening ox at the higher rate assumed in the table, namely, 15 ℔ per 1000 ℔ live-weight, or 1.5% per week. It is seen that whilst of the nitrogenous substance of the food the amount stored up in the fattening increase of an ox would be only 1.13 ℔, the amount carried off as such in the milk would be 6.6 ℔, or nearly six times as much. Of mineral matter, again, whilst the fattening increase would only require about 0.22 ℔, the milk would carry off 1.35 ℔, or again about six times as much. Of fat, however, whilst the fattening increase would contain 9.53 ℔, the milk would contain only 6.33 ℔, or only about two-thirds as much. On the other hand, whilst the fattening increase contains no other non-nitrogenous substance than fat, the milk would carry off 8.32 ℔ in the form of milk-sugar. This amount of milk-sugar, reckoned as fat, would correspond approximately to the difference between the fat in the milk and that in the fattening increase.

It is evident, then, that the drain upon the food is very much greater for the production of milk than for that of meat. This is especially the case in the important item of nitrogenous substance; and if, as is frequently assumed, the butter-fat of the milk is at any rate largely derived from the nitrogenous substance of the food, so far as it is so at least about two parts of such substance would be required to produce one of fat. On such an assumption, therefore, the drain upon the nitrogenous substance of the food would be very much greater than that indicated in the table as existing as nitrogenous substance in the milk. To this point further reference will be made presently.

Table VI.—Constituents consumed per 1000 ℔ Live-Weight per Day,
for Sustenance and for Milk-Production. The Rothamsted Herd
of 30 Cows, Spring 1884.

	Total Dry Substance.	Digestible.
		Nitrogenous Substance.	Non- Nitrogenous Substance (as Starch).	Total Nitrogenous and Non- Nitrogenous Substance.
	℔	℔	℔	℔
3.1 ℔ Cotton cake	2.76	1.07	1.50	2.57
2.7 ℔ Bran	2.33	0.33	1.09	1.42
2.8 ℔ Hay-chaff	2.34	0.15	1.18	1.33
5.6 ℔ Oat-straw-chaff	4.64	0.08	2.21	2.29
62.8 ℔ Mangel	7.85	1.01	5.73	6.74
Total	19.92	2.64*	11.71*	14.35
Required for sustenance		0.57	7.40	7.97
Available for milk		2.07	4.31	6.38
In 23.3 ℔ milk		0.85	3.02	3.87
Excess in food		1.22	1.29	2.51
Per 1000 ℔ Live-Weight.
	℔	℔	℔	℔
Wolff	24	2.5	12.5†	15.4

 * Albuminoid ratio, 1-4.4.
† Exclusive of 0.4 fat; albuminoid ratio, 1-5.4.

Attention may next be directed to the amounts of food, and of certain of its constituents, consumed for the production of a given amount of milk. This point is illustrated in Table VI., which shows the constituents consumed per 1000 ℔ live-weight per day in the case of the Rothamsted herd of 30 cows in the spring of 1884. On the left hand are shown the actual amounts of the different foods consumed per 1000 ℔ live-weight per day; and in the respective columns are recorded—first the amounts of total dry substance which the foods contained, and then the amounts of digestible nitrogenous, digestible non-nitrogenous (reckoned as starch), and digestible total organic substance which the different foods would supply; these being calculated according to Lawes and Gilbert’s own estimates of the percentage composition of the foods, and to Wolff’s estimates of the proportion of the several constituents which would be digestible.

The first column shows that the amount of total dry substance of food actually consumed by the herd, per 1000 ℔ live-weight per day, was scarcely 20 ℔ whilst Wolff’s^[1] estimated requirement, as stated at the foot of the table, is 24 ℔. But his ration would doubtless consist to a greater extent of hay and straw-chaff, containing a larger proportion of indigestible and effete woody fibre. The figures show, indeed that the Rothamsted ration supplied, though nearly the same, even a somewhat less amount of total digestible constituents than Wolff’s.

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

1. Landw. Futterungslehre, 5te Aufl., 1888, p. 249.