formed. The development of the mammary glands after puberty varies considerably with the species. As a general rule, further development of the mammary gland is closely associated with the onset of pregnancy, although in some cases, as in the mare and bitch, the mammary glands occasionally develop to the milk-pro- ducing stage even in the absence of pregnancy. The development of the mammary gland and the secretion of milk are closely correlated with the formation and subsequent atrophy of a body called the corpus luteum. This body is found in the ovary as the result of the shedding of an egg cell or ovum, which, if fertilized, develops into the embryo or foetus. Provision is thus made for the adequate nutrition of the young animal after birth, and it is owing to this correlation between the development and atrophy of the corpus luteum and the development of the mammary glands that the mammary glands are enabled to secrete milk shortly after birth of the young.
The milk first secreted after the birth of the young is abnormal in appearance, and contains a large amount of albumin and globulin, together with a large number of cells. This first-produced milk is called colostrum or " biestings " and is generally used by the young calf. The change from colostrum to normal milk is gradual, generally taking about five days. The following table gives the chemical com- position of colostrum and ordinary cow's milk :
Colostrum
Milk
Water Casein Albumin \ Globulin J Fat Milk Sugar .... Ash
73-1? 2-65
16-56
3-54 3-00 1-18
87-75 3-oo
0-50
3-40 4-60 o-75
Two views are held as to the manner in which milk is secreted. The first view holds that the cells of the alveolus enlarge, the enlarged end breaking off with its contents and forming milk, the lower half of the cell regenerating again and repeating the process. The second view is that milk is a true secretory fluid, the secretion from the cells being produced in the same way as in any other gland. Recent work tends to confirm the second view as being correct.
Factors Influencing Milk Yield. The various causes which affect the yield of milk nearly always affect the percentage com- position of its ingredients owing to the fact that the fat of milk is secreted independently of the other solids, and the various causes leading to alteration in yield affect only the water and solids, not the fat portion. Any factors therefore which alter the yield of milk will generally lead to variation in the percentage fat content. For this reason the variation in fat content of milk may range from 1-7 to 6-0 per cent. Under normal conditions, however, the percentage of fat present in milk is a breed charac- teristic, and the capacity to produce milk containing a high per- centage of fat is an inherited characteristic. This fact has been utilized in Denmark to grade up the cow population, and by careful control of breeding allied with fat tests on the milk of milk-recorded cows, the average fat content of milk produced by cows in that country has been increased. The chief difficulty in carrying out the same grading-up process in England is due to the common practice of discarding bulls after a few years' use, the result being that the bull is killed before its real value as a milk-getter is known. The following table gives the average fat percentage of the various breeds of cows:
Breed
Fat Per- centage
Breed
Fat Per- centage
Jersey Guernsey Devon Shorthorn
5-35 5-16 4-60 4-05
Brown Swiss Ayrshire Holstein Friesian .
4-25 3-66
3-42
The various circumstances that affect the yield and composi- tion of the milk of cows are as follow:
(a) Period of Lactation. The amount of milk secreted rises for the first two or three weeks after calving, then falls slightly for about six weeks. The rate of fall then gradually increases, accelerating greatly as the cow approaches the next calving period, until near calving the milk flow ceases altogether. In the case of cows not pregnant the milk flow may continue for as long as two years, and it is said that spaying greatly prolongs the period of yield. As the yield falls off, the fat percentage tends to increase.
(ft) The Act of Milking. Much of the milk obtained at a single milking is actually secreted at the time of milking. The first-drawn milk is generally poor in fat (it may be as low as I %), and the percentage of fat rises gradually until in the strippings the fat content may be as high as 15 per cent.
(c) The Frequency of Milking. The longer the interval between the milkings, the larger is the amount of milk secreted at each milk- ing. This milk, however, is poorer in fat owing to the fact that the quantity of fat secreted at each milking is approximately the same, whereas the quantity of solids not fat and water secreted is depend- ent upon the interval between the milking. The pressure of the secreted milk in the udder inhibits the secretion of fat, and to a certain extent the secretion of solids not fat, so that a cow milked three times a day at equal periods will yield more milk relatively richer in fat than if milked twice a day.
(d) The Time of the Year. A comparison of milk records, after allowing for effect of lactation period, will show that cows yield most milk in May and June and least in Nov. and December. The greater yield of milk in the spring months is undoubtedly correlated with the abundance of fresh green grass in the feed, and there appears little doubt that the yield of milk is partially dependent upon the palat- abilty and succulence of the feed. For this reason, the practice among dairymen of giving wet mashes to cows is sound.
(e) Age of the Cow. The yield of milk given in a lactation period increases up to the sixth lactation period, remains constant for the next two or three lactation periods and then declines.
(/) Other Conditions. Carefully controlled experiments have shown that cows may perform a useful amount of muscular work without diminishing the milk yield to any serious extent, whereas the fat content may be actually increased.
The cow giving a fair yield of milk is producing large quantities of heat in excess of her requirements, and may therefore be subjected to fairly low temperatures without in any way decreasing her yield of milk or increasing her food requirements. In view of the tendency for extension in England of out-of-door cow-keeping, this fact is important. An accumulation of milk in the udder will cause an atro- phy of some of the milk-secreting cells, leading to a decrease in the milk-yielding capacity which cannot be regained until the next lactation period. The same effect will be produced by long periods of under-feeding. For this reason it is extremely important that the udder should always be stripped at each milking, that the intervals of milking should be approximately equal, and that the cow should be adequately fed in proportion to her milk yield.
Feeding in Relation to Milk Production. The magnitude of the dairying industry has directed the attention of research workers to the study of the relationship between milk yield and food requirement. It is obvious from a commercial standpoint that the establishment of such a relationship is the first impor- tant step towards the production of cheap clean milk. It is no exaggeration to say that the health of a nation is dependent largely upon an abundant supply of good cheap milk, and, to obtain this milk, production must be made profitable.
Under the auspices of the British Ministry of Agriculture the first essential step was taken by the introduction of milk- recording schemes. Under this scheme cows are ear-marked and the daily supply of milk yielded carefully weighed and recorded. In order to render the records accurate and of com- mercial value, surprise visits are made by official milk-recorders, who inspect the weighing of the milk, ear-marking and record- ing of the individual cows in a herd. The farmer who records his cows can thus obtain the annual record of the milk production of each of his cows, and this enables him to weed out of the herd the unprofitable cows. Before the World War it was established that, with cows yielding less than 600 gallons a year, milk could not be produced at a profit, and yet at that time the average milk yield for the country was well below this figure. In other words, the farming community was producing milk at a loss. The milk-recording of dairy cows is, therefore, the first impor- tant step towards the economical production of clean milk.
Having satisfactorily obtained the record of the individual yields of milk of cows, the next important item to consider is whether this milk is being produced at the minimum necessary expenditure of food material. Evidence has already shown that the herd giving the largest yield of milk does not necessarily produce the milk at the cheapest cost per gallon. It consequently becomes very important to investigate the possibility of feeding cows in such a way as to ensure the production of the milk with the least possible expenditure of food material.
This problem has been given increased attention since 1910 and, owing chiefly to the labours of keen men of science, has been satisfactorily solved. In this connexion brief mention may be made of Wolff, Lehmann, Kellner, Haecker, Savage, Armsby, Pott, Henry Morrison and their numerous co-workers, to whom our present knowledge on feeding standards is due.
