596 THE PHYSIOLOGY OF REPRODUCTION 



THE INFLUENCE OF DIET AND OTHER FACTOKS ON THE 

 COMPOSITION AND YIELD OF MILK 



The composition of the milk in any one species is subject to 

 some amount of variation, which is due to various causes. Thus, 

 the differences in the composition of cow's milk are said to depend 

 on the following factors: (1) The breed, (2) The advance in the 

 period of lactation, (3) The season of the year, (4) The length of 

 the interval between the times of milking, (5) The occurrence of 

 sexual excitement, (6) Situation and climate, (7) Meteorological 

 changes, and (8) The character of the food. 1 These factors may 

 now be briefly discussed. 



That the yield and composition of the milk varies in the different 

 breeds is generally admitted. Thus Jersey cows yield a larger 

 proportion of butter fat than Ayrshires. Within the limits of the 

 breed also there is great individual variation and heredity plays an 

 important part. 2 But of all the factors enumerated above, diet is 

 perhaps the most important. The richest and also the most 

 abundant supply of milk is usually yielded when the food supply 

 is literal. As a result of giving food rich in protein substances, 

 the milk supply tends to contain a larger quantity of protein, sugar, and 

 fat (especially the latter). Sharpey Schafer has pointed out, however, 

 that because an excess of a particular organic principle in the food 

 causes an increase of certain constituents in the milk, it must not 

 be supposed that these constituents are necessarily formed directly 

 from such material, " for the effect may be produced indirectly by 

 the functions of the gland-cells becoming modified, according to 

 the nature of the pabulum they are receiving. Looked at in this 

 light, certain substances may be said to stimulate the cells of the 



1 Crowther, Milk Im-cftigations at Garforth, Leeds, 1904. Droop Richmond, 

 "The Composition of Milk." . I /m///.--/ 1 , vol. xxxi., 1906. Lander, "The Variation 

 in the Composition of Milk," Bulletin XI. issued by the Edinburgh and East 

 <>f Scotland College of Agriculture, 1906. Crowther, "The Chemical Composit inn 

 of Butter," Trout. Highland and Agric. Soc., vol. xix., 1907. Gilchrist and 

 Jones, "Dairy Investigations in the North-East of England," Trans, ifighland 

 and Agric. >SV., vol. xviii., 1906, and vol. xix., 1907. 



2 Gavin has dealt with the question of estimating a cow's milking capacity 

 by her first lactation yield, and has come to the conclusion that with cows 

 giving a fairly high or fairly low first lactation revised maximum, this figure 

 should l)e used to determine whether they should be kept or not, but with 

 cows giving a medium first lactation revised maximum, it is worth waiting to 

 obtain the increased accuracy of an estimate based on the mean between the 

 first and second lactation revised maxima. (The "Revised Maximum" is 

 defined as the maximum day-yield of the lactation which is three times reached 

 or exceeded, i.e. the highest figure common to the three highest day-yields of 

 a lactation.) Gavin thinks that the maximum yield is a better indicator of 

 physiological capability than the average yield, and therefore a better guide as 

 to whether a cow should be kept. ("Studies in Milk Records," .l>,,i,\ Agric. 

 Science, vol. v., 1913.) 



