114 MILK 



the physical condition of the cow seemed to have some influence 

 on the quantity of plasma sohds and fat. When the animals 

 were in good condition the sohds were relatively high in the be- 

 ginning of the lactation period, but in a short tune the normal was 

 reached. 



4 . Variation in Composition of Milk During the Lactation Period. 

 — The change in composition of milk during the lactation period 

 has been determined by a number of investigators in this country 

 and abroad. The variabihty of milk constituents diu-ing lactation 

 naturally affects the food value and digestibiUty of milk. How- 

 ever, this is probably of consequence only when milk is derived 

 from one or a few animals; usually market milk is obtained from a 

 large herd of cows, and experience has shown that the composition 

 of milk from a large herd is fairly well equahzed. 



The results of investigators are harmonious, on the whole, and 

 may be summed up as follows: The quantity of milk diminishes 

 as lactation advances, but the percentage of sohds increases. ' 

 This change is quite marked toward the close of lactation. Some 

 observers have stated that dm-ing the first weeks there is a decKne 

 in the percentage of solids, and that after this period there is a smaU 

 increase up to the eighth or ninth month, and all agree that dur- 

 ing the last two or three months of the lactation period there is a 

 marked increase in sohds. The fat is usually considered the most 

 variable factor, the proteins vary to a considerable extent, while 

 the milk-sugar and ash remain nearly constant. The Reichert- 

 Meissl, iodin, and Koettstorf er numbers generally decrease with the 

 progress of lactation. Hunziker's investigation led to the conclu- 

 sion that after an initial rapid decrease during the colostral period 

 and a few days afterward the protein decreases for two months, 

 then remains constant for about three months, and finally increases 

 up to the close of lactation. 



The work carried on at the New York Experiment Station also 

 showed conditions similar to those reported previously, namely: 

 the quantity of milk diminished with progress of the lactation 

 period; the fat content remained approximately stationary for 

 about five months and then increased constantly. The number 

 of small fat globules increased for the whole period, while the 

 number of large ones decreased. The total sohds declined shghtly 

 for one month and then increased to the close of lactation. The 

 casein followed the decrease and increase of total sohds, while the 

 milk-sugar was relatively variable. The sugar at first decreased, 

 then increased up to the tenth month, and finally decreased again. 

 The percentage of ash remained practically constant. 



The variability in composition during the lactation period 

 has been extensively studied by Eckles and Shaw, whose results 



