46 



MANUAL OF MILK PRODUCTS 



are the average for the whole period of lactation. The percent- 

 ages of fat for the different ages are as follows : 



Influence of stage of lactation on fat-content of milk (Van Slyke). 

 From the time a cow "comes fresh in milk" up to the time 

 she becomes "dry/' the composition of the milk undergoes 

 gradual changes, which are quite independent of other factors. 

 The period of lactation varies in length with different individual 

 cows, but, for practical purposes, lasts about ten to twelve 

 months. The changes observed in the percentage of fat during 

 the progress of the lactation period are quite marked and 

 fairly regular, without reference to individual or breed. The 

 colostrum, which is the secretion produced by a cow soon after 

 calving, is very different in composition from normal milk, and 

 is not considered at all in our discussion of the constituents 

 of milk. The figures presented in the following table repre- 

 sent the monthly averages of nearly 100 different lactation 

 periods : 



