VAEIATIONS IN COW'S MILK 29 



little fat and may not exceed 2.25 per cent, to 2.75 per 

 cent.* 



While it is scarcely possible, as will be discussed 

 later, to influence the composition of milk materially or 

 permanently by alterations in the quantity and com- 

 position of the food, still it may not be entirely impossi- 

 ble to influence the young animal by skilful or by 

 appropriate feeding in such a way as to encourage it 

 later to produce milk rich in fat. But thoroughly 

 reliable data on this point are not available. 



3. The age of the cow. Although the quantity of 

 milk varies according to the age of the cow, so that it is 

 usually greatest during the third to the fifth lactation 

 periods, the chemical composition appears to remain 

 almost unchanged from year to year (Hittcher) ; yet it 

 is apparent that fats as well as other ingredients 

 decrease with great age. 



4. The stage of the lactation period. The colostrum 

 secretion is not especially important. The true milk 

 secretion very quickly attains a considerable volume, but 

 falls off gradually after a longer or shorter time, until 

 the secretion ceases altogether or is insignificant to- 

 wards the end of the period of gestation. Some cows, 

 however, continue to milk very well up to calving and 

 to the new period of lactation. It was emphasized 

 above that colostrum differs essentially, both morpho- 

 logically and chemically, from normal milk. Further- 

 more, the composition of milk during the first part of 

 the lactation period differs from that secreted toward 

 the close. 



a. Colostrum. The first colostrum is whitish, yellow- 

 ish, or even reddish to brownish; it is slimy and has a 



* It should be borne in mind that the cattle here referred to 

 are of heavy milking lowland breeds not represented in America. 

 [L. P.] 



