CITY MILK SUPPLY 



tion are needed for nourishment of the fetus so that the flow falls off and 

 usually dries up. The udder then enters a resting stage, and the 

 animal goes through a period of recuperation, or conditioning for 6 weeks 

 or more, when she drops another calf and enters a second lactation 

 period. If this rest is not given the cow she begins the new lactation 

 period at a low level of production. Animals that remain farrow usually 

 give milk for about 2 years, though some farrow cows continue in milk 

 for a long time. 



Average Chemical Composition of Milk. The average chemical 

 composition of milk is indicated in Table 2 which is made up from analyses 

 from several sources, but in reality it is impossible to state the average 

 composition exactly for the milk of every dairy district is determined by 

 the dominant breed of cow; thus in a Holstein the average would differ 

 from that of a Guernsey section and that again from one where the cows 

 were mostly natives. 



TABLE 2. AVERAGE COMPOSITION OF Cow's MILK 



The composition of milk varies. As a rule, the percentage of milk 

 sugar and ash are most constant and that of the fat is most variable while 

 the protein varies with the fat but to a smaller extent. According to 

 Van Slyke the butterfat in the milk of an individual cow varies from less 

 than 2 to over 10 per cent., the casein from 2 to 4 per cent., the albumin 

 from 0.5 to 0.9 per cent., the casein and albumin together from 2.5 to 6 

 per cent., the lactose from 4 to 6 per cent., and the salts from 0.7 to 0.9 

 per cent. These variations in the constituents of milk may be regarded 

 as usual: butterfat 3 to 6 per cent., solids-not-fat 8.5 to 9 per cent., pro- 

 teins 3 to 5 per cent., milk sugar 4 to 6.5 per cent, and ash 0.7 to 0.8 

 per cent. The milk that reaches the consumer is usually the mixed prod- 

 uct of several cows so that variations in the milk of the individual cows 

 counterbalance one another with the result that considerable fluctuations 

 rarely occur. 



Commonly an analysis of milk includes only a determination of the 

 specific gravity, the butterfat and total solids; more complete analyses 

 give the milk sugar, casein, albumin, ash and index of refraction. The 



