LACTATION 599 



volatile acids, but it tends to improve rapidly during the first few 

 months, the improvement being maintained until near the close of 

 the lactation period, i.e. in most cases near the approach of the next 

 parturition.^ 



It is well known that there is a change in the amount of milk 

 produced -by a cow as it grows older. Pearl has dealt with this 

 matter statistically and has arrived at the following general con- 

 clusion : " The amount of milk produced by a cow in a given 

 unit of time (seven days, one year, etc.) is a logarithmic function 

 of the age of the cow." 



The law may be stated verbally in the following way : Milk flow 

 increases with increasing age but at a constantly diminishing rate 

 (the increase in any given time being inversely proportional to the 

 total amount of flow already attained) until a maximum flow is 

 reached. After the age of maximum flow is passed the flow 

 diminishes with advancing age and at an increasing rate. The 

 rate of decrease after the maximum is, on the whole, much slower 

 than the rate of increase preceding the maximum. In general the 

 law above stated applies to the absolute amount of fat produced in 

 a unit time as well as to the milk. ^ These conclusions agree with 

 those of Gavin. 



The Duration of Lactation 



The duration of the lactation or nursing period in the different 

 species of animals is governed mainly by the needs of the young. In 

 such animals as the guinea-pig, in which the young are born in a 

 sufficiently advanced state of development that they are able to fend 

 for themselves, the length of the lactation period is relatively short 

 and inconstant, while in other animals, belonging to the same order 

 of Mammals, the young are born helpless, and are dependent for 

 some weeks upon their mother's milk. In the larger animals the 

 period of nursing is of course longer, but in them also its average 

 duration appears to depend largely upon the necessities of the 

 offspring. 



The natural period of lactation in the cow is between nine and 



1 In cows which are "drying off," the percentage of volatile acids in the 

 butter fat is very low. See Crowther, loo. cit. 



2 Pearl, " On the Law Eelating Milk Flow to Age in Dairy Cattle," Proc. 

 Soc. Exp. Biol. aiid Med., vol. xii., 1914. See also Pearl, "The Change of Milk 

 Plow with Age," Agric. Exp. State Papers, Orono, Maine, 1917 ; and Gowen, 

 "Studies in Milk Secretion," V. and VI., Genetics, -vol. v., 1920. These papers 

 give numerous references. For further information about variation in quantity 

 and quality in milk of dairy cattle see papers by Pearl and by Gowen, Jour, of 

 Agric. Research, vols. xvi. and xvii., 1919, and Gowen, Jour, of Dairy Science, 

 vol. iii., 1920. For "Transmission or Inheritance of Milking Capacity" 

 see Gowen, Jour, of Heredity, vol. xi., 1920. 



