GESTATION AND LACTATION 83 



is hereditary. The present highly productive dairy breeds 

 owe their greater ability to produce large quantities of milk 

 to their inheritance of this quality from their ancestors. 

 In the same breed certain families are known to possess the 

 quality of large capacity for milk production to a higher de- 

 gree than the average of the breed. It is also true that the 

 duration of the period of lactation is influenced by heredity. 



79. Exercise. An excessive amount of muscular 

 exertion of any kind must be regarded as unfavorable 

 to the maximum production of milk. Cows that are 

 required to travel long distances over sparse pastures 

 in order to secure sufficient food for their needs cannot 

 produce their maximum quantity of milk. In many 

 European countries, cows are generally employed as 

 draft animals in the ordinary farm operations of plowing, 

 harrowing, reaping and other work. Investigations in 

 Germany have shown that so long as cows are employed 

 at moderate work the milk flow is not decreased. When- 

 ever cows were compelled to work at heavy labor and 

 for long hours, invariably the flow of milk was decreased. 



80. Climate. Exposure to extreme dry cold or to cold 

 driving storms will have the effect of decreasing the normal 

 milk yield of a herd of cows. In general, a lack of ade- 

 quate shelter in a cold and humid climate may seriously in- 

 terfere with the highest development of the milking func- 

 tions. A herd of cows will produce more milk in winter 

 if provided with water which has been slightly warmed. 



81. Unusual lactation. In general, lactation is closely 

 associated with reproduction. Pregnancy followed by 

 parturition normally precedes the secretion of milk in 

 the mammary glands. Males and sterile females possess 

 rudimentary mammae but seldom secrete milk, although 

 a number of cases are on record where males have been 



