122 Maine Agricultural Experiment Station. 1920. 



the total milk yield of the first five lactations (8 months lacta- 

 tion periods) will be for a Jersey herd of similar milk produc- 

 tion to the herd here studied. 



In a previous bulletin the variation of milk yield with age 

 has been determined. In the present paper the phase of the 

 problem dealing with one lactation in its relation to another will 

 be considered. 



The functioning of the mammary glands may be considered 

 dependent upon three main factors, taken in order of their posi- 

 tion in time, heredity, development through feeding, etc., (en- 

 vironmental circumstances of these organs up to their commenc- 

 ing to secrete) and lastly environmental factors in their widest 

 sense acting during the months when the gland is active. It is 

 reasonably clear that on our ability to distinguish the relative in- 

 fluence of these three basic variables depends many of the com- 

 mon a priori dairy practices as well as furnishing a solid founda- 

 tion for the analysis of the causal mechanism of milk production 

 itself. The analysis is a complex one and needs to be attacked 

 "by many channels. The present investigation was undertaken 

 in the hope that by an analysis of the intra individual variation 

 ■of milk secretion from lactation to lactation some light would be 

 given on the relative merits of these three variables. The homo- 

 geneous nature of the material is such, however, that the investi- 

 gation necessarily deals chiefly with the first of these variables. 



Little work on milk secretion has been done that approaches 

 the problem from this viewpoint. Of the available data those 

 on the English herds analyzed by Gavin are undoubtedly the 

 best. This investigation on a mixed herd of British Holsteins 

 :and grade Short-Horns furnishes data of the value for the rec- 

 ords of the first lactation in comparison with the yields of sub- 

 sequent lactation. In all of this work the' measure of the lacta- 

 tion used is what he designates as the "revised maximum," this 

 term being defined as the maximum day yield of the lactation 

 which is three times reached or exceeded. These results are 

 considered largely for their strictly practical bearing. They are 

 of little use to the American farmer in that he is accustomed to 

 deal with records over a certain limit of time and not maximum 

 productions. The constants derived by Gavin will be of a good 



