232 Bulletin 397 



Variation of groups in fat production. — All the preceding studies have 

 been made with regard to milk production only, since that is the factor 

 given the greatest consideration in feeding trials. It seemed desirable to 

 ascertain whether the variation in fat production is similar to that in 

 milk production. In order to determine the variability of groups of cows 

 in fat production, groups were made up of cows having approximately 

 equal fat production. Since 100 pounds of milk was the range allowed 

 in the group limits in studying variation in milk production, a correspond- 

 ing range of 3.5 pounds of fat was allowed in the group limits in this study, 

 this being the approximate amount of fat in 100 pounds of average Hol- 

 stein milk. The results are shown in table 9: 



TABLE 9. Variation of Groups in Fat Production 



It is seen from table 9 that the coefficient of variability for fat pro- 

 duction is practically identical with the value shown in table 2 for milk 

 production. 



STUDIES OF HOLSTEIN RECORDS: COWS IN VARIOUS STAGES OF LACTATION 



As has been pointed out, the beginning of an experiment on a selected 

 date, using cows that have freshened during the preceding six months, 

 represents the procedure usually followed in a feeding experiment. There- 

 fore studies were made of records taken on this basis in order to furnish 

 data as to variability under this method of selection and to obtain infor- 

 mation by comparison with the results of the preceding method of selection 

 as to the effect of stage of lactation on variability. The general methods 

 used in these studies were the same as those employed in the former 

 ones, except that, as previously stated, the time elapsing between fresh- 

 ening and the beginning of the twenty-weeks period was not fixed. 



Individual variation 



The individual cows were here, as before, divided into classes of approxi- 

 mately similar individual production. Inasmuch as the cows were in 

 different stages of lactation, the average ntmiber of weeks after calving 

 is given for each group. The results of this study are shown in table 10: 



342 



