156 MASS. EXPERIMENT STATION BULLETIN 255 



mineraJ" group as against 10.4 per cent in the "mineral" group. Among the 

 heifers, if No. 88 were of the same age as the other three (4 months younger than 

 she actually is) her rate of increase in weight would in all probability be enough 

 higher to offset the difference of 13.3 per cent actually found. All things con- 

 sidered, the growth of the young animals was probably not materially influenced 

 by the addition of the mineral supplement. 



Another angle from which growth records have been viewed is that of weight 

 of the young stock born and reared during the course of the experiment and now 

 in the herd, as compared with the weight of their dams at a similar age. Of 

 the ten animals listed in Tables 3 and 4, two are as large as their dams were at the 

 same age, six are larger, and two are smaller. The grouping in the experiment 

 is as follows: 



Mineral supplement No mineral supplement 



Larger than dam 3 Larger than dam 3 



Same size 1 Same size 1 



Smaller 1 Smaller 1 



All are Holsteins and sired by the same bull, except the two which are the same 

 size as their dams were, which are Jerseys. These facts dispose of possible varia- 

 tion due to influence of different sires; and the identical make-up of the two groups 

 is further evidence that the mineral supplement had little, if any, effect on growth. 



Milk yield 



The average production of the herd has been well maintained all through the 

 experiment. The corrected^ daily yield per cow for the whole herd and for the 

 two groups is shown in Table 5. 



Table 5. — Daily Yield of Milk, Total Solids, and Fat. per Cow 



No. of Milk Total Solids Fat 



Cows Lbs. Lbs. Lbs. 



Whole Herd 



1st year 10 26.47 3.32 1.07 



2d year 11 27.59 3.46 1.09 



Mineral Group 



1st year o 25.03 3.14 1.00 



2d year 5 25.38 3.25 1.02 



Entire experiment 10 25.21 3.20 1.01 



Non-Mineral Group 



1st year : 5 27.92 3.49 1.14 



2d year 6 29.64 3.65 1.15 



Entire experiment 11 28.85 3.58 1.14 



The "non-mineral" group was somewhat superior to the "mineral" group in 

 production but it would be unwise to conclude from this that the mineral supple- 

 ment had a detrimental effect in this respect. The probability is that the differ- 

 ence is due to variations in the inherent capacity of the individuals in the two 

 groups to produce milk. An endeavor has always been made to keep the groups 

 as evenly matched in this respect as possible, but due to the impossibility of 

 predicting in advance the performance of a cow, or more especiallj^ of a heifer, 



* Corrections for differences in length of lactation and stage of gestation have been made 

 according to the method of Gaines and Davidson, described in Illinois Agr. Expt. Sta. Bulletin 

 No. 272. Corrections for age have not been necessary as the groups were of approximately the 

 same age, and the age of the herd as a whole did not increase, as some older cows were sold and 

 heifers took their places. Corrections for breed are taken care of by including in the corrected 

 summary the total solids and fat produced as well as the total milk yield. 



