NORMAL VARIATION IN I?AT CONTl^NT OF MIXJjD MILK. 303 



milking-s of the clay, and usually is highest in the evening milk, 

 although the month of March forms an exception to this rule. 

 That this diurnal change in percentage fat content is significant 

 is shown by table 3, which compares the differences with their 

 probable errors. 



TABLE 3. 



SHOWING THE DIFFERENCES BETWEEN MORNING AND EVENING MILK 

 IN MEAN PERCENTAGE OF FAT. 



Month. 



Difference between mean fat 



percent of morning and 



evening milk. 



Difference divided by its 

 probable error. 



March. 



May (as given) 



May (partially corrected) . . . 



July 



October 



. 205 -t. 023 

 .546 ±.042 

 .580 ±.034 

 .353±.032 

 .267±.031 



8.9 

 13.0 

 17.6 

 11.0 



8.6 



The probability that the percentage fat content of the even- 

 ing milk is really higiher than that of the morning milk is 

 obviously so great as to amount to certainty for all practical 

 purposes. This result agrees with the findings of Richmond.' 



2. Without exception the absolute amount of fat is greatest 

 in the morning milk, least in the evening milk, and interme- 

 diate in amount at the midday milking. This, of course, means 

 that the amount of milk produced in the long interval between 

 milkings is greater than in the short intervals. The differ- 

 ences are large and significant. 



8. The difference between morning and evening milk in 

 percentage fat content is significantly smaller in March and Octo- 

 ber, than in May and July. Owing to the uncertainty respecting 

 the May evening milk constants stress cannot be laid on the very 

 high difference in that month. July, however, shows the same 

 relation, though to a less marked degree. It would on general 

 grounds be expected that this difference would be greater on 

 pasture than on the more exactly controlled stall feeding. 



4. Taking into consideration the total day's milk it is seen 

 that the percentage fat content rises steadily from I\Iarch 

 through October. The milk of this herd was a little over one- 

 half o.f a per cent richer in fat in October than it was in March. 



*Cf. Analyst, Vol. 2,7, PP- 298-302, 1912. 



