The percentage of fat in cows' milk varied definitely with seasonal changes. The fat test was highest during the first half of winter, or January. It gradually declined to the second half of summer, August and early September, when the lowest test occurred, and then it rose rather rapidly in the fall.
Ayrshire and Holstein tests were approximately 0.6 percent lower in the second half of summer than in the first half of winter. Guernsey and Jersey tests were approximately 1.1 percent lower.
The butterfat tests were found to be lower with higher outside and inside temperatures.
As measured by regression coefficients, butterfat tests were affected more by changes in environmental temperatures than by the other factors studied.
It was not determined whether variations in outside or inside temperature had the greater influence upon the fat test, altho there was an indication that variations in the fat test were more closely related to variations in outside temperature.
The butterfat test, during the lactation period, tended to be high immediately following freshening, declining for two or three months, and then rising during the rest of the lactation.
An increase in the fat test followed an advance in the stage of gestation.
There was considerable variation in the butterfat test which could not be attributed to the effect of the factors studied in this trial.
Weaver, Earl and Matthews, C. A.
"The influence of temperatures and certain other factors upon the percentage of fat in milk,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 8
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol8/iss107/1