Monthly Variations in the Composition of Milk

Results of analyses of milk produced by cows of the Ayrshire; Brown Swiss, Guernsey, Holstein and Jersey breeds of dairy cattle and of Guernsey-Holstein crosses (the Bowlker Herd) have been reported in bulletins (1, 2) of the Illinois Agricultural Experiment Station.The complete plan of the experimental work, including detailed descriptions of the method used to obtain samples and of procedures used in the analyses of the samples, and complete tabulations of the analytical results obtained, are included in those bulletins.The purpose of the present publication is to study further the data presented in the two bulletins and to show the monthly variations in the composition of milk. EXPERIMENTAL DATAThe samples of milk were taken usually at four-or five-week intervals during the lactation periods.All samples were three-day composites.Each sample is tabulated in the month in which it was obtained, and its composition is considered to represent the composition of the milk of the cow during the four-or five-week interval.The periods of time used for the samples were determined by dividing the time according to the weekly production records, considering the time represented by a sample to extend from the week-end nearest midway between it and the last preceding sample to the week-end nearest midway between it and the next succeeding sample.