Although the primary object of recording cows' milk yields is to provide a means of comparing one individual with another, it is not satisfactorily achieved when the records have been obtained, since yields are influenced to a large extent by environmental factors which vary from cow to cow. The lactation record is the result of two sets of factors—genetic and environmental—and for purposes of selection and breeding it is important to be able to make accurate allowance for the one, so as to arrive at a good estimate of the other.

Leaving aside the variation due to feeding and management (which, whilst undoubtedly large, is minimised for the cows of the same herd, and which it is hardly possible to study statistically in the existing data) the chief factors operating on the lactation yield (i.e. the measurable environmental factors) are the following:

(1) Season of the year; the lactation yield is influenced to a certain extent by the month of the year in which the cow calves.

(2) Service; i.e. the stage of the lactation at which the cow again becomes pregnant. The interval between calving and the next fertile service is here termed the Service Period (S.P.); thus if a cow calves on June 1st, and becomes pregnant again on July 1st, her S.P. for that lactation is 30 days.