Oestrogen Function in Established Lactation in the Cow

SUMMARY In a series of six experiments the effect on milk yield and composition of injecting oestradiol monobenzoate (OB) at five dose levels has been studied on seventy-six cows of the Ayrshire, Guernsey, Friesian and Shorthorn breeds. The doses ranged from 6·25 to 100 mg, and all produced increases in the percentage of non-fatty solids. These changes were due mainly to increases in the protein, lactose and ash fractions and persisted for periods of several weeks. When more than 25 mg OB were injected, fat percentages also increased and milk yields declined. At the 12·5 mg level, significant increases occurred both in the yields of fat (lb.) and non-fatty solids (lb.). These results indicated that galactopoietic responses could be obtained only within a restricted dose-range, the limits of which were affected by stage of pregnancy and differences in breed. The considerable similarity which exists between these results and the changes occurring in the milk of pregnant cows during a normal lactation suggests that the latter are probably determined by a progressive rise in blood oestrogen levels as pregnancy advances. The results are discussed in relation to the effect of pasture oestrogens on milk composition and with regard to the possibility of improving the quality of milk by oestrogen administration.