Oestrogens in milk.

The steroid oestrogens oestradiol-17 beta, oestrone and oestradiol-17 alpha have all been adequately identified in various body fluids and tissues of cattle. There is also good evidence for the presence of oestrone sulphate. Oestriol (or similar triols) may also be present. The oestrogens found in the systemic plasma of cattle are present in milk in similar concentrations; whether their passage into milk involves metabolism by the mammary gland is uncertain. Oestrone sulphate, at least at relatively high levels of secretion, is believed to be found only in pregnant cattle and measurement of its concentration in milk is in use as a practical test for pregnancy. A close correlation has been found between the concentrations of oestradiol-17 beta in systemic plasma and milk of non-pregnant cows and levels of this oestrogen in milk may, together with those of progesterone, now be used in studies of post-partum ovarian function.