Superovulation in the cow: comparison of oestradiol-17 beta and progesterone patterns in plasma and milk of cows induced to superovulate; relationships with ovarian responses.

Free oestradiol-17 beta, free + conjugated oestradiol-17 beta (total oestradiol-17 beta) and progesterone in milk, and free oestradiol-17 beta and progesterone in plasma were measured in 16 cyclic cows injected with FSH to induce superovulation during the treatment and periovulatory periods. The patterns of steroid secretion were the same in milk as in plasma but at different concentrations. Among oestrogens, the highest concentrations were measured for total oestradiol-17 beta in milk, followed by free oestradiol in plasma and free oestradiol in milk. Progesterone concentrations in milk were higher than in plasma. The peak concentrations of oestrogens were related to ovulation rate: Spearman Rank Correlation coefficient (r.s.) = 0.87 (P less than 0.001), 0.78 (P less than 0.001) and 0.69 (P less than 0.001) for total oestradiol, free oestradiol in milk and free oestradiol in plasma respectively. The increase in progesterone concentrations in milk between the beginning of treatment and prostaglandin injection was negatively correlated with the percentage of viable embryos among those recovered (r.s. = -0.68; P less than 0.001). This was not observed for progesterone in plasma. These results therefore show that the steroid pattern in milk gives a better indication as to the ovarian response to a superovulatory treatment than does the steroid pattern in plasma. In addition the fact that milk samples are easier to obtain and handle than blood plasma have led us to conclude that, to follow the effect of gonadotrophin stimulation, it would be more informative to assay oestradiol-17 beta and progesterone in milk rather than in plasma.