Plasma hormone levels in the cow

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Abstract
Progesterone and oestradiol concentrations in the peripheral plasma of 12 dairy cows during the oestrous cycle were determined by competitive protein-binding and radioimmunoassay respectively. The levels of progesterone reached a peak of 6-7 ng/ml in mid-cycle, then began to fall 4 days before oestrus to values of about 1 ng/ml at oestrus and remained low for a further 4 days. Peak levels of oestradiol (11.2 ± 1.3 pg/ml, s.e.) occurred on the day before oestrus and a second peak (7.9 ± 2.9 pg/ml) occurred on Days 6-8 of the cycle.