Oestrogen excretion in mares in relation to various reproductive states

The rate of urinary oestrogen excretion was studied in four ovariectomized mares. In two animals ovariectomized when urinary oestrogen concentration was high, levels fell to a minimun within 48 hr. Intramuscular injection of oestradiol-17beta produced maximum urinary oestrogen levels within 3 to 6 hr and a return to minimum detectable levels within 24 hr. The major urinary oestrogen in mares was oestrone. In nine maiden mares studied between September and June, the cycle, the durations of dioestrus and oestrus, and the time of ovulation were similar in autumn (September to December) and spring (January to June). All nine mares had periods of anoestrus and 'shallow' and 'deep' forms of this condition were observed. In all cyclic mares, urinary oestrogen excretion showed a gradual rise from dioestrous levels beginning 6 to 7 days before ovulation. A rapid and highly significant (P less than 0-001) fall in oestrogen excretion occurred on the day of ovulation and the day after. Maximum urinary oestrogen levels were twice as high in spring than in autumn cycles.