The Effect of Treatment with Oestradiol Benzoate on Oestrus Expression and Endometrial Oedema in Anovulatory and Cyclic Mares

SIMPLE SUMMARY: Anoestrous mares treated with oestradiol are often used to aid in the collection of stallion semen and as recipient mares to receive embryos when combined with progesterone. The aims of this study were to investigate the dose rate effect of oestradiol benzoate on the duration and persistence of endometrial oedema and oestrous behaviour in anoestrous and cyclic mares. The administration of oestradiol benzoate to seasonally anoestrous mares induced expression of oestrous behaviour and endometrial oedema within 48 h of treatment. Persistence of expression was proportional to the dose and the initial response. A dose of 2 mg was enough to induce oestrous behaviour and endometrial oedema in the majority of anoestrus mares. Regardless of dose, however, there was a strong individual mare effect on the intensity and duration of the response, which was not explained by the weight of the mare. In cyclic mares, treatment with 3 mg of oestradiol benzoate failed to induce endometrial oedema in dioestrous mares; therefore, it can be used as a rapid test to rule out the presence of elevated progesterone concentration in a mare that shows no endometrial oedema despite having a large follicle. ABSTRACT: Oestrogens treatment is often used to induce oestrus behaviour in anoestrous mares to aid in the collection of stallion semen and as recipient mares to receive embryos when combined with progesterone. However, there are no studies to describe the effect of dose and individual mare on the intensity and duration of the response, in both anoestrous and cyclic mares. In Experiment 1, 13 anoestrous mares were treated with one of five doses of oestradiol benzoate (OB) (1, 1.5, 2, 3 and 4 mg) per mare in five consecutive treatment periods (n = 65), to determine the response in terms of endometrial oedema and oestrous behaviour. Experiment 2 and 3 used 3 mg of OB in cyclic mares to confirm or deny the presence of an active corpus luteum (CL). There was a dose rate of OB and individual mare effect (p < 0.05) on the intensity and persistence of endometrial oedema and oestrous behaviour. A total of 2 mg OB was enough to induce endometrial oedema and oestrous behaviour within 48 h in most mares. Mares with an active CL did not show endometrial oedema following treatment of 3 mg OB.