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A total of sixty ovariectomized beef heifers were involved in two trials to study the effects of oestradiol benzoate (odb) and human chorionic gonadotrophin (hcg) on induced oestrous behaviour, Both hormones were administered in single injections, odb subcutaneously in sesame oil and hcg intramuscularly in physiological saline. Dosages evaluated were: odb—0·2, 0·3, 0·4, 0·5 or 0·6 mg; hcg—0, 2000 or 5000 i.u. Animals were observed twice daily and scored for oestrous behaviour on a scale from 0 to 5.

The minimal dose of odb sufficient to induce 'standing' oestrus was 0 ·3 mg. The proportion of animals in heat, length of oestrus and total score tended to increase with increasing levels of odb. hcg administration (concurrent with odb) significantly delayed the onset of oestrus following injection in one trial, with the same tendency (non-significant) observed in the other trial.

Inexplicable oestrous behaviour was observed in several heifers after ovariectomy and hormone treatment.