Control of the oestrous cycle in mares with altrenogest

The clinical effectiveness of the synthetic progestagen, altrenogest, was evaluated in field trials with 449 mares during the 1980 breeding season. An oral dose of 27 mg altrenogest was administered daily for 15 days. In the first trial treated mares were compared with controls, and in the second trial the effectiveness of treatment for prolonged or erratic spring oestrus was evaluated. Oestrus was suppressed in 94% of the treated mares in the first trial. The post-treatment response was related to the stage during the transition from winter anoestrus to the spring breeding season and degree of ovarian activity when mares were treated. In mares treated during the early transition period (before 15 March), the mean diameter of the largest follicle was about 1.4 cm. There were no differences (P greater than 0.05) in the duration of oestrus or interval to conception between treated (13.0, 41 days) and control (16.6, 39 days) mares. In mares treated after 15 March (late transition period), the mean diameter of the largest follicle was about 2.1 cm and the duration of oestrus and interval to conception was less (P less than 0.05) in treated (7.7, 28 days) than in control (12.1, 40 days) mares. The duration of oestrus and interval to conception was also less (P less than 0.05) for mares treated during the late transition period than in those treated early. More (P less than 0.05) mares treated during the late transition period (75%) exhibited regular post-treatment oestrous cycles than did controls (57%) or mares treated during the early transition period (55%). The mean interval to oestrus after treatment was 4.4 days. There were no differences in pregnancy rates among any of the groups. In the second trial, 78% of the mares had exhibited oestrus for 9-30 days before treatment. All mares returned to oestrus within 10 days after altrenogest treatment: 56% conceived within 18 days and 80% within 45 days.