Five tests were conducted at intervals of 3 months to determine the seasonal variation in fertility following treatment with two types of intravaginal sponges impregnated with 30 mg of a synthetic progestagen (SC-9880; Cronolone, Searle). Sponges were prepared with 5 ml ethanol to release 20 mg SC-9880 over a 16-day period or with 1 ml ethanol to release 10 mg. Ewes were inseminated twice, 48 and 64 hr after withdrawal of the sponges.
There was a marked seasonal variation in all parameters associated with fertility and best results were obtained overall with the sponges releasing the higher dose. Treatment in the spring was followed by a low incidence of ovulation, a high incidence of 'silent heats' and few pregnancies following insemination (13·6%) ; most of the ewes that were not pregnant became anoestrous. Treatment in the autumn was followed by a high incidence of ovulation and oestrus, and a high pregnancy rate following a double insemination (82·5%), and few animals became anoestrous.