Observations on the anti-gestation effect of Aimax (methallibure) in recently bred gilts.

AN ANTI-GESTATION effect has been reported by Barker (1) in recently bred gilts fed Aimax.1 It was suggested (1) that the first 20 to 21 days of the gestation period may be the specific period during which the estrous synchronizing dose (100 mg/gilt/day) of Aimax acts anti-gestationally. While this may be the first report on an anti-gestation effect in swine, Paget et al (8) first reported the effect in 1961 in rats. They observed termination of pregnancy with oral doses of 100 mg/kg/day on any three consecutive days of pregnancy. Harper (4) presented results in rats that showed the compound has a marked effect on fertility. Oral doses of 100 mg/kg/ day for three consecutive days did not inhibit implantation when the treatment was initiated on days 1, 0, 3 and 4 of pregnancy. Treatment on days 2 to 4 did completely prevent pregnancy. Harper (4) also commented that in rats treated on the days previous to days 2 to 4 of those eggs that implant very few develop into viable fetuses. Thus treatment on days 0-2, 1-3 or 2-4 was most likely to demonstrate the greatest effects of the compound on pregnancy in rats. Administration at earlier or later stages produced much less effect on the number of eggs implanting. Groves (3) reported birth of a normal litter of piglets from a gilt accidentally assigned to a 20 day period of treatment at mid-pregnancy. Barker (2) has reported a similar observation at near term and in unpublished data on feeding trials within 10 days of the termination of pregnancy no effect was noted. The observations noted in (1) suggest that a brief period exists during early gestation in gilts when the compound exerts its greatest effect and the accidental or intentional feed-