FERTILITY-CONTROL USING INTRAUTERINE-DEVICES - AN ALTERNATIVE FOR POPULATION-CONTROL IN WILD HORSES

The purpose of this study was to develop a contraceptive method for fe ral horses. The feral horse population has increased significantly in recent years despite attempts to control numbers. As in most wild anim al population control programs, contraceptive methods must be easy to apply, cause minimal disruption to the social structure and be fully r eversible. In the present study, we tested the effectiveness of an int rauterine device (IUD) for fertility control in mares. Six mares were fitted with a silastic O-ring-shaped IUD on July 1 of Year 1. The IUD- treated mares were turned out with 12 nontreated mares and a fertile s tallion in a large pasture until October 20 (112 d). None of the IUD-t reated mares and all the nontreated mares became pregnant. The IUD-tre ated mares were maintained separately from the stallion during the win ter. Following removal of the IUD on April 27 of Year 2, the mares wer e again introduced to the pasture with the stallion together with 6 no ntreated mares. For the 6 mares previously treated with an IUD, the me an interval from introduction to the stallion to conception was 17.5 /- 5 d or 1.3 cycles per pregnancy, and all mares produced a normal fo al at term. Subsequently, 19 recorded mare breeding seasons resulted i n 18 foals. Uterine cytology and histopathology indicate that the IUD causes mild chronic endometritis without permanent changes in the endo metrium. We conclude that based on our observations, the O-ring-shaped IUD is an effective, safe and practical contraceptive method for mare s.