Reproductive response of mares after treatment with progestogens with and without the additional of estradiol

Summary A study involving 60 light-horse mares was conducted both to evaluate the response of mares to injectable progester- one or altrenogest and to determine ifestradiol in combination with either progestogen provided any added benefit. Treatments were initiated at either early estrus, late estrus, early diestrus, mid-diestrus or late diestrus in order to assess the effect of stage of cycle at onset of treatment. Within each of these stages of the cycle, mares were randomly assigned to 1 of 4 treatments: 150 mg progesterone injected i.m. (P); 150 mg progesterone + 10 mg estradio11713 injected i.m. (P+); .044 mg altrenogest per kg body weight orally (A); and .044 mg per kg body weight orally plus 10 mg estradiol 1713 i.m. (A+). All treatments were given daily for 7 days with 10 mg PGFaCt given on day 7 to all mares. The number of mares ovulating by day 14 after treatment (N=15/group) was 13, 7,11 and 8 forA, A+, P and P+, respectively. The response of mares to progesterone and altrenogest was similar. Fewer (Pi0.05) mares given combined steroid treatments ovulated within 14 days (15 of 30) than those given progestogen treatments. Stage of cycle had no affect (Pi0.05) on response of mares ovulating within 14 days or after 14 days of treatment. Mares that ovulated within 14 days of treatment had larger foUieles after progestogen treatment than those not ovulating by 14 days.